Thanks for all your help. Im going to test all fuses and relays then if no luck start checking grounds with this tool. As long as I can figure how to use it!

 

Testing the grounds can be informative, but if you even suspect a ground problem on a Jaguar, the best course of action is to disassemble every ground you can access, clean the studs and nuts and washers and eyelet terminals with a small brass-bristle brush and zero-residue electronic cleaner, dry and reassemble and snug the nuts or bolts without over-tightening them. You just want a gas-tight assembly. You don't want stripped threads or a stud snapped off the body.

 

The Electrical Guide shows all manner of things including grounds.

 

Let's revisit "bridge the relay" and the starter responds. Just to be clear, you're talking about the small starter relay in the front power distribution panel under the hood, right? If so, you connected a test jumper between sockets 30 and 87 and the starter spun up? That mimics the starter relay receiving the start command and sending power to the starter solenoid via socket 87.






If that tested good, let's look at the command to the relay. You had mentioned the relay did not click with the key to start, so let's figure out why. Energizing the magnetic coil inside the relay is a two-part process. Battery power is send to socket 86 via the ignition switch and the transmission range selector. At the other side of the coil, the circuit is completed by the switched ground at socket 85. The instrument pack is what switches this ground on or off. Both sides of the circuit have to be completed for the relay to respond.

It's easy enough to check the voltage input to socket 86. Remove the relay. Connect the red lead of a voltmeter or test light to socket 86. Connect the black lead to a good ground. Turn the key to start. You should see battery voltage present at socket 86.

Checking the relay's ground via socket 85 is a little trickier. But if you've confirmed good voltage into 86, and substituted a known good relay, the ground signal is the culprit.

If the ignition switch is suspect, there's a simple test you can do before even touching the relay or a meter. In the diagram above, look at fuse #2 in the primary junction box. This sends a power off command to the stereo system while the key is in the start position. To test this feature, turn the key to run and turn on the stereo. Make sure the display is illuminated and your favorite tunes are cranking. (We don't judge) When you turn the key to start, the music should stop and the display goes blank, as if you had manually turned off the stereo. If this does not happen, there's a problem with the ignition switch or the fuse feeding it.

 

A few relevant pages of X200 security from my student guide 684.

 

Hey again! Thank you so much for your help and the diagram. I have just tested all fuses and relays with test equipment, and they are all good. It looks like I need to check this socket 85 to the ground. i have a pb100 circuit tester I just got. Do you know the best way for me to check this? I don't want to bust anything or the circuit tester as I believe the starter is very high voltage. But everything you said and the diagram really make sense that it is the starter ground that is bad. It's raining at the moment, but when it stops I am going to jack up the car and get under and check that the wires are on right and snug.

The radio does switch off when I turn the key also. I have found thwbrelay does click when i tey and start with the key

Is it possible that the guy who helped me has mixed around the live and ground on the starter? if so, would it make sense that it still turns when I jump the relay with a wire, but also cause the problems I have? Just a thought as the wires look alike, the big silver shielded ones. Although hopefully they wouldn't both reach the starter positive.

 

OK I have checked all my grounds etc, every single thing seems ok.

I did begin with an ABS fault that cleared on its own twice months and months ago. before my starter packed up I had an ABS fault for 2 days solid.
Today I went into the IDS Legacy software with the Mongoose cable, I know my battery is a bit low, its on charge now.

I has a LOT of codes, some seemed to clear, some are persistent, and may be down to me having a low battery at the moment. I will check them all later to educate myself now I think I know how I am meant to clear them.

Anti-Lock Braking system (01 DTC)
B1646
U1262

General ECM (01 DTC)
B1676

Audio Unit (01 DTC)
U2008

Instrument Pack (03 DTC)
B1676
U2152
U2162

Message Centre Control Module (03 DTC)
B1676
U1131
U1262

Powertrain Control Module (01 DTC)
P1000
P1233
P1260

Restraints Control Module (02 DTC)
B1318
B1342

Rear ECM (Err 0x100b)
Serial Communications Error
-----------------------------------------
Antilock Braking System (01DTC)
C1805

Instrument Pack (01DTC)
B2328 (apparently Radar system but I don't have one)

I have had a brief look, online but will look more, so far looks like codes ending in "26" may be just due to low battery voltage when I checked so I will re check and try to clear them all again tomorrow.

