V12 ECU failure

My 1990 V12 XJS consumes the Electronic ECU units making the car unusable. Within 20 miles of purchase the car refused to start, all the dash lights were dead. Replaced the ECU number DAC4586 with a used one, all the lights came on and engine started. 47 miles and several successful starts the dash lights went dead again and no start. While running the engine runs perfectly and has never stopped involuntarily, it is only upon a restart.
Has anyone any idea what is causing this repeated failure??


Is the resistor pack in good working order?

Excellent guess with resistor pack!

You will need a wiring diagram to figure out which paths should show resistance. If you find a bad resistor in the pack, get a used pack, test it, and you can then try yet another ECU with some confidence.

Another option is to mail it to Richard Bywater and maybe he can tell you based on what he sees inside.

If it’s the resistor pack is missing or bad the zener diodes in the ECU will likely be obviously burnt. Take the back off and you’ll see them. Check the archives I am sure there are pics.

No diagram in front of me at the moment but I’m struggling with how the dash lights are tied-in with an ECU failure.


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Excuse my ignorance but what and where is the resistor pack?


If the row of dash lights do not light up the engine will not crank over.
A replacement ECU has the lights come back on and the engine starts.

We are talking about this thing:

I am curious about resistor pack also. Could be corroded connection? But what about a bad ground to ECU? Could that harm it?

This one is to fit up to 1986, mine is a 1990???

They are visually identical. You’ll see it near the radiator, on the engine side, passenger side, kinda down low.

Smarter people than me will chime in here. But: the resistor pack is used to limit the current that passes through the injectors. The ECU provides a ground for the injectors, the injectors are triggered by the ECU intermittently providing this ground to 3 injectors at a time. The other side of the injector is hot all the time. The resistor pack limits the current, because the injector is just a solenoid, and has very little built-in resistance.

I am a little fuzzy on the details, but I think, for some reason, the car will actually run without the resistor pack, which means you’re passing too much current through the ECU and the injectors. The failure point is the Zener diode inside the ECU.

Or it could be that a common failure mode for the resistor pack is a short, which bypasses the resistors. I don’t recall exactly.

@V12JagGuy and SD Faircloth can provide a better explanation.

Before I did anything, I would crack open one of the failed ECUs and see if the diode are obviously burnt.

The ECU in my car right now is actually my spare, I sent it to Bywater for his enhanced tuning and to have it modified for my manual transmission. It had burnt diodes, which he fixed.

@Roger_Slater a bit of history would be good here. Was the car running great, than this problem started to happen? Was there some work or modifications done? Did the car ever run right?

Hi Roger

What I’m driving at is the fuel injection ECU does not control or have any tie-in with any of the dashboard lights. (I’m presuming you mean the various warning lights— brake warning, ignition, low coolant, etc)

How did you determine the ECUs have failed?

When you say the engine refuses to start does it crank OK? Crank-crank-crank, but no signs of firing?

When the dashboard lights die, does anything else die?


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The car ran very well but then failed to start, a replacement CPU and the main relay in boot top right got it going again. No modifications have been done to the car.
Now heres a thing. Two hours ago the car was stuck outside in the back yard in no start mode. With nothing better to do I changed the main relay. Car fired up. I am now wondering if the problem all along has been with the main relay failing and not the ECU??? . The relay is warm with engine running, my guess now is that the relay is burning out while car is running but then after switching engine off it goes into no start mode until the relay is replaced again. ??? The Thot Plickens.

CPU? You mean ECU?

Bob Brackney

Another bit to toss in. When I first got the car the CPU fitted was DAC6337.
I replaced it with same. Now I discover that that part number is for none catalytic models, mine has cats. Wonder what effect this has on things.

Correct, I failed to put brain in gear. Engine Control Unit is correct terminology.

None to the extent that it would cause a ‘no start’ situation


The main realy powers-up the ECU and injectors, and triggers the fuel pump relay. So, sure, it could be the problem.