Code #41 and #55

[Rc 415]

I'm reading my computer and its showing a code #41, No distributor reference pulses at specified engine vacuum. And, a code #55, Grounded V rev (term21), hush voltage on oxygen sensor circuit ECM. Can anyone tell me what these mean and how to fix them?

[Rc 415]

any ideas?

[ChevyTech]

A bad ECM would cause the code 55 and that should be repaired before working on any other code.

What are you working on?
What year?
What vehicle?
What fuel system?
Is it stock?

[Rc 415]

1983 Chevy C10 Stepside
its a crate 350, bout 10 years old with 75k miles on it, was previously a 305. its a carbed engine, with an original rochester carb on an edelbrock manifold, and everything else about the engine is stock as far as I know.

[ChevyTech]

Check for corrosion at the ECM connectors.

Here are some tests you can do:
Test the reference voltage to the TPS, MAP or BARO sensor. This tests terminal 21 voltage. It should be 5 volts. 4 to 6 volts is considered acceptable.

With the O2 sensor unplugged, and the engine idling - Test the voltage on the purple ECM wire for the O2 sensor. I can’t remember the exact value but it should be around .4 to .6 volt.

Reply back with your results.

[Rc 415]

Ok I'll try to get to that tomorrow if it doesnt rain, and if I figure out what all those things are heh

[rfmaster]

Originally Posted by ChevyTech

Check for corrosion at the ECM connectors.

Here are some tests you can do:
Test the reference voltage to the TPS, MAP or BARO sensor. This tests terminal 21 voltage. It should be 5 volts. 4 to 6 volts is considered acceptable.

With the O2 sensor unplugged, and the engine idling - Test the voltage on the purple ECM wire for the O2 sensor. I can’t remember the exact value but it should be around .4 to .6 volt.

Reply back with your results.

When measured with a DVM between engine ground TPS reference voltage (circuit 416, Gray wire) should be 4.75 to 5.25 Vdc. At Idle, TPS signal (circuit 417, Dark Blue wire) back to ECM should be 0.20 to 0.95 Vdc, but typically 0.50 to 0.60 is considered a norm.

ECM supplies 450 mV bias voltage - this may be different were you place you ground lead. This value maybe as low as 320 mV.

//RF

[ChevyTech]