I have connected a jumper lead to my batt negative and tested that connected to my starter motor ground wire and got continuity, also got continuity when it was reattached to the starter. I have the same issues not starting etc, but I am now leaning towards it being me needing to clear codes. But, I am no expert at all obviously.

So, in brief, the way this happened in my eyes was,
1 broke down,
2 disconnected battery neg,
3 disco all wires to starter,
4 recon neg to batt so I could lock car, needed to leave job and lock car
5 disco neg to battery as I noticed rear lights were on (sure this was the start of the error)
6 Changed starter, recon batt, steering locked, rear lights on, other issues etc which I am sure all happened at "4"

Does this make any more sense to any of you amazing and helpful people.

Also, just want to add, it is so so nice of you all to spend your time trying to help me, and everyone else. Really restoring my faith in humanity.

 

Quick note: starter is battery voltage but high current. Such high current that the battery voltage will drop (sag) a bit depending on its condition.

That sag can upset modules but doesn't sound like that's the main problem. (Or maybe?)

One test is to have someone watch the battery voltage (meter on battery) as you try to start. Shouldn't drop much. Can be worth having the meter on a place in the engine bay if a drop is suspected.

 

P1260 is the key transponder NOT RECOGNIZED!!!!!!!!!
The car will NEVER START if it is a hard fault. (cannot be cleared)

 

Oooh! Making some progress.

I'm still a bit baffled, though. After poking around online, P1260 generally seems to clear itself once a properly coded key is present and the transponder is powered up. For example, the fuse to the transponder could have been open, etc. Or maybe the magic bean had fallen out of the key. But once the fault was corrected, no reset wizardry was needed. But in this present case, the fault seems to continue, even though the fuse checked good, 2 known-good keys were tried, etc.

Take a look at this similar thread for P1260:

https://www.jaguarforums.com/forum/s...-stuck-249755/

Very interesting read. The fault was a bad wire near the ignition switch. The gears in my head are turning, trying to think of some specific tests we can suggest for the latest situation.

 

Plan of attack formulated! Punt on the first down!

Okay, got a couple of quick things to check. Previously I had you check some fuses, but we didn't actually check if those fuses were receiving power. Both circuits are switched on with the ignition, so are not always hot.

The first is the circuit powering the key transponder. This is the ring antenna where the key is inserted. Turn the key to Run. See if you have power at fuse F24 in the Primary Junction Box, the little panel inside the cabin. No need to remove the fuse, if you can touch either of the little openings in the head of the fuse.

Next is fuse F17 in the same panel. Same thing, key to Run and see if battery power is present. This is the circuit that powers the "OK to start" portion of the instrument cluster, which sends the ground signal to the starter relay.

That same F17 fuse also powers the Airbag indicator light in the instrument cluster. With the key to run, the airbag warning light should illuminate for a few seconds (as a lamp test) and then go out. Does this light illuminate? If not, we can narrow our search without even using a meter.

 

Thanks. I will try these in the morning!

 

Some of those DTCs look to be low battery voltage.

These cars HATE a tired battery and can throw just about any codes. And not start.

 

Excellent point. Along that line, inspect and clean the connection where the battery ground cable is attached to the body. The tip of the bolt extends into the wheelwell. Moisture can wick up via the threads and corrode the mating surfaces. Remove the cable, clean both sides to bare metal, and then reassemble. Add a little dielectric grease to inhibit further corrosion.

