[v12-engine] Trouble Codes, ECUs, and Marelli

Hi folks…

I have a '94 XJ-12 in for service and it threw an FF 13 code which I gather
from an aftermarket source is the MAP sensor.

I looked for the ECU which is supposed to have a vacuum line to it.

I found three processers…

One of them is in the boot and has a GM style BARO sensor attached to it
and says Delco Electronics on it.

I’m assuming that this is the processor for the 4L80E trans…

Under the dash on the right side, there are two processors that both have a
vacuum line to them. One is pretty big and I am assuming that this one is
the EFI processor.

The other one is right next to it, smaller, but also has a vacuum line to it.

My questions are…

What is this second ECU?

Is this one for the Marelli ignition?

I am familiar with MAP sensor failures/codes from a detached vacuum hose or
fuel clogging the line but this is not the case AFAICT.

I have good strong vacuum to both hoses.

Anyone have any thoughts as to what may be the cause of my FF 13 code?

I cleared the code and it didn’t instantly reset but I haven’t run the car
for very long either.

Any thoughts would be greatly appreciated.



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At 12:23 AM 2/20/2002, you wrote:

I would guess the small one is for the heating controls, and the larger one
for the

Nope! It appears that the smaller ECU is for the ignition.

I’m unfamiliar with the newer XJ40 XJ12 cars. Here’s a test, while car
is running,
unplug the main connector for one of the units. I would try small one first.
If it keeps running, it’s not the ECU.

Actually, I unplugged both to check for vacuum source and they both had
good vacuum. Disconnecting the one from the EFI ECU did not shut it off.

There should be some laberlling on
the cases.
Does one of them have bright yellow or red tape all over the harness?
If so, leave it alone, as it is for SRS or ABS.

Nope…the ABS and the trans ECU are in the boot.

The SRS and climate control computers are not in quite the same area as
these other two processors.

How about under the seats? Front and rear. Is there any other control units

I didn’t see anything under the seats. These XJ-40s have a pile of assorted
computers that range from a fault finding processor for the electrical
circuit to one for the doorlocks. They have one for the climate control,
trans, EFI, ignition, ABS, and a trip computer.

I’m amazed at what a crash course I’ve been getting with these cars since I
bought my '91 VDP and a customer who just bought this '94 XJ-12.

They’re very cool cars.


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