XJS revival with some strange problems

Unfortunately I must concede and make a post. It was not my knowledge of diagnostics that got me, but rather my lack of complete understanding of these vehicles and their systems. Much learning to do.

A small back story: it’s an 85 XJS V12, I bought it for very cheap from a youtuber. It’s missing a sizable chunk of floor on the drivers side, but the car is mostly complete and in much need of love. The previous owner had it running, but VERY poorly and only on a few cylinders (or one side?). I have done a lot of initial repairs: fuel hoses, seized distributor, plugs, new vacuum advance module and some wire repairs.

The main problem is injectors. as it sits, the crankcase is full of fuel (again!!). Last night it ran fairly well for the first time after new plugs and a full fuel system cleaning. then about 45 seconds in, it died and would not restart. Today I went to try it again and it’s hydro locked from fuel in the cylinders. only a couple from what I can tell. So I noticed last night after it died, when I checked voltage at the injector pins, On the B bank, I was getting 12V on the constant and ~10V on the other pin. what this automatically told me was that the injectors are going to be dumping gas. I believe you want to see 12V on both pins, with the ignition ON.

Today, I pulled the front 2 injectors to see what they are actually doing. as I figured, B bank would start spraying the second I turn the key. then, I cycle the key again and NOTHING. Then I cycle a few more times and it would start spraying again. A couple of times it would spray for a moment, then stop, then when I crank the engine It would start spraying constantly.

so to conclude: Does this indicate a bad ECU?

some further exploring: The ECU (side opposite of the MAP sensor) has a strange looking burnt out bridge between the 4 “on” circuits and the 2? “injector ground” circuit. Can anyone confirm if this is supposed to be there and/or what a good ECU looks like? I believe it is pictured below.

Thank you for reading!

1 Like

burnt area is top right

Hi Aaron and welcome.

First thing I would do is to energize each injector with a 9v battery to see if they are stuck or that they are clicking as they should.

The ECU definitely needs some attention and/or replacement.
Try fixing the burned contact and see.
What are the corresponding components on the other side and how do they look?

It’s most likely the component adjacent to the burnt track is short circuit. I would check that before repairing it and plugging it back in. An alternator often puts out negative spikes & it is important to protect the ECU. It’s common to put a diode across the 12 volt to protect the circuits, that diode may have gone short circuit.

No. You should conclude there is a short to Gnd on the injector trigger wires.
That short MAY be caused by the Ecu, but could be for another reason.

Everything looked decent except for that one spot. I’d like to fix the burnt contact, but I need to know exactly what to replace it with. wire? diode?

The corresponding side doesn’t seem to attach to anything

just some further findings from last night. probing across the injector pins on B bank, It would intermittently go from 10V to 12V as I cycle the key. it seems to me like whatever is controlling that ground source is sticking.

The track has burned off the board, so a healthy current has passed through it.
Ascertain what the issue is then replace the track with some wire. What components are on the other side of the board and does it connect to one of the output connectors?

I’m confused as to where you are measuring the injector voltages. The injectors are fed a constant +12v via the relay. The control side is first fed to Gnd directly, then the Ecu switches to the ballast resistor pack to reduce the current being drawn.

There is nothing on the other side of the board. I dont have a picture currently.

I am measuring at the pin that connects to the injectors. 12V from relay is always good, but the other one (from ECU), when not running should also show 12V. If that is incorrect, I would like to know. Once you turn the key, that number stays 12V until the ECU grounds that circuit, then the number goes to 10V or less. This is what I’m experiencing.

If I’m getting 10V non running on one bank intermittently, what does that tell you is the issue?

Obviously, the ECU needs repair, but you should carefully examine the injector harness for frayed, open or shorted wiring. Is almost 40 years old, and every owner I know of, has either repaired, bought, or made an injector harness.

An easy test for the injection system is using a NOID light to test the signal to each electrical injector connector that plugs into each injector. On the injection harness. A NOID light for a Bosch style injector. Add to your profile, where in the world you are.

Cheers, SD Faircloth www.jaguarfuelinjectorservice.com

The ECU just grounds the injectors and you should check that the second pin has no continuity to Gnd when the engine is not running.
You should definitely check the engine portion of the injectors harness, a common failure, somewhere there is a short and most probably that’s why your ECU got fried.

I don’t see how a short to Gnd on the Ecu side of the injector harness can cause the Ecu to burn, after all, shunting the injector wires to Gnd is what the Ecu does.
A short to +12v might burn the Ecu as it will be trying to dissipate whatever current is being supplied, with no limiting resistance of the injectors.

To Loggato.
The injectors are fed +12v via the relay, so you should always see that on the supply side of the injectors, but the Ecu shunts the return side to Gnd, initially directly, then via a set of ballast resistors, hence the lower voltage you see once the engine is running.