This connection is a known problem area. It came to mind, wondering what could have changed. Since you'd have disconnected the battery, this short stiff cable may have flexed and affected the connection. The corrosive effect is even worse in areas with heavy road salt, etc. One might think the climate in Australia isn't like that, but don't forget one critical detail. This is the country that foisted Air Supply on the world. Even to this day, I'm sure the atmosphere is still highly saccharine and corrosive:

 

Hey. I tested All the fuses in both sides on run they are all good. Today with a charged battery instead of codes I got a serial comms error for every module. Im at my wits end. Looks like im going to have to scrap her and just get used to life without a car. Such a shame as mechanically she is great. But I cant afford to have anyone look at her Story of my life. I sadly think I have to give up. Nothing has worked. And nothing makes sense just from changing a starter motor. Its ridiculous. I dont think I'll be buying a Jaguar again. So sad loved this car

 

Sorry to hear the problem continues. Not sure of your situation, what tools you have available, workshop space, time pressure, etc.

Please be aware long-distance troubleshooting like this can be a very slow and plodding experience. A shop might be able to figure this out fairly quickly with the right diagnostic capabilities, but that doesn’t sound feasible right now.

I do know every mechanical issue does have a solution. It’s just not always straightforward getting there. I highly doubt the root cause is something exotic (read: expensive). It’s mostly likely something to be found working through the basics, bit by bit.

 

I have been doing some more research and had an epiphany. I was completely unaware of the Mega Fuses! It is dark outside at the moment, but you bet your bottom dollar I am going in the trunk to find these in the morning. My method of failure is this.

1 I undid all the starter wires and left them that way as I had to come back to finish the job.
2 I reconnected the battery so I could lock up the car.
3 Issues started then (rear lights stayed on, couldn't use the key fob except to unlock drivers door, all other faults were noticed the next day after installing the new starter).

I reckon, that when I reconnected the battery, some off the starter wires were touching either themselves or something else and have blown the mega fuse at the front that powers some things. Google is telling me that it would be normal for my interior lights, radio and front drivers door to still be working with this fuse blown, and that the ecu's would lose communication as would PATS.

I have high hopes that tomorrow I will test the mega fuse with a multimeter and find it blown! In the words of that Irish band The Script "Just prayed to a God that I don't believe in", but it would make complete sense! Cheap fix if so, omg I have fingers toes and legs crossed!

Who's with me?

 

This is my Google AI result, bit long winded, but makes sense:


1999 jaguar s type v8 if I can jumper the relay and turn the starter but suspect a mega fuse is blown which one is it. would it normally allow some systems such as radio and interior lights to carry on working?

On a 1999 Jaguar S-Type V8
, if jumping the starter relay works but the key doesn't, a blown megafuse is a prime suspect.
1. Which Mega Fuse is it?
The starter megafuse is located in the luggage compartment (boot).

• Location: Behind the right-side trim panel in the boot, near the battery. Look for a junction box containing three high-amperage megafuses.
• The Specific Fuse: The front-most of these three fuses is typically the one that powers the starter circuit.
• Appearance: These are large, bolt-down fuses (typically 175A or 450A depending on the specific battery cable configuration).

2. Would systems like the radio and interior lights still work?
Yes. The S-Type's power distribution is split across multiple megafuses and independent circuits.

• Isolated Fault: Because there are three separate megafuses in that rear junction box, one can blow—cutting power to the starter—while the others continue to power the passenger compartment and engine management systems.
• Typical Symptom: You will have "everything light up" (dash, interior lights, radio), but the car will fail to crank or even click when the key is turned to the start position.

3. Verification & Next Steps

• Visual Inspection: Check for melting or heavy corrosion on the starter fuse terminal in the boot junction box.
• Multimeter Test: With the ignition on, measure voltage on both sides of the fuse. If you have 12V on one side and 0V on the other, the fuse is definitely blown.
• Immobilizer Check: If the fuse is good, observe the security light on the dash. If it flashes rapidly when you try to start, the PATS (Passive Anti-Theft System) might be blocking the signal to the relay.

Do you have a multimeter handy to check the voltage across the megafuse terminals, or would you like a guide on how to bypass it safely for a temporary test?

would this also be the cause of a serial communications error across all modules

It is highly likely that a blown megafuse (or a failing junction box) is the root cause of a
Serial Communications Error (U-codes) across your modules.
In the 1999 Jaguar S-Type, these high-amperage fuses don't just feed the starter; they provide the "backbone" power to the primary fuse boxes (engine bay, passenger side, and trunk).
Why a Fuse Causes Comm Errors
When a megafuse blows or—more commonly on the early S-Type—the terminal nut vibrates loose or corrodes, the voltage supplied to the rest of the car becomes "dirty" or insufficient.

