Where does the fuel injector get it's signal from?

SableSal · Jul 21, 2011

Well, if you've seen my other thread, you'll see I'm in search of a fault in cylinder #3.

http://www.stangnet.com/mustang-forums/842247-misfire-woes-i-need-help-please-read.html

1995 Mustang GT 5 Speed. Completely Stock.

I'll be placing a test light at the injector harness and checking for the injector pulse. I'm curious to ask, does anyone have any information on what the pulse should look like? I'm assuming the pulse should look similar to a timing light?

Also, where does the injector get it's signal from? I'm assuming the injector gets it's signal from the ECU.

Thanks for all the help and opinions so far!

HISSIN50 · Jul 21, 2011

Yep regarding the flash. An LED test light or noid light is best because of the dwell control.

The injectors get constant accessory 12V, and each has its own modulated driver that comes from the EEC, as you noted.

94GTCoupe · Jul 21, 2011

Hissin,his computer works flawlessly in my car.

SableSal · Jul 21, 2011

Hissin, so you're saying it gets a constant 12v, the switched signal is the ground?

HISSIN50 · Jul 21, 2011

SableSal said:
Hissin, so you're saying it gets a constant 12v, the switched signal is the ground?

You've got it. If the noid light does not flash like a timing light, do a continuity test on the wire. If the noid light is on solid, look for the driver (ground) wire being pinched between metal parts (and shorted to ground, thus exciting the injector constantly).

SableSal · Jul 21, 2011

HISSIN50 said:
You've got it. If the noid light does not flash like a timing light, do a continuity test on the wire. If the noid light is on solid, look for the driver (ground) wire being pinched between metal parts (and shorted to ground, thus exciting the injector constantly).

Yeah, I got a hold of a noid light and will be using that this weekend. Also, question for you. The ground appears to be a red wire, for all the injectors. I'm wondering how they are switched? Also, sounds like the ground retuns to the ECU. How is the ECU grounded? I'm thinking I might have a bad ground somewhere... I guess I gotta get a hold of a wiring diagram to see how it's switched and so on...

Update:

Found this:

So if I'm seeing this correctly, brown/yellow is suppose to be the switched ground for the injector? Looks like the red wires are hot at all times.

Hmmm... interesting.

HISSIN50 · Jul 21, 2011

Red is the power to the injectors. Brn/yellow should be the ground for injector 3. Remember, they share accessory 12 volt power but have individual grounds from the EEC.

The big EEC ground is below the EEC in the kick panel. This is not where I'd be looking since only one cylinder is apparently affected.

64post · Jul 21, 2011

Remember, you can have noid light "pulse" or flash and still have a bad injector (pintle failure) so check that injector with a tester, stethascope/long thin screwdriver compare to others, it should "click" each time it's energized and that means grounded by ecm, common failure in EFI cars.

SableSal · Jul 21, 2011

HISSIN50 said:
Red is the power to the injectors. Brn/yellow should be the ground for injector 3. Remember, they share accessory 12 volt power but have individual grounds from the EEC.

The big EEC ground is below the EEC in the kick panel. This is not where I'd be looking since only one cylinder is apparently affected.

Yeah, that's true. If only one is affected, then the brown/yellow wire is the one I should be worried about. Also, when I last put a volt meter to the brown yellow wire, it was reading battery voltage. Tell me I'm wrong?

I have a stethoscope at home, I'll grab it and check it. I forgot about the sound test.

Thanks for the info guys, some more opinions would be great!

HISSIN50 · Jul 21, 2011

I thought you had swapped the injectors around already......

SableSal · Jul 21, 2011

HISSIN50 said:
I thought you had swapped the injectors around already......

I have. But I'm just saying, if it's not making sound, then I know it's not getting switched on and off.

94GTCoupe · Jul 21, 2011

Iirc,brown/yellow wire had fluctuating voltage.
We tested at firewall harness connector.

SableSal · Jul 21, 2011

davis3 said:
Iirc,brown/yellow wire had fluctuating voltage.
We tested at firewall harness connector.

Yes, Harv... You're right. Maybe the connection with the DMM wasn't good. I think next we need to test it from the EEC connector.

But first thing is first, I need to hook up that noid light tomorrow night

HISSIN50 · Jul 22, 2011

The ground-side can show weird things with a meter because of the involvement of the EEC, injector coil, and hysteresis in the DMM. The noid makes it easy.

SableSal · Jul 22, 2011

HISSIN50 said:
The ground-side can show weird things with a meter because of the involvement of the EEC, injector coil, and hysteresis in the DMM. The noid makes it easy.

I got the noid light, I'll be checking that and compression again today. Wish me luck.

SableSal · Jul 23, 2011

Is this normal? Noid Lite test.

Cyl#3
‪Cylinder 3‬&rlm; - YouTube

Cyl#2 & 3
‪Cylinder 2 & 3‬&rlm; - YouTube

I'm curious if I have a bad ground. It shouldn't get that much brighter, right??

HISSIN50 · Jul 23, 2011

Looks ok to me.

And the test with both looked very symmetrical to me. If that result is good enough for cylinder 2, it should be good enough for number 3.

SableSal · Jul 23, 2011

HISSIN50 said:
Looks ok to me.

And the test with both looked very symmetrical to me. If that result is good enough for cylinder 2, it should be good enough for number 3.

in person the light output seems VERY low in person. I still think I have a ground issue, because I highly doubt I have a full 12v at idle. I'm curious to think what if the shortage in voltage is causing an issue with the electronics. I'll be running my own grounds today and see if that helps.

SableSal · Jul 23, 2011

Ok HISSIN50, Harvey and I just checked the car again. We added two new grounds to the system, one to the accessory bracket near the idler. One at the factory point on the block near the oil pressure sending unit. She ran the same, although I did notice a steady voltage. I planned on better grounds, so I will keep them... So Harvey and I were thinking, maybe recheck the timing on the car. I think we may be getting closer to finding out what is going on. We removed the spout and started up the car, and AMAZINGLY ENOUGH! The car is running GREAT!!!

We turned off the car, and put the spout back on, and it was running really bad. Seemed to miss and the car just didn't seem right.

On one of the cylinder balance tests, there was a code that came up. The code was for A Ground Issue with the spout. We blew it off thinking that it was just because we ran the car without the spout connected. It looks like it was trying to tell us something. What do you think? Ever ran into this before??

HISSIN50 · Jul 23, 2011

Be really sure that the PIP is ok.

Where does the fuel injector get it's signal from?

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