[FIXED] cylinder #4 fuel injector has constant ground signal

Hi All. Hoping someone can help..
My wife's 2003 Audi A4 1.8T cabriolet (58000miles) began to run rough with whitish smoke with gas smell being blown out of the exhaust.
Initial diagnosis confirmed problem due to cylinder #4 fuel injector getting constant ground signal. Swapping injectors made no difference and when engine was cranked verified injector on cyl4 was staying open. Disconnecting cyl4 injector and running engine eliminated the smokefrom exhaust and car ran ok but only on 3 cylinders.

Carried out the following tests to identify whether problem was due to a short to ground in the injector wiring harness or due to ECM injector driver short.. However the results of the tests have provided confusing results and I am no closer to identifying cause of problem.

Tests:
1. with cyl4 injector plug disconnected and using test light across the injector plug terminals with engine cranking , the test light stayed on confirming constant ground signal. There is no light when the ignition key to only on position.
2. using ohmmeter with car battery disconnected found continuity between the ground terminals of ALL fuel injector plugs and engine. Continuity of ALL the injector plug ground terminals was also confirmed using a test light connected between the +ve battery post and injector plug ground terminals with the light staying on..This test was also done with the ECM disconnected and the light stayed on.

The above tests would suggest that there is a ground to short in the injector wiring harness for all the injectors, however the car runs ok on cylinders 1-3 and only the cylinder #4 injector stays open when engine cranking or running.

I'm stumped.. Can anyone tell me what's going on? I originally was all ready to send the ECM off for repair but then I did the tests to check whether the constant ground signal was due to possible wiring short to ground and now am only more confused by the results of the test.

Thanks in advance..

Before replacing the ECU you could consider this try cutting the No4 ground wire close to the ECU and testing it engine running with a test light to battery +.

Thanks ANDY.. I just rechecked a number of things to verify my original tests and to test something that I read somewhere about fuel pump relays potentially causing ground faults..This is what I found:

1. With everything connected but with the fuel rail off and cranking the engine there was about 25mm fuel ejected from cylinder4 fuel injector.
2. the above test remains the same when injectors are swapped ie. problem stays with cylinder4
3. using test light connected between + battery post and ground terminal on all injector plugs produces LIGHT ON..
4. However disconnecting Fuel Pump fuse (#28) and using test light connected between + battery and injector plug ground terminals results in NO LIGHT..

I plan to buy a Fuel Pump Relay and see if this fixes the problem.. I'm still stumped and confused by the results. Why do the tests demonstrate continuity to ground on all the injectors yet when the car is running only cyl4 injector has the constant ground signal? It appears that the conventional tests for ground shorts in wiring harness aren't reliable.

I'll try your recommendation of cutting the wire corresponding to pin 88 (cylinder4 ground injector) on the ECM harness and running the engine to test for ground short in the wiring harness after I replace the Fuel Pump Relay. I just hate cutting wires, but I appreciate and I like your suggestion as it likely is the only conclusive test for ground short in the wiring harness unless there is something else going on, given the weird test results I've gotten.

Cheers...

alfredo wrote: I just hate cutting wires

You could disconnect the connector from the ECM and the Injector. This SHOULD isolate the wire. Now, check for continuity to ground. Or, test-light to battery positive, test the wire. If it lights, that wire is shorted. If it doesn't, it isn't.



If that checks out, THEN cut the wire, as Andy suggests.

Thanks Pole71 for the schematics and your help..

I had planned on doing what you recommend but I haven't figured out yet how to disassemble the ECM harness connector to unplug wire 88. I would like to do that if after installing the new Fuel Pump Relay makes no difference. I forgot to mention that by either removing the fuse 28 (Fuel Pump) or fuse 32 (fuel injectors) the continuity to ground disappears using a test light, however removing fuse 34 (engine management) which also has connection to Fuel Pump Relay makes no difference.. This is the weirdest set up I've come across. I'm beginning to think that AUdi manufactured a wiring harness that isn't what it should be according to the schematics..

Technically, according to the wiring diagram, the ground wires from the injector plugs should be uninterrupted going directly to the ECM. Then how is it possible for me to be getting continuity to ground for all the injectors when using a test light with ECU disconnected and yet when car is running cylinders 1-3 injectors work fine..?.. This is driving me crazy..

Cheers..

alfredo wrote: I haven't figured out yet how to disassemble the ECM harness connector to unplug wire 88.

