So, I was checking my fuses again with a test light and # 10, 12, 14, and 16 under bonnet RH side don't light up the 12V test light with car off and battery attached. All the others in that tray do.

Also, #10 & 14 LH Under bonnet, but those both appear to be for the horn and I think I remember that the horn grounds the circuit when pressed.

Could that be part of my problem?

Like I said, I've tried everything I know to do at this point and messing with electrical systems has never really been in my wheelhouse. I'm college-educated, not ignorant...but, honestly, the whole "electricity" thing has never really made much sense to me. Some lights need to be grounded, others don't? It just never stuck with me.

So, any help here would be much appreciated. Definitely out of my comfort zone now.

 

This is my first Jaguar so I am not an expert in the field , I just play one on the internet . I assume you are not using a meter so I will approach this in a manor without using a meter as the easiest steps first . Not necessary at this point , but if you want a electrical picture to see how it works the electrical wiring schematic is available on the above X300 series " How To " topic --- DOWNLOADS ( assorted Tech files ) --- Jaguar Documents ---- Electrical Guide . Once you pull it up you can print of the pages you need to take to the car .

Fig 1.04 page 36 and Fig 20.1 page 168

RH fuse # 10 ( Ignition control plus others )
# 12 ( Engine management system plus others )
# 14 ( Oxygen sensors heaters / idle speed control valve )
# 16 ( Engine ancillaries / supercharger or V12 stuff )
LH fuse # 10 ( horn 1 )
# 14 ( horn 2 )

Fig 1.01 page 30

Where the power comes from

You don't need to look at this at this point but it gives you a way to look at it .

EASIEST THINGS FIRST ( least intrusive ) :

To better under stand how electrical things are " controlled " some items are power seeking that always has a ground and other items are ground seeking that are always powered . Just depends on what side of the items 2 power wires the control ( switch , relay , dimmer ) is on . An example of both a power and ground seeking is the horn relay . A relay has 2 halves to it , the pulling coil and the mechanically but electrically isolated contact points . The pulling coil is ground seeking through the horn button and the horn trumpet itself is power seeking as the relay contacts are closed and grounds on the other side of the horn. There is power on both halves of the relay with the key off ( hot at all times ) . So you are seeing both examples as you read the horn relay in the LH fuse box.

A quick overview of the X300 electrical system is that there are large relays behind the radiator that are for ECU , headlights , fog lights , etc . The smaller blue sometimes black relays under the LH / RH fuse covers are what I refer to as king ( airline term ) relays that control many other items at the same time through there individual fuses next to them . The same king relays are it the trunk and 2 under the back seat so there are 5 . The king relays do not make contact without the key on . To confuse this the LH king relay only controls the 2 horns and is hot at all times .

From what I can infer you have given some results of your findings without defining your cars problem so I'm guessing you have a engine control issue . As a least intrusive try you might swap the large ECU #5 relay behind RH radiator with the Headlights #1 ( good to know where that one is ) or the # 3 A/C clutch . The relays are hard to remove so you might spray the separation seem beforehand . I will get into other areas of work but you have to define the issue . Not to be hard on you at all for not everyone works on there own cars but I need more definition of the cars principle issue . Keep in mind that the ECU will act up as the battery drops below 11.5 volts .

Thanks , Joe

 

Thanks, Joe, for taking the time to respond and break things down into layman's terms. So, I guess it did seem like a pretty flimsy post, as I didn't actually say what my issue was. I had just made a post moments before and in that one I just directed any readers to look at the post below it for the whole story.

But, basically, the car had been running great until January when, out of the blue, I had loss of power/surging/hesitation. However, the problems would only occur once I'd fully reached operating temp. Out of the gates, and for the first several miles, the car still drove like a champ. Once it had reached normal operating temp, it would then have these issues when under load/heavy acceleration/at high rpms.

So, I used vacuum gauge and found excessive backpressure. Took it to shop to be flow tested. The underfloor Cats were pretty clogged. So, I gutted the two underfloor Cats. Things seemed a bit better but not like it should be.

I had changed the Thermostat. New Fuel Filter. Replaced Plugs. Replaced all 6 Coils. Again things seemed a bit better but, after fully warming up, the car would still buck at higher speeds/under heavy load.

Finally, got to the point where I was pretty confident it was either a failing fuel pump or the Crankshaft Position Sensor. I was getting ready to leave for TX in a couple days and it was going to take a few days to get the sensor in, so I decided I'd replace the Fuel Pump. I figured, if that fixed it, great. If not, I'd chalk it up to preventative maintenance, and then order a CPS, which would be quick and easy to replace when I returned from my trip.
.
Well, after replacing the fuel pump, it seemed as though it might have been fixed but, as I was cruising on the Interstate, it reared its head again. Same symptoms once it had warmed up. I ordered the CPS and left town for a month.

When I returned, I installed the new CPS, but then the car wouldn't even start anymore. It had always started just fine. I bought a new battery charger and made sure I had a full charge. Also, installed new ECT sensor. But, still can't even get it started now.

It is getting fuel. I cracked the nuts at the rear of the fuel rail and let shoot into jug. Removed plugs and they smelled of fuel. Let Dry overnight. Tested for spark at the coil and got very weak spark. Tried starting with pedal depressed fully.

It's trying hard but just won't catch.
Oh...let me back up a minute. When I returned from trip and put new CPS on, it started just fine but, as I gathered my tools and was putting them away in the boot, it died. I haven't been able to get it started since.

I did try putting the old CPS back, but no luck. Tried installing some of old coils and tested for spark, but no improvement.

So.....right now my issue is that I can't even get the car running. Who knows? The new CPS may have fixed my hesitation under load when warm issue but I can't get the car started to find out.

It would seem electrical/sensor related. I've pulled the box from under the RH passenger flloorboard area to look for corroded pins, but things looked ok so I just cleaned them with electronics cleaner.

I do have a meter.
I have a code reader, which can do some basic graphing.
I've checked compression.
I've got an infrared thermometer.
And a few other tools.

I hope that helps to better explain my situation. If you've got any ideas, I'm all ears.

Like I said, electricity is not my strong suit, but I'm pretty good at following directions. If there's anything you think would be a good place to look, please do share.

Thanks again.

 

One other point of interest: When I was trying to get it started earlier today, I tried starting in N as well. Then, I noticed I could change gears without turning the key. The P safety switch wasn't locking. I could move through all the gears with the key in my pocket.

I removed the slope but didn't see the switch and didn't want to start down another rabbit hole.

But, I will chase that furry little f--ker if people are reasonably confident that could/would be the cause of my current no start situation.

 

My bad I didn't see your main thread . My Penelope has a spot of stumble under her bonnet as well . I'll get back with you later on your main thread .

Thanks Joe