Some of you may recall the shifting issue I'm having with my XJR. I'm in the process of doing pinpoint tests for voltage and resistance, using the Workshop Manual that I downloaded from JagDocs several years ago, and the 1998 Wiring Diagrams manual that I also got from JagDocs (I think). I am attempting to find a fault in the wiring circuit that would have caused the high speed fan fuse to blow. I am hoping that whatever the fault is, it is causing the ECM to choose the wrong shift pattern when the high speed fan circuit is activated by the ECM. That's when my shift issue surfaces.

Anyway....

My pinpoint test readings are wrong for the high speed fan circuit. WAY OFF, for both voltage and resistance. There are two options here as I see it. Either:

1) The results I'm seeing truly indicate a fault, and I've found the problem, or
2) The readings are ok for my car and I need to keep looking for the problem.

If it is the first option, I am going to have to figure out how to trace the wires through the harness to find the fault. Obviously, that could be a massive undertaking. Before I do that, I would like to know for certain what test results I should be expecting for my early 98 model.

The reason I question it, is because I have replaced two relays that the manual told me to replace based on my test results. But there was no change in the readings. Because the pinpoint tests for resistance and voltage are SO FAR OFF, I'm wondering if I even have the correct Workshop Manual. The workshop manual shows me which points to test, and shows me the expected results. But man, they are WAY off, even after replacing the relays as indicated!

I have the 1998 Wiring Diagram manual dated Fall 1997, and all of the wire colors and locations jibe with what I'm seeing in my car. Therefore, I think I have the right Wiring Diagram Manual for my car.

But the revision date on the cover of my workshop manual says 2014. Does anyone know if there is an earlier version of the Workshop Manual available? Say, from the fall of 1997? I would love to look at the pinpoint test section and see if it shows different expected test results than my manual says. I know there were some changes from the very early 1998's, and I'm wondering if this might be one of those things that was later changed, and the later manuals do not reflect it.

My next step is to disassemble the engine wiring harness to find the fault. As I said, before I do that, I need to make darn sure that I truly have a fault, and not just the wrong manual.....

If anyone has a copy of an early X308 workshop manual, I'd love to take a look at it. At the very least I might ask you to take a screen shot of two particular pages.

Thanks again!

 

If the fuse 'blows' when the fan is activated, then one or both fans are drawing too much current.
I have several old fans that draw more than 30 amps. The protection fuses are both 30 amp.

They will sometimes work but the circuit soon fails when the fuse(s) 'blow'.

Test the current draw through each fan and if one or both exceed about 25 amps, you should find some good-used replacements.

(could also be a wiring fault?)

All sedan fans are compatible from about 1994 to 2003.

bob

 

I'm with you, but that doesn't explain why the supply side voltage and resistance are wrong with the engine turned off and the key on, which is what it calls for in these tests. The manual calls for 10 volts at Pin 8 of the fan module. I have 13.7 volts. If it is over 10 volts, it told me to replace the fan module. I did that, and it still reads 13.7 volts. It also calls for "over" 10,000 ohms of resistance on two of the other wires running from the ECM to the fan module. I have 300 ohms on both wires.

I will test the current draw at the fans, but all of these readings are taking place BEFORE the relay. The relay then tells the fans to run on low or high speed, based on the input from the wires in question.

It is possible that one of the fans is pulling too much current, which may have blown the fuse. But would that explain why these other tests are wrong in the circuit before the relay? I suppose everything is tied together though. I'm searching for answers, obviously.

 

One other thing: I think the fuse blew 3 years ago when this issue originally surfaced. I did not even know there were two fan circuits until a month ago, so I didn't even know the high speed fan was not running. Whenever I opened the hood, both fans were running. Apparently I never paid attention when the fans were on high speed, that one of them was not running.