You don't need to disassemble the connector. Just unplug it. The WHOLE ECM connector and the injector. You don't need to start the engine.

I had removed the harness connector from the ECM as part of my tests that I first posted.. With the ECM disconnected there is continuity to ground for all the injectors, yet when I have everything connected and engine cranking or running the cylinder 1-3 injectors work fine.. There shouldn't be continuity to ground for all the injectors.. I had expected to only find continuity to ground for cylinder4 injector. This is why I am stumped..

alfredo wrote: according to the wiring diagram, the ground wires from the injector plugs should be uninterrupted going directly to the ECM. Then how is it possible for me to be getting continuity to ground for all the injectors when using a test light with ECU disconnected and yet when car is running cylinders 1-3 injectors work fine..?.. This is driving me crazy..

Just a theory...If the #4 injector control wire is shorted to ground, the ECM is disconnected, but the injectors are connected. Then, there would be continuity to all injectors, albeit high resistance.

alfredo wrote: I had removed the harness connector from the ECM as part of my tests that I first posted.. With the ECM disconnected there is continuity to ground for all the injectors

Did you have the #4 injector disconnected, too?

Yes, I had all (1-4) fuel injector plugs removed when I carried out the continuity to ground tests using either a multimeter or test light.

Continuity to engine ground existed on ALL the injector plug ground terminals whether the ECM was connected or not. Yet when the car is cranking or running cylinder4 fuel injector gets a constant ground signal while cylinder 1-3 fuel injectors get the normal pulsed ground signals.

Also, when fuses 28 (Fuel Pump) or 32(Fuel Injectors) are removed, the continuity to ground on the injectors is eliminated., yet fuse 34 (Engine management) has no effect.. All these fuses get power thru the Fuel Pump Relay..

I"ll have the new relay tomorrow to check whether a faulty relay could unexplicably be the cause of the fuel injector problem.

Is it possible that the wiring harness or actual wiring installed by Audi isn't what it should be according to the manufacturer's schematics for the Audi A4 1.8T? Where are all the fuel injectors getting their continuity to ground and why would the Fuel Pump Relay affect the results when the Fuel Pump Relay is only supposed to provide + power to the injectors?

I think, it is time to follow Andy's suggestion. Snip the wire and see what the ECM is doing.


I will post the complete Engine Performance schematic, in the morning.

pole71 wrote: I think, it is time to follow Andy's suggestion. Snip the wire and see what the ECM is doing

Yup.

Keep in mind that continuity to ground doesn't, necessarily, mean shorted to ground. In this picture, I attempted to draw a light bulb with an open on the power side. The principle is the same for your injectors. You can read continuity to ground THROUGH an UN-energized component.

Schematics pages 1 and 2

Schematics Pages 3 and 4

Try disconnecting all 4 injectors and the connector at ecm and checking for ground at each injector plug

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Thanks Pole71 et al for all your help...I'll post results of the final test after I change the Fuel Pump Relay ie. checking for constant ground on injector 4 after disconnecting the ground wire (88) for cylinder4 injector from the ECM and running the engine.

pole71 wrote: Keep in mind that continuity to ground doesn't, necessarily, mean shorted to ground. In this picture, I attempted to draw a light bulb with an open on the power side. The principle is the same for your injectors. You can read continuity to ground THROUGH an UN-energized component.

This is exactly right. I bet this concept is what confuses people the most when using a power probe. "Why is there ground on the power side of the bulb?" "Is it a short to ground on the power side?" "Why isn't it blowing the fuse?"

I love my power probe btw, I think it's one of my favorite tools.

Well I changed the fuel pump relay and everything remains the same.. I was being hopeful it had something to do with the continuity to ground results I originally carried out because when either fuse 28 or 32 were removed the test light when out.. However nothing has changed and I'm confused as ever to explain my findings/test results other than I must have 2 separate issues going on. Given that the ground terminals in the injector plugs are supposed to be separate and go directly uninterrupted to the ECM, the test light should not come on with ECM disconnected and test light connected between + battery post and ground terminals for all the injector plugs and yet it does.

Tomorrow, I'll finally cut or disconnect the suspect ground wire from the ECM and test again with engine running.

Cheers...

Continuity to ground on the red/blue wires while the connector is unplugged is through fuse 32 then fuse 28 and through the fuel pump motor. That connection is broken when removing either of those fuses. I suggest ignoring continuity to ground on those wires when unplugged because that is normal. You should only focus on a short to ground on the grey/Violet wire for fuel injector #4.