On circuit 430, connector location B5, there's the purple/white wire for the distributor reference.

The ECM voltage chart that I got from DIY-EFI (which looks like it was scanned from a GM service manual) indicates that I should see 2.3 volts with the engine running. I'm only seeing ~.6V.

Is this a typo or do I have a problem?

Thanks in advance!

 

Anyone??

 

According to my diagram the distributor reference pulse circuit 430 should be D8 on the harness connector. Your voltage should be 2.3 volts with engine running. Location B5 should be circuit 452 throttle position sensor ground.

 

I'm pretty sure about the location... I'll double check. I do know the position varies from vehicle to vehicle, but the voltage is the same.

The question is, why am I only seeing .6V?

 

If your certain about the wiring locations the only thing i can think of is are you getting and maintaining at least 12 v to the ecm while running?

 

well, is the car actually running? this is the squarewave output from the distributor to the ecm, and the actual voltage you measure will depend on the voltmeter you are using. Its difficult to measure an AC signal on a DC meter, so they give you an approximate mid range voltage becuase the average voltage should be the average of 0-5v, which is about 2.5v, but only with a meter slow enough to average that out. if you meter has a fast average, or the square wave is not near 50% duty cycle, it won't read around 2.5v.

the correct device would be a scope, but they aren't too cheap.

what are the problems you are having?

you could use your meter in ac mode, or better, if you have a frequency or duty cycle mode, you could use that for better insight.

 

The car runs. I take the measurement with the engine running, just like the manual says.

I'll go back and try the AC mode.

 

Where are you grounding the meter? It's best to ground to the ECM grounds.. to maintain a neutral reference to ground.

But going by your sig.. I think you know this..

 

Well, I made some more measurements:


- Verified that I'm only getting roughly .6V with respect to any ground (ECM, battery, chassis). I've also verified that I have less than 1 ohm from ECM ground to (-) terminal on the battery. In fact, I went ahead and verified every ground on the harness(sensor, ECM, chassis, engine, etc.) just to be safe.

- Switching the multi-meter to AC voltage, I get a value bouncing around between approx 1.3V and 1.5V. The sample rate is pretty quick on the meter, on the order of less than .25 seconds

 

Fired up my ECM Bench...

It's a 749.. but that doesn't matter.

Dist Hi to ground 2.5 VDC as measured with my DVM.

Dist Hi to Dist Lo 2.5 VDC.

Dist Hi to ground 2.8 VAC as measured with my DVM.

Dist Hi to Dist Lo 2.8VAC.

Bob

 

I just pulled out the module and ran it across my Actron sensor/module testor (it has specific tests for various ignition modules including the GM HEI 7 pin). It passed the tests. I have another module in a box on the shelf. Tested that one too and it is also good. I also ran continuity checks across all of the wires from the distributor back to the ECM. All show 0.0000 ohms (which is not surprising since it's a brand new harness from last year).

If the ECM is supposed to be sending out 5V on this reference line, then is my ECM bad? If so, before I go out and slap on another ECM, what could cause it to fail like this? I've verified every other ECM input voltage and they are all right on the money. I don't want to get another ECM and ruin it too.

EDIT: Saturn5, thanks a bundle for taking the time to do those measurements on your bench. I appreciate it!

 

As measured to ground.. at the module.

Ref Lo (G) ~0VDC
Bypass (B) 4.75VDC
Ref Hi (R) 2.7VDC
Est (E) 1.3VDC

Now the following is inside the dizzy.

(+) 13.25 VDC (batt)
(P) 1.9 VDC (pick up)
(P) 2.1VAC(pick up) (technically a square wave pulse)
(N) ~0VDC (pick up)

HTH Bob

 

Yes, all of the other voltages/grounds check out just fine.

Swapped modules. Same thing.

Did some more expirementation.

When I first start the car, the voltage is at ~.9V, then drops to ~.6V.

If I rev the engine, the voltage will climb to 1.5V, but no higher- even 4000+ rpm.

