How do I test for faulty ground lead on injector harness?
 

I suspect that I may have a bad spot in my engine harness that is causing an injector to see a continuous ground, thus holding it open all the time. How do I test for this? Checking continuity of the ground lead would not tell me if it is shorting out to another wire or the chassis or whatever. I'm having a hard time understanding how to diagnose this.

what happens if you place your probe on one of the injector plug pins and the other one on a good chassis ground? is there continuity?

Looking at the primaries only:

-All four leads for the two primary injectors have continuity between the plug and the chassis (checking with the ground on the passenger side of the fire wall, above the brake lines.)

-The plugs do not have continuity across the positive and negative on the same plug.

-There is continuity from the neg. of one plug to the neg. of another (makes sense) but no continuity between plugs using any other lead combination.


I don't know if any of this helps you, but that's what I've got so far. These checks were all done with the battery disconnected.

-James

hard to give advice on this one without having the car in front of me

I understand.

As I recall the injector grounds thru the ECU.

If you run from the ECU pin to the voltage connection (it's up near the battery / fuse box) you should see only the resistance of the injector plus a couple of ohms for wiring resistance. Check all 4 and you should see similar results on all 4. I'm no electrician but I was able to figure that out using the factory wiring diagram. If the harness is shorting to ground the resistance between the injector wiring and ground will be small. If you download that and check the diagram you should be able to figure that out.

The 2nd thought other thing you could do is disconnect the ECU plug, then apply 9v at the fuse box end of the injector wiring and look to see if the 9v finds it way to the ECU terminal. If there's a short to ground it won't be there.

David

This is excellent, thank you. I really appreciate it. I will reply later with results.

Are your injectors sticking open permanently? Or intermittently? I'm having a fuel issue now where my cars bogging from dumping fuel in, which I'm suspecting are the injectors getting stuck open. However mines happens sporadically. I'm thinking of doing the same test on mine and see what I get

you said:

Looking at the primaries only:

-All four leads for the two primary injectors have continuity between the plug and the chassis (checking with the ground on the passenger side of the fire wall, above the brake lines.)

/\ this cannot be right. or if it is the fuse for the injector power is blown and the + lines are shorted to ground.
or you made a mistake doing the procedure
the batt+ will not have continuity to ground.
the ecu cntrolled side will show continuty to ground with the key off and the ecu plugged in
if you disconnect the ecu, the ground(control wire) for each injector will be open. if not the wire is shorted

Thanks for the reply.

Here is a picture of what I'm doing, showing the continuity tester light shining in the upper left corner for both leads of one of the primary injectors. I've now tested all four injectors this way and all of the injectors leads, + & -, show continuity to the chassis. This being done with the key off and the battery disconnected.

Before I removed the UIM the car did run. But it sounds like you are saying that the car should not be able to run given the results of my continuity tests.

What are your thoughts?

James

http://www.mcdonaldindustries.com/ja...continuity.jpg

I re-did all my tests this morning and wrote everything down.

http://www.mcdonaldindustries.com/james/rx7/img002.jpg

Now I am testing all the injector wires for voltage. This is testing at the connector with the battery connected and key ON.

http://www.mcdonaldindustries.com/james/rx7/voltage.jpg


The small values on the positive side of the primaries may be rounding errors, or effectively zero, on my digital multimeter. But I don't know if it is correct for them to read this way. I suspect that all 8 wires should show approximately the same voltage, and that this is wrong.

Does anyone know? I am not great with electricity and I'm having a hard time finding this info in the wiring manual.

These measurements were taken by putting one probe on an injector connector receptacle and the other on a chassis grounding point.

OK, looking again at Z-30 in the wiring diagram book, ALL of the + sides go to the main EGI relay, so with the key on they should all read about the same voltage. Obviously they do not, so it seems I have a short to ground somewhere between the + side of my Primaries and the main relay, and that would be why I am seeing continuity to ground but no voltage on the + side of the primaries.

This however, doesn't seem to jibe with the resistance value I recorded for the + side of the primaries, above.

James

Alright this is what I got. I did everything with the battery connected and Key on, and checked voltage running through the injector plugs with the MM

ECU disconnected: No Voltage through any plug

ECU connected: No Voltage through any plug

ECU disconnected - Injector pins on ECU side grounded to chassis: (Mimicking what the ECU does to turn on the injectors)
I get 12v through all injector plugs

Here's what I see, there's always 12v being fed to the injector plug (+) EGI relay side. The injector plug (-) side goes to the ECU. The ECU keeps the circuit open until it needs to be grounded. If there is a short somewhere causing the injectors to be grounded, the only possibility is on the (-) side of the injector plug going into the ECU. If there was a short on the (+) EGI side, then the injector wouldn't operate at all.

So if the injectors are sticking open, there's a short somewhere on the (-) ECU side, and you would see 12v on a MM going through the injector plugs all the time with the ECU connected or not.

Where I could be wrong is if the ECU side supplies voltage also, so if it does, somebody please correct me on that