'89 central microprocessor problem?

(Grooveman) #1

Everything was fine but I just couldn’t leave well enough alone !!!

For some reason I decided to adjust how my ski slope fit around the radio/clock/climate controlled unit and after pushing the assembly back in it’s “cave” I noticed the lights were out in the A/C panel and clock.
I checked the fuses and found that one in the console box was blown along with a circuit 3 failure warning. Replaced the fuse and the warning went out but the problems remained. On further investigation I found more nonfunctioning systems and lights then Tom Hanks had in Apollo 13.


I checked for a pinched or broken wire in the “cave” but everything looked good. All the wires actually looked fresh and pliable. Nothing brittle or suspect.

After some inter-web research it appears that the “Central Microprocessor” controls a lot of these failed items. So right now that’s my main suspect.

Has anyone had any experience with this component? I can find plenty of used processors for later model XJ40s but can’t even find a part number for my '89. Actually I’m not really sure exactly where it’s located.

So … A little help here please …My beloved daily driver is in the hangar until I can figure this out and my wife seems to be always on the go in “her” Uber Germanmobile. So I’m doomed to watching reruns of “Gilligan’s Island” until I sort this out.

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( Larry ) #2

Try watching Green Acres instead of Gilligan for tips. Make sure you have a One, Three and another One plugged in or you might be in trouble.

I think I saw something labelled as the Central Microprocessor under the passenger kick panel - not sure, but I think that’s where it is …pretty sure it has a label on it with that designation …maybe you pulled the harness at the slope and slightly dislodged the plug from that unit?

(Grooveman) #3

Larry …

Thanks for the quick input. I never was much of a Green Acres fan, so the age old question for me remains … Ginger or Mary Anne :heart_eyes:

What bothers me about this problem is the blown fuse. Something must have caused that and I’m afraid it may have also effected the microprocessor. I think the next logical step will be to pull the passenger (right side) kick panel and look for that little devil.

I can only hope that it’s something as simple as a loose plug but they seem to be old school industrial strength plugs that are tough to get apart even when you’re trying.

(Mike Stone) #4

Might be worth checking the connections to the grounding stud in the center console as well as the connections to the back of the climate control and clock panel.

(Grooveman) #5

I’m having trouble even finding the central microprocessor !

Opened up the area under the glove box and here’s what I have …


I point out the blower assembly just for orientation. There is a large ECU with a faded stamp size label that says DBC 3285 155 (as well as I can read it). Online search of that designation turns up nothing. I did find a picture that look very close to it and it was the XJ40 engine ECU.

It’s very strange that I can’t find a part number anywhere for the central microprocessor.

(Casso) #6

Hi groove, Just a thought, is it possible the central processor is soldered or plugged into the ECU as a permanent fixture, rather than a separate component situated elsewhere ?

(David Martin) #7

I hope this helps, 1989 xj40 us passenger side knee bolster!

(David Martin) #8

All my bolsters are off so if you need part numbers I can probably get them for you.

(Grooveman) #9

David …

Thanks for the picture and it’s right on. Here’s a schematic that I finally tracked down from British Auto Works.

I’d really appreciate it if you could get the part number for your CPU (number 10, the big copper colored box) I believe my number is DBC 3285 199 but it’s barely readable.

Here’s one I found online so the 199 is really ISS.

(David Martin) #10

1989 us xj40

(Grooveman) #11

Thanks for the picture David …

That confirms the numbers on my box are the same as yours. So that means I’ve finally tracked down the location of the CPU which I’m confident is the “Central Microprocessor” that the wiring diagrams refer to.

Since I have so many seemingly unrelated systems that are not working or lighting they all have one thing in common, the CPU. So my theory is either the CPU is malfunctioning or it’s not being powered.

So my next step is to figure out if the CPU is actually being powered … ARGH !

(Grooveman) #12

Casso …

“is it possible the central processor is soldered or plugged into the ECU as a permanent fixture, rather than a separate component situated elsewhere ?”

I found that the CPU (central processor unit) and the ECU (engine control unit) are indeed two separate boxes as this diagram shows. I also checked it visually. But that was a good thought.


(David Martin) #13

If you want the ecu numder it won’t come from me, need to remove from what I can tell, no numbers visable.

(Grooveman) #14

David …

No thank you I don’t need the ECU number, it’s working fine. I just needed to confirm that your '89 CPU had the same number as mine … and it does.

Already ordered a correct used replacement so when It arrives Monday I’ll know if that was the problem.

(Grooveman) #15

YES-S-S-S !!

What can I say, the Jaguar Gods are being kind to me.

After seemingly eliminating everything else I thought my problem must be the Central Microprocessor.
So I purchased a "used* unit on Ebay. As we all know NOTHING is easy with these cars so what are the chances if it actually was the CPU that I would get a good used one that would work.

SO Wham … I plugged it in, shut my eyes, turned on the ignition, and … EVERYTHING worked again. Most of the time the bear gets me but today I got the bear.

Thanks so much to everyone who gave me inputs on this problem.

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