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Do I need a new ECM for my car?


Jan 13, 2002
kirkland, wa
1981 Black
I'm (still) getting code 15 from my ECM.
I changed the coolant sensor.
I checked the ohms at the ECM pin 3 and 7 (cold 5.80K ohms hot 257 ohms )
I removed the fuse to reset the computer code.
The check engine light is still coming on!

Is it possible the I have a bad ECM. Is there any further tests I can do to verify this.

Sounds like you have followed the procedure to check "Code 15" the only other suggestion is to clean all connections, also my GM shop manual states to perform the checks for "Code 21". ... Throttle Position Sensor Circuit...

If you need the proceedure I can post it or email you with it..
Extreem measures

I would 1st put a fixed resistance at the ECM 500 ohm.
( by cutting the 2 sensor wires @ the ECM) and see if it still throws the code. Now the only way you can see if the coolant temp sensor is doing it's thing is it modifies the DWELL ( ign timing )

So if you remove the resistor, & have a dwell meter you should see a 5 deg change between Hot & cold resistor on & off

The trouble flow chart states if the sensor measures OVER 1000 ohms replace the coolant sensor.

You state 5.8K cold
250 (ish) hot

Then the book states @59 degrees
4.1K to 4.7K

In the intro it states the operation of 100,000K Cold & under 1K hot.

So If the resistance Goes LOW when HOT I do not think the values are that critical.

Measure the resistance of the sender @ the ECM, the sender might be OK but the wiring could be funky ( if you decide to break the harness & put some mating connectors in)

The O2 sensor has some interaction in the circut and if you have a GOOD meter you could measure it's output.

.2 volts@ normal idle .7 rich runnning.

If you disconnect the O2 sensor @ the pipe & jump the 2 O2 pins you should see an under 10 deg dwell change...

All the ECM wires are HOME run ( no firewall sub connector problems)

This is a VERY confusing system.....

Hope this helps

I would like very much the procedure on how to check for code 21, I really think my cooland system is working fine since I tested the ohms at the ECM and it's within range.

If you can post or send me the code 21 procedures that would be great.

If that doesn't work I will try to figure out vigman suggestion about the dwell since it's a new territory for me!
If Mike (vigman) doesn't post it first, I'll post it later today when I get home.. Good Luck

Hope this helps you Rocketkid

We are getting in to uncharted territory here for ole Yoda.. I'll post here what I copied from my 1981 GM Shop Manual..

Trouble Code 21

“Check for stuck or misadjusted TPS Plunger.
(repair as necessary)

1. Ground “test” terminal.
2. Connect dwell meter to M/C solenoid – use 6cyl. Scale
3. Disconnect TPS harness connector terminals “A” to “C”
The “B” to “C” with voltmeter and ignition “ON” engine

Either or Both under 2 volts

Check voltage from the ECM terminal’s “21 to 22” and
“2 to 22”
If either or both these test under 2 volts … then you have
Faulty ECM connections or ECM

If both are over 2 volts … then you have an open point in
Wiring harness to the TPS

Both are over 2 volts

1. Start engine and let idle.
2. Jumper harness connector terminal “B” to “C” and note dwell

If under 10 degree … replace the ECM

If over 10 degree … Check TPS connector terminal connections

Good Luck Rocketkid.. wish I had more experience with this.. I'm learning too.. :(

Not to dwell on this

Old school.....
The Dwell is the amount of time the coil is charging or the flat spot on the breaker of the cam for the points, when you break the circut
is when the feild collapsed and you threw the spark. The Dwell meter would tell you that time in degrees.

The new ECM's control that time to vary the timing of the spark to meet engine needs.

The ECM would look at a few things ( and more later) to set the fuel/ air /spark conditions.

BASIC sensors

1) The TPS( throttle position sensor) lets the computer know how far the ( in your case) the CARB was open
2) Water temp.. colder engine=richer mixture
3)The O2 sensor to see whats coming out the back door.

These are all modified by the vehicle speed sensor/ gear selector/distrib timing( EST )

So by looking at the dwell you can see how the ECM is reacting to engine conditions by modifing the spark ( varying the dwell)

So the center point of the dwell should be 35 degrees ( in a perfect world)

So you need to tell us what the dwell is BEFORE we can guide you futher.

The hook up point is off the M/C( mixture control) solenoid on the carb.

There is a Light Blue wire with a MALE single pin connector just hanging loose.

That goes to the + on the dwell meter and the other goes to ground.

Get that hooked up and tell us what the reading is.

It's a scary thing I know.


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