Pulled out an old ECM that I had on there a while ago- same thing. Though, this ECM was run on the car in the same config that it's in now so it might also be shot.

This is crazy!

 

If I unplug the ECM and crank the engine over, I get about ~1V.

 

There is a difference between the ref+ signal on the bench and in the car. In the car the REF+ is less then a 50% duty cycle (DC). On the bench it is probably close to 50% which would show the 2.5 volts or there abouts.

In the car the DC will be lower then 50%. The signal is still a square wave. Hence a lower voltage.

RBob.

 

OK, made some measurments on the car. Pin D8 is the correct pin on a '730 for REF+, pin B5 is a sensor ground reference (as mentioned by 884+3).

With the DVM on DC volts scale the voltage was 1.6 volts at idle and 2.0 volts at 2,000 RPM.

With the DVM on the AC volts scale the voltage was 2.5 volts at idle and at 2,000 RPM.

RBob.

{edit: page 6E3-A-8 of the '92 Camaro Helm manual states there should be 1.5 volts on pin D8 with the engine running. This is with the engine at idle and the DVM on DC volts. This is close to the 1.6 volts I found}

 

RBob, Thanks!

You guys are right, it is in fact D8.

Well, my voltages are still off compared to yours.

Any ideas on what it could be? Is there anything in the eCM programming that could cause this?

 

are you having some trouble with the car? that voltage that you are measuring is the OUTPUT from the ignition module, so it might vary from the following reasons:

you meter isn't cal'ed perfectly

the inductive pickup is old/connection aren't perfect

the module itself just puts out a slightly different duty cycle based on its triggering window, i've seen aftermarket ones actually wiggle around alot at low rpm, drove me crazy for a few months tracking down a weird stalling issue.

maybe the distributor reluctor points aren't perfect, causing a lower voltage to be generated.

all in all, if the car isn't having problems, you shouldn't have any issues with the voltage reading a little low. the ecm shouldn't have an effect from running code, although if there is a pull up resistor or something in the ecm, and it varied a little, it could change the voltage swing, but not very much.

 

A rough and smelly idle often with a misfire. And believe me, I've gone over EVERYTHING else

This module voltage is the only thing I've found that is not per spec.

The cam is a Comp Cams roller 212/218 @.050 so I know it's supposed to idle smoothly.

The distributor is brand new (including everything in it)- within the last year. But I'm also going to check the voltage with a different volt meter just to see if I get the same reading.

 

In the ECM the REF+ signal is filtered with a small capacitor, then a 270 ohm series resistor. It is then buffered with a 33K resistor in series before being fed to an Op Amp. It is possible the small filter cap is bad. That can pull the signal down.

There is no code/software in teh ECM that can affect this signal.

I would start with the distributor: check that it is getting a full 12+ volts from the coil feed line (small cap or big cap distributor?). Check the polarity of the pickup coil to module signal. The DRP duration is dependent upon the pulse from the pickup coil.

Then the module itself. I trust the OEM GM moduels more then any others. Although I do have a Holley module in one vehicle. A scope would help immensely. The REF+ signal (DRP) is a positive going square wave of 0 to 5 volts.

RBob.

 

Most of the wierd ign problems I've seen turned out to be the pickup coil. Unlike a module, they seem to be able to sort of work...

 

Well since there's no way to really check the ECM (unless I'm mistaken), I'll have to assume that since I'm on my 3rd ECM, the ECM is not the problem.

I am getting a full 12V on the coil feed line. It's a large cap distributor. However, as far as the polarity, the pickup coil wires (green and beige) are fed into pins P and N on the module. What is the correct polarity? I'll go and check it out. Is it Green to P or Green to N?

 

On a small cap distributor the green is P and the white (beige once their old) is N.

RBob.

 

you really need to check the ref hi signal with a scope. the signal is a on/off signal so to check it with a voltmeter is not an accurate way of doing it. the signal is on about a third of the time then off for two thirds.