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

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.

EricGoodrich wrote: "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 that green light.

OMG.. Was it possible I was testing the wrong terminal in the injector plugs as Eric had alluded to?

Well having a schematic (Thanks Chad) with the numbers marked 1 or 2 for the injector plugs and having my wife verify the colors since I'm color blind, ERIC was right, I was testing number "1" power terminals on the injector plugs all along...

I apologize for having wasted everyone's time..

Re- testing with battery connected and using test-light between + battery post and terminal "2" (ground) on the injector plugs now resulted with light ON when ECU connected and light OFF when ECU disconnected.. I'm now confident that the problem lies with the ECU and not the wiring harness. I assume I no longer need to disconnect or cut the ground wire from the ECU and test with the engine running.

Thanks Chad, Eric and all for your help...

I sincerely apologize to all for having got my wires crossed in not identifying which were the actual ground terminals "2"at the outset.. I feel like such an idiot... Again thanks to all..

A final question or two...Could anyone recommend whether it is better to get a new ECU (programmed to plug and play) or is it OK to get the original ECU repaired? I've found a number of places online that can repair ECU's. Could anyone recommend a place they've had good results with? Also, I should probably get new fuel injectors as I've read that working but bad injectors can cause the injector drivers on the ECU to go.

Cheers...

alfredo wrote: I apologize for having wasted everyone's time..

If you came out of it with a better understanding than you went into it, then nobody's time was wasted.

I'm glad you got to the bottom of it. As for replacing or repairing....I have sent modules out to be rebuilt a few different times. I don't think I have ever used the same place twice. Once, through Autozone. :ohmy: Once, from an ebay listing. And, a couple different websites. (I can't recall, specifically). Usually, though, the reason I chose rebuilt over new was because I couldn't get new. On one occasion, the module was returned to me as "un-repairable". Beyond that, I have had no bad experiences with rebuilds.

Monde wrote: Did you do the test with the car running? If you did not, you have to start the engine to see if the light flashes, which will tell you the pcm driver and the wiring are fine.

There should be no ground KOEO. If disconnecting the ECM removes a constant ground, it is safe to say the control driver is shorted to ground. The ECM should PULSE a ground only when running or cranking. It should not supply a constant ground with the engine not running. In this case, the light will not flash. It will stay lit, continually.

Be sure to check the injector for a short circuit before you replace the ECM. A shorted injector coil can allow more current to flow than the ECM driver can handle. It's a possible underlying cause of the failed driver.

Thanks..Yes will be putting in new injectors after I get the ECU repaired.. Aftermarket injectors ok?

Will post outcome after repaired ECU installed..

Then will need to deal with another issue..The TCM for the CVT also needs to be sent off for repair which has the classic PRND light flashing on dash with no reverse going into limp mode... Wish I knew all the problems AUdi's have before I got the car for my wife.. The only good thing I can say about the A4 cabriolet is that it looks nice..Oh well live and learn..