• Module Brown-outs: Control modules (like the ECM or TCM) require a steady 12V to stay "awake" on the network. If the voltage drops too low due to a failing connection, the module shuts down.
• The "Silent" Module: When one module loses power, it stops talking. Other modules on the CAN-bus or SCP-bus try to communicate with it, fail, and then log a Serial Communications Error.
• Network Termination: In some cases, if the module providing the network termination (the "end of the line") loses power, the entire communication bus can crash.

The "High Resistance" Trap
Often, the fuse isn't fully "blown" (melted in half), but the connection at the megafuse block has developed high resistance.

• The Symptom: You have enough current to run low-draw items like the radio or interior lights.
• The Failure: The moment you try to crank the engine or activate a high-draw system, the voltage at the fuse block collapses. This "spike" or "drop" confuses the digital networks and triggers the communication errors you are seeing.

Comparison of Symptoms Symptom Blown Fuse Loose/Corroded Mega Fuse Radio/Lights work Yes Yes Starter clicks/turns No Intermittent Dash "Christmas Tree" Possible Very Common Comm Errors Yes (Missing Modules) Yes (Voltage Instability) Recommended Action
Before replacing modules or chasing wiring:

1. Disconnect the battery.
2. Open the Megafuse box in the boot (near the battery).
3. Physically remove the nuts holding the fuses down.
4. Clean the eyelets and the fuse surfaces with a wire brush or sandpaper to ensure shiny metal-to-metal contact.
5. Re-torque them (don't over-tighten, but ensure they are snug).

Would you like the specific torque specs for those megafuse nuts, or should I help you identify which specific communication codes (U-codes) you are seeing?

I have the following codes, would these be related? Anti-Lock Braking system (01 DTC) B1646 U1262 General ECM (01 DTC) B1676 Audio Unit (01 DTC) U2008 Instrument Pack (03 DTC) B1676 U2152 U2162 Message Centre Control Module (03 DTC) B1676 U1131 U1262 Powertrain Control Module (01 DTC) P1000 P1233 P1260 Restraints Control Module (02 DTC) B1318 B1342 Rear ECM (Err 0x100b) Serial Communications Error ----------------------------------------- Antilock Braking System (01DTC) C1805 Instrument Pack (01DTC) B2328

Yes, these codes are absolutely related to a power supply issue, specifically centered around the battery, megafuses, or the rear junction block.

The presence of B1676 (Battery Voltage Out of Range) and B1318 (Battery Voltage Low) appearing across multiple modules is the "smoking gun." These aren't component failures; they are the modules screaming that they aren't getting the voltage they need to operate.
Breaking Down Your Codes

• The Voltage Group (B1676, B1318): These are recorded by the ECM, Instrument Pack, and Restraints Module. They trigger when voltage drops below ~9V or spikes. This happens during a crank attempt if the megafuse connection is poor.
• The Network Group (U1262, U2152, U2162, U1131, U2008): These "U" codes are communication errors. If the Rear ECM (RECM) is losing power—which is common given your "Serial Communications Error" note—the whole SCP (Standard Corporate Protocol) network fails because the RECM is a major hub for data.
• The Security Group (P1260): This code means "Security Input Received, Engine Blocked." Because your voltage is unstable, the instrument cluster (the PATS master) cannot "handshake" with the PCM. This is likely why it won't start with the key even though the starter works when jumped.
• The Fuel Group (P1233): This indicates the Fuel Pump Driver Module is offline. Again, this module is powered from the rear of the car; if the power backbone is failing, the pump won't run, and the car won't start even if you get it to crank.

The Connection to the Rear ECM (RECM)
Your error "Rear ECM (Err 0x100b) Serial Communications Error" is the most significant.
In the 1999 S-Type, the RECM sits in the right-hand side of the boot, right next to the battery and the megafuses.

1. Water Ingress: Check the spare wheel well and the area around the RECM for water. It is very common for tail-lamp seals to leak, drowning the RECM and the megafuse block, causing exactly these codes.
2. The "Front" Megafuse: The megafuse you suspected supplies the primary power to the front of the car, but another fuse in that same block supplies the RECM. If that block is corroded, the RECM loses its "ground" or "power," causing the "Serial Communications Error."

Summary of the "Chain Reaction"

1. Faulty Megafuse/Connection → 2. Voltage Drops → 3. RECM goes offline → 4. Network crashes (U-Codes) → 5. PATS Security locks the engine (P1260).

Your Next Step
Since you’ve already confirmed you can jump the starter relay, your starter motor is fine. The problem is "Upstream."
Do this immediately:
Check the Black Ground Cable where it attaches to the car body in the boot, and the Power Distribution Box (where the megafuses are). Look for a nut that looks slightly "burnt" or has white/green crusty corrosion. Even a slightly loose nut here will cause every single code you listed.

is it likely to be only the front mega fuse?

In short:
No. It is more likely a fault with the entire Megafuse Junction Block or a loose connection at the Battery Terminals, rather than just the single "front" fuse.
While a blown front megafuse would explain the lack of crank, it doesn't typically cause the mass "Serial Communications Errors" (U-codes) across all modules if the other fuses are healthy.
Why it's likely the whole block or a loose connection:

• Shared Infrastructure: The megafuses are housed in a single distribution block in the boot. This block acts as the "power backbone" for the car. If the main nut holding the positive battery cable to this block is loose, or if the block has internal corrosion, all downstream systems will suffer from voltage drops.
• Voltage Instability: Your codes B1676 (Battery Voltage Out of Range) and B1318 (Battery Voltage Low) appearing in multiple modules suggest the voltage is "rippling" or dropping momentarily. A clean "blow" of one fuse usually just kills a circuit; a loose or corroded connection causes the digital noise that crashes the communication networks (U-codes).
• The Rear ECM (RECM) Factor: Since you specifically have a Serial Communications Error with the Rear ECM, and that module is physically located right next to the battery and megafuses, it suggests the power issue is local to that boot area.

Most Likely Culprits (In Order):

1. Loose Main Power Nut: The 13mm nut that secures the thick red battery cable to the megafuse block. If this is even slightly loose, the car will have "ghost" electrical issues.
2. Corroded Megafuse Terminals: Moisture in the boot often causes white/green corrosion between the fuse and the busbar. This creates high resistance—enough power for lights, but not enough for modules to "talk" to each other.
3. Failing Battery Ground: Check where the black negative cable bolts to the car body in the boot. A weak ground will cause the exact same "Serial Communications" and "Voltage Low" codes.

Checklist for you:

• Check the nuts: Do not just look at the megafuses; put a wrench on the nuts. If they move at all, that's your problem.
• Inspect the "Protection Module": This is the plastic housing for the megafuses. Look for signs of heat (melted plastic) around the studs.
• Hard Reset: Once you tighten everything, disconnect the battery cables and touch them together (away from the battery!) for 30 seconds to clear the "hung" communication modules.
  

Do you see any signs of water or dampness in the spare tyre well or behind the right-hand boot trim where the fuses live

no

Succinctly:
It is unlikely to be just that one fuse.
The "Serial Communications Error" and the "Battery Voltage Out of Range" codes across multiple modules point to a systemic voltage drop rather than a single severed circuit.
Since you don't see water damage, the most probable cause is high resistance at the common power source. In these early S-Types, the three megafuses share a common busbar. If the main input nut (from the battery) is loose or the busbar itself is corroded, it starves every module on that network of stable power.
Why it's the "Whole Block" and not just one fuse:

• The Chain Reaction: A single blown fuse wouldn't cause the B1676 (Voltage Out of Range) in the Instrument Pack or the B1342 (Internal Fault) in the Restraints module. Those modules are seeing "dirty" power—fluctuating voltage that crashes the communication lines.
• The RECM Error: Your Rear ECM failure is the giveaway. The RECM acts as a gateway for the communication network. If its power supply is fluctuating (due to a loose connection at the megafuse block), the entire car's "conversation" stops, causing your U-codes.

The "Dry" Fault Check:
Since there is no water, focus on Mechanical Failure:

1. The Main Nut: Use a 13mm wrench to check the nut where the thick red battery lead connects to the megafuse box. It often loosens over 20+ years of vibration.
2. The Earth Bolt: Check the battery's negative cable where it bolts to the car floor. If that connection is painted or slightly loose, it creates a "floating ground" that triggers every code you listed.
3. The Hidden Fuse: There is a 10A fuse in the Primary Junction Box (passenger footwell) that sometimes blows if the megafuse has been arcing, which will keep the PATS security system from "handshaking."

Next Move: Physically tighten the battery terminals and the three nuts on the megafuse block. After tightening, perform a hard reset (touch the disconnected battery cables together for 30 seconds) to "reboot" the communication modules

I have serial comms issues across all modules not just the recm

Yes, if all modules are showing serial communication issues, the problem is almost certainly not a single "blown" fuse, but rather a failure of the power or ground backbone that all those modules share.
1. The "Floating Ground" (Highest Probability
Since your car has "no water damage," the serial comms errors across all modules (U-codes) are typically caused by a bad battery ground.The Symptom: When you try to crank, the high current demand from the starter finds no easy path back to the battery. It "backfeeds" through the small communication wires of the modules, crashing the network.

• The Check: Locate the black negative cable where it bolts to the trunk floor. Even if it looks tight, remove the bolt, sand the contact area to bare metal, and re-tighten. A "clean" ground is essential for these modules to "talk" to each other.
  2. The Mega Fuse Junction "Arcing"

If one of the mega fuses in the boot has a loose nut or a hairline crack, it creates "electrical noise."Modules like the PCM, ABS, and Instrument Pack are extremely sensitive to voltage ripples. If the power supply to the main busbar is intermittent, the modules will repeatedly "reboot" or lose sync, leading to the "Serial Comms" error you see globally.

• Action: Do not just look at the mega fuses. Use a 13mm wrench and physically ensure the nuts on all three mega fuses are snug.

3. The SCP Network "Short"
The 1999 S-Type uses an SCP (Standard Corporate Protocol) network. Unlike modern CAN-bus, this is a simpler system where all modules are linked in a "daisy chain".

• If one module has an internal short (common in the Driver’s Door Module or Rear ECM), it can "pull down" the entire network, making it look like every module is dead.
• Testing: Try disconnecting the Rear ECM entirely. If the "Serial Comms" errors on the other modules (like the dash or engine) disappear, then the RECM was the one crashing the network.

4. The PATS / P1260 Security Lockout
The P1260 code you have means the car is in theft lockout

• Because the communication is down, the Key (PATS) cannot tell the Engine (PCM) it's okay to start.
• This is why you can jump the relay and the starter turns, but the engine won't fire—the PCM has disabled the fuel injectors.

Quick Test to Confirm:
Turn the ignition ON. Watch the small red LED on the top center of the dashboard (near the windshield).

• Solid/Off: Communication is okay.
• Rapid Flashing: The security system is blocked because it can't "talk" to the other modules.

 

Glad to see you haven't started scrapping the car just yet.

One caveat: The Google is not infallible. Some of the details sound good, but others only apply to 2003+ cars. One big difference:

1999-2002 models only have a single megafuse. It's called the Clean Power Fuse for those years, and is located next to the rear power distribution box:



In the wiring diagrams, you can see this big fuse in figure 01.3. It supplies battery power to the front power distribution box.

2003+ models do have two megafuses, as The Google is telling you. The first one feeds the front power distribution box, basically the same as the clean power fuse on the earlier models. The second megafuse is in the starter/alternator feed (not present on earlier models).

So I'm not quite sure what to make of these AI results, other than some of it is not quite correct. I do think it's time to start checking power and ground at various modules, specifically the RECM since that one seems to be acting up.

 

Did a little poking around the wiring diagrams. For a quick check of the clean power fuse on an early model, see if the windshield wipers work, including the spray function. Many circuits go through the clean power fuse. That's one of the easiest to check.