This 2001 3.0L with 124K miles arrived with the Check Engine Light illuminated. The owner stated that it was running fine and that the battery had been replaced recently. I began with computer diagnostics and found that twenty-eight DTCs were stored, many related to low battery voltage or loss of module communication. The most relevant codes were:

PCM P1260 Theft detected, engine disabled
PCM P1587 Throttle control unit modulated command fault
IPK B1681 Passive Anti-Theft System (PATS) transceiver module signal is not received

When starting the engine, I noticed that the engine cranked and started sluggishly.

I measured the battery resting voltage and it was extremely low at 11.88 volts. I then measured the alternator charging voltage at the battery terminals and it was very low at 13.2 volts. I connected my battery-charging system analyzer and found that the battery capacity was just 597 Cold Cranking Amps. There was no rating label on the battery, but typically H8/Group 49 batteries carry ratings of 850 – 950 CCA, so this battery’s capacity was significantly diminished, possibly due to poor charging.

The alternator output was clean with relatively low AC ripple of just 75 mV, so I suspected high resistance in the battery power and ground circuits between the battery in the trunk and the alternator and starter in the engine bay. This is a very common issue on all Jaguars with batteries in the trunk/boot.

I detached the negative battery cable and inspected the terminal and battery post. Both exhibited white lead oxide corrosion.

I then detached the negative cable ground connection to the inner wheel well by removing the 8 mm screw.




The battery negative cable ground eyelet terminal and screw both exhibited white oxide corrosion. Both lead oxide and aluminum oxide are insulators and inhibit the flow of electrical current.




I detached the positive battery cable and found the terminal and post both covered in oxidation. I cleaned the cable terminals, battery posts and ground screw with battery brushes, a small brass-bristle brush, and CRC zero-residue electronic contact spray cleaner. For now, I left the battery disconnected.




Next I gained access to the Bulkhead Power Stud by raising the vehicle, removing the front right wheel, detaching the rearward half of the wheel arch liner and folding it forward over the brake rotor.




Upon inspection, I found that a wad of wet, decomposing leaves was wedged above the bulkhead power stud.




I used a pick to clean out most of the leaves. Then for improved access to the stud cover, I cut the zip tie retaining the electrical harness:




The red plastic power stud cover is secured by three slotted “legs” that must be pried outward until they unclip from wedged tabs.




Remove the 13 mm nut and the power cable will pull off of the stud. Both the nut and eyelet terminal exhibited white oxidation.



The threaded stud is now accessible to clean. On the other side of the bulkhead/firewall is a similar connection, though because it is sheltered from weather, it tends to not suffer the same corrosion and contamination as this side.




The small brass-bristle brush and electronic contact cleaner are good for cleaning all these eyelet terminals, threaded studs, nuts, washers, etc.




I don’t know how important the zip tie is for immobilizing the electrical harness, but if you are a thorough mechanic, you’ll replace it. Jaguar didn't bother to cut the end off - it was just routed up into the convenient hole:




The engine ground strap body connection is just below the bulkhead power stud.




I removed the 8 mm screw and inspected the eyelet terminal and screw and found more crusty white oxidation:




The other end of the engine ground strap is connected at the rear engine flange just below the starter motor:




Upon closer inspection, I found that the engine ground strap stud was wet with oily gunk. This is a 13 mm nut.




With the ground strap detached from the engine, it did not appear that the oil had seeped into the electrical contact area covered by the nut flange, but white oxide was clearly visible there.




The other side of the terminal was in similar condition but the white oxide in the electrical contact area was even crustier:




I used the brass-bristle brush to clean away all the oil and oxide and then flushed the terminal with contact cleaner.




For access to the starter and alternator on this 2001, a small supplemental wheel arch liner must be removed.




The starter electrical connections are protected by a red plastic cover.




The battery power cable eyelet terminal is secured to the starter by a 13 mm nut. Various tools can work, but I used a ¼” drive long-handle flex-head ratchet and socket.




The battery power cable eyelet terminal was badly corroded with a trifecta of red rust, white oxide and green copper oxide.




The alternator battery power connection is protected by a rubber boot, which can be pushed up the cable for access to the 12 mm nut.




I accessed the nut with my ¼” drive long-handled flex-head ratchet and socket.




The alternator power cable eyelet terminal was also covered in white oxide.




After cleaning all of these connections and reassembling in reverse order, the engine cranked and started noticeably faster. The alternator charging voltage at the battery had risen significantly to 13.77 volts. I put everything back together and took the vehicle for a 20-minute test drive, then measured the battery resting voltage again. In just 20 minutes, it had increased from 11.88 volts to 12.25 volts. I re-scanned the vehicle and only one Climate Control Module heater sensor code recurred. None of the PATS or voltage-related codes recurred.

I do not know if this battery’s capacity will improve, but cleaning all of these battery and ground connections made a measurable improvement in battery charging. Some of these connections did not look terribly "bad," but the resistance added by corrosion on each connection is cumulative, and the loss of more than half a volt of charging voltage at the battery had a very significant adverse effect on this vehicle.

If you have difficult-to-diagnose electrical gremlins, I recommend this service as basic maintenance. If you continue to have electrical gremlins, it may help to also clean the other side of the battery power circuit, at the Clean Power Megafuse, the Secondary Junction Box and the Front Power Distribution Box.

Cheers,

Don

 

Good findings, Don!
I assume that this would explain many of the mysterious faults reported here on the forum.
Your findings are in regards of the 2001 S-Type.
If I have any kinds of mysterious issues, I would have to find out, where all those connections are on the 2004/2005 S-Type, especially the positive bulkhead connection (where the old leaves were composting)... - I guess, this would be different on the 2004/5.

 

That’s great, but 12.25 volts is still a bit low. Suspect battery may need replacing. You should be seeing about 12.6 or more as standing voltage.

 

Great write-up!

Many of the power/gnd are the same across the years or if not are (mostly) shown in the appropriate Electrical Guide. (There's MUCH more in there than people tend to notice!)

 

The 12.25 volts was after only a 20-minute test drive, but up from 11.88 volts, so trending in the right direction. The fact that the charging voltage had increased from 13.2 volts to 13.77 volts at the battery indicates that at least the charging system should now keep a battery fully-charged. We typically look for charging voltage to be at least one full volt higher than desired battery resting voltage of 12.6+ volts.

I have my doubts about the battery in this car. It has no manufacturer's label, only a label from a local battery store. For all I know, it is a "recycled" battery. We'll see how it does as the owner of the car gives the battery time to fully recharge.

Cheers,

Don

 

Hi Peter,

You can download the wiring schematics for most S-Type model years at the website maintained by our member Gus. Bob (motorcarman) provided the files from his days as a Jaguar dealership technician. Thank you Bob and Gus!

JagRepair.com S-Type Electrical Guides

The Electrical Guides typically have pages associated with each schematic that list the Components, Harness In-Line Connections, and Grounds associated with that specific schematic and give a textual description of their locations which will at least get you in the right vicinity.

Cheers,

Don

 

Very nicely documented findings!

I wonder if that mass of wet leaves was causing a slight overnight drain on the battery. If so, that might muddy normal troubleshooting logic. Typically you measure the drain with a clamp-on ammeter, and then pull fuses one by one and watch the meter. But in this case, if the wet leaves could conduct some current, you'd never get the drain to fall off completely simply by pulling fuses.

 

Great diagnosis and complete write up. I think the white oxide forms from the iffy ground connections and progressively gets worse. Funny how people skimp on a battery when all modern vehicles say 2000 and newer rely heavily on a premium manufacturers rated battery. Remember when all major electrical eyes were pure copper. Best of the best