Bonehed

According to the GM ECM installation instructions, resistance between specific ECM connector pins should be 20 ohms or above.
Mainly pin A6 should have more than 20 ohms between A2,A3,A4, A7, C1, C2. D 15,16,17 do not list the values. I get 2-4 ohms between A6 and all of the pins the instructions specify to check. Pretty good place to start since it kept blowing computers
Question is..
HELP, what is the most likely place to start finding out where this thing is shorted out?
Any one sensor or something could cause this?

Vader

Bone,

It would really help to know what kind of engine and fuel system you have (or at least the ECM part number). Each system uses different loads and sensors on the connection points, so the answer can vary between TPI, TBI, CCC, etc. Even V-6 and V-8s have different loads and sensors.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Bonehed

Car is 85 IROC, the 305 appears to be from an 83 firebird, and the stock 85 TPI is still present, minus the ECM, and there used to be several alarms installed, which were all stolen along with the stereo.
By the way, I am looking for an 86 ECM, the dash and engine compartment wiring harnesses, as well as the one sensor that changed in 86, I have that information at work.

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85 IROC, 305 TPI, block from 83 firebird, 3" exhaust, unknown heads/cam, no ECM, Urban Assualt flat green paint, 245/50/15 eagle GT II's. Car was FREE, and worth every broken knuckle.

Vader

Bone,

Thanks, that helps a lot.

The A6 connection is the ignition circuit power feed to the ECM.

A2 is the Air Injection diverter valve PORT solenoid (which dump air to the exhaust manifolds for warm up);

C2 is the Air Injection DIVERT solenoid, which dumps air from the system;

A3 is the EVAP canister purge control solenoid;

A4 is the EGR vacuum solenoid;

A7 is the torque converter lockup clutch relay (or "SHIFT" light on a manual trans car);

C1 is the radiator fan relay;

D15 and D16 are the fuel injector banks;

A1 is the fuel pump relay;

D12 is the MAF burnoff relay;

A5 is the SES lamp.

Unfortunately, I don't have a schematic that shows D17.

The relays should have a resistance of at least 30 ohms. The fuel injector banks could have some very low resistances and still be O.K., as low as 3 ohms.

The various solenoids should all have a resistance of greater than 20 ohms.

I hope that helps. Make sure the ignition is OFF and the battery is disconnected for the test.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Bonehed

Geez, gotta love the message board. I had the ignition off, BUT the battery was connected. THANX, I'll try it again today.

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85 IROC, 305 TPI, block from 83 firebird, 3" exhaust, unknown heads/cam, no ECM, Urban Assualt flat green paint, 245/50/16 eagle GT II's. Car was FREE, and worth every broken knuckle.

Bonehed

Well, battery is unhooked, I still get hardly any resistance at all, anywhere.....I am going to try a different multi-meter just in case, but it seems to me something is screwed up bad. There is basically no resistance between a6 and anything.

Vader

Bone,

Try the different DMM. Incidentally, by saying "no resistance", do you mean "zero ohms" or "no reading" (infinite resistance)?

Just a little clarification. I understood from the earlier post that the resistance was too low, but that was with a battery path and may have confused your meter. Do you have a resistor to test the meter calibration?

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Bonehed

Meaning very little resistance. When I have 0 ohms, my analog meter (I prefer the analog) shoots all the way to the rt side, there you can set the meter to "0" and continue to perform your test. Ignition off, battery out, I went from pin A6 to all the others mentioned above, and they all had 2-3 ohms, none were anywhere near 20 ohms.

The car had a history of blowing ECM's.

I will check the meter against a resistor, but I am confident it is accurate. (wouldnt be the first time I ate my words though)

Is there any one part sensor or solenoid that could make the whole thing appear shorted out?

I am currently shopping for a new harness and computer....86 with the burnoff relay would be my preference. In the meantime, I am going to yank the dash harness to begin with, and go through it to be sure there isnt something like a meltdown situation or any catostrophic idiocy going on here. Not sure if I mentioned it, but I believe the car has a history of some un-professional wiring....oh yeah, had three alarms in it all of which were stolen along with the stereo, amplifier, ECM and who knows what else. Glad the car was free.

Vader

Bone,

Roger that. You are using an analog, which is just fine if the scale is correct. Hopefully it's a low current meter, not a Simpson 260 or older Triplett industrial meter - they have a fairly high current on the RX1 scale. The solenoids often have a diode for flyback current spike reduction on de-enrgization. It's possible that the battery was once connected incorrectly and toasted several of these diodes on the solenoids. It's also possible a high current ohmmeter could damage these diodes during testing.

As an alternate test, you could check each device's resistance directly at the load with the harness disconnected. Unplug the EGR vacuum solenoid and test the resistance there. Unplug the electric cooling fan relay and test the coil resistance, etc. If there is a discrepancy between the readings at the loads and at the ECM harness, you may have some serious wiring issues.

I may have steered you wrong. I understood that you were testing an '86 ECM, but now I see that it is an '85. The ECMs are slightly different, and I'll check the loads at the appropriate I/O points if you give me the ECM part number.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Vader

Bone,

O.K. If you have a 1226870 ('85 TPI) the I/O assignments are as follows:

The A6 connection is the ignition circuit power feed to the ECM.

D5 is the Air Injection diverter valve PORT solenoid (which dump air to the exhaust manifolds for warm up);

C7 is the Air Injection DIVERT solenoid, which dumps air from the system;

A3 is the EVAP canister purge control solenoid;

A4 is the EGR vacuum solenoid;

A7 is the torque converter lockup clutch relay (or "SHIFT" light on a manual trans car);

D2 is the radiator fan relay;

D14, D15 and D16 are the fuel injector banks;

A1 is the fuel pump relay;

C3 and C4 are the IAC "B" coil hign and low steps;

C5 and C6 are the IAC "A" coil hign and low steps;

A5 is the SES lamp driver.

Unfortunately, the schematic I have shows only 16 pins on the "C" and "D" connector, so there is no "D17".

The MAF power and burnoff functions on that ECM are controlled by the MAF module attached to the outside of the ECM, not internally conrolled.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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Bonehed

Well I'll be Darned, I went out, bought a few resistors from radioshack, Multi-meter says the same thing, no resistance. Time for a new one. (I even had it set to the right scale this time too)