When I replaced the 30 amp fuse recently, I test drove the car several times, and the fuse has not blown. But now that I'm aware of the high speed circuit and how it works, I pay attention to when the high speed fans kick on. The shifting issue surfaces each time the high speed fans are turned on. That's why I'm going down this diagnostic path. My theory is that when this first happened 3 years ago, the ECM panicked after it called for high speed fans, but the car did not cool down as expected (the fuse was blown). It then changed the shifting pattern of the transmission to compensate. I have no idea if that is even possible, but it's the only theory left at this point. And it explains why, when it gets hot and calls for high speed fans, the tranny goes bonkers. It also explains why there has never been a trouble code of any kind. The ECM thinks it's doing the right thing.

I may need an ECM, but I really want to know why these readings are so furiously wrong in each case. There's either truly something bad going on, or I have the wrong manual.

But I am going to dig deeper into the fans themselves based on your suggestion. Heck, I'm open to anything!

 

Send me a private message and I will send you a link to my drop box where you download the 1998 manual if you like.

 

Done. Thank you!

I will report my findings, and continue to search for the solution.

 

Thanks to avern for sharing an earlier version of the manual. The pinpoint tests are exactly the same as in my manual. Soooo....that means I have an issue. That’s either good news or bad news, depending on what I can figure out!

For a wire to have 10 volts, it has to go through some kind of resistor, I assume. My voltage measures 13.7 at that particular wire. That’s what the battery measures, so there is basically no step-down in voltage. I replaced the fan module as the instructions said, even though the voltage I’m measuring is on the incoming side of the module, which made no sense to me. How could replacing the module step down the voltage AFTER the pin I tested?? Well, replacing the module did not step down the voltage. It still measures 13.7 volts, just as I figured it would.

So that brought me farther back in the wiring harness. I then landed at the engine management relay. That relay also has resistors in it, so I replaced that relay. No change. Still 13.7 volts.

Not sure what else to do. I still have the ohm issue, which is grossly wrong as well. The book calls for “over” 10,000 ohms, and I measure 300 ohms. I figured whatever resistor is supposed to step down the voltage to 10, would also provide resistance of over 10,000 ohms. Yet....nothing. There are no more relays to replace in that circuit.

How else can voltage be stepped down? I don’t know a ton about electricity, so maybe need to be looking for something besides resistors in that circuit??

Thanks a ton, everyone! I really think I’m in the right vicinity here. I now believe that my book is accurate, and my readings are indeed indicating a problem. I just can’t figure out why the voltage and resistance is so wrong....

 

The voltage on Pin 8 (Pin LF9-8) is the ignition supply from the battery (see first pic) so the voltage there should be whatever the battery voltage is. There is no step-down resistor. This pin provides positive supply to one side of the solenoids of all three relays which are then activated by grounding (by the ECU) of the other sides of the relay solenoids, as required for the Low and High fan speeds. The 10V you found in the manual is certainly not applicable here. It could be for something else or it could simply be an error.

The resistors shown inside the relays, connected in parallel to the relay solenoids, are not for any voltage step-down but are there to "eat" the high voltage spikes (200-300 volts, "flyback voltage") generated by the relay solenoids on disconnection.

To measure the Ohms at the two wires coming from the ECU (wires to LF9-9 and LF9-7 on the diagram below), you have to disconnect the connector to the fan relay module. If you measure resistance between the two pins on the relay module, you will get about 300 Ohms which is the resistance of the two relay solenoids in series to which the two pins are connected. If you measure resistance between the same two pins inside the connector (the resistance inside the ECU), you will probably get the "over 10,000 Ohms".



The Low and High fan speed switching points are as shown in the table (based on engine temp and/or the AC system pressure):

 

Small quibble: I think those "resistors" inside the relays are (flyback) diodes

 

Here are pictures from the workshop manual. They posted in the wrong order here, but I started with pinpoint test AB and followed it through because I thought I had a problem in the high side circuit. I may or may not have a problem, but why do these instructions show me results that cannot be true? What you said in your post makes perfect sense to me, and is what I’m seeing in my tests. So now my question is, what is the purpose of the pinpoint test in these pictures? This is exactly why I was asking about an earlier version of the workshop manual. None of my results were matching what the manual shows. Obviously there’s something critical about these tests that I’m missing. Looking at the wiring diagram I have, there is no reason the test should show 10,000 ohms, which again, led me to question the manual version.

In any case, I probably don’t have a problem with the high speed fan circuit. The shifting problem likely lies with the ECM somehow.

One other question: your diagrams show nice voltage readings in red. Mine doesn’t. Where did you pull these pictures from? Thank you, this is all very helpful!

Edit: One other thing. My wiring diagram calls the fan relay LS31, and my workshop manual (posted here) also calls it LS31. Yours appears to show it as LF9. Now I’m back to wondering about the version of my manual. Where did you get the pictures you posted here??

Edit 2: I should point out that my pinpoint tests are failing at AA3 and AA4. It seems obvious to me why they are failing the voltage test, because as you mentioned, that wire is coming from the battery and should measure what the battery does. Which it does, 13.7 volts. I cannot figure out why the instructions are calling for 10 volts, and what difference changing the relay would make, and, as I suspected, changing the relay made no difference. I am prepared to let this mystery go unsolved, as I am somewhat convinced I do not actually have a problem here. It just looks like I do based on the instructions.

The car drives and shifts perfectly, until the high speed fans kick on. Then the shift pattern changes, and it refuses to upshift until around 3,000 rpm. When coming in to a stop light, it's like a jake brake - the transmission downshifts and does not "coast" to a stop. I'm still convinced it has something to do with the high speed circuit, but it may be a fault within the ECM itself when it orders the high speed fans to turn on, instead of in the relays or wiring.

I really appreciate all the input. I will get to the bottom of it eventually!

 

Some relays use diodes and some use resistors in parallel to their solenoids to suppress the flyback voltage. According to the symbol used in Jaguar diagrams, they are resistors (and I have verified this in a number of Jag relays that I opened up for cleaning). If a flyback diode is used, then diode symbol is shown on the diagrams and on the side of the relay housing. In the case of relays with a resistor, it does not matter whether the positive supply is to pin 85 or pin 86. In the case a diode is used, the positive supply must be to pin 86, otherwise either the diode or the fuse (if there is one) will be blown. below are examples of relays with a resistor and with a diode (the blue one).

 

I didn't know that. What ohms (roughly)?

 

Usually 470 or 560 Ohm.

 

I've resumed my search for a compatible ECM with a VCATS suffix of /001. I'm having no luck so far. Is there any chance an incompatible ECM can be reprogrammed to my early 1998 XJR with an AJ26 engine?

Thank you.

 

Aaaaaand just like that I found one! Jaguar Heaven in California called. I had given them the part number last week, and when they called back today I made THEM tell ME the VCATS number, and bingo! They're sending me an ECM LNC1410AD/001, which is exactly what I need!

I'll have more questions when it comes time to do the immobilization setup, I'm sure. Thanks all.

 

You WILL need to perform an 'immobilization-setup' when you install the replacement ECM!!

The engine will NOT start until the setup is complete.

bob

 

How exactly do I accomplish that? What kind of scan tool will do what I need?

 

The voltage test just seems to be written very ambiguously. The way I understand it, if you're getting at least 10v at the probed point, then all is good on the ign power side (and means that your fans are getting power?) and your fan motors are dead. If you're getting less than 10v it means there's a fault between the battery and the probed point, probably a frayed wire, or voltage drop somewhere. 10V is an odd number to pick though, I'm not sure exactly where that probe point is, but I'd be concerned to see anything more than ~.5V below system voltage (12.5ish car off, 14ish car on) anywhere linked to ign circuit switching. 10V might make sense if you have a dying battery, with the ignition on but engine off, accessories and lights on, etc but then everywhere would be 10v.

Basically, your measured results are what is expected with ign power functioning properly, and answers "YES" to AA4, meaning a fan motor's dead, assuming the previous tests were "NO" since AA1-3 eliminate other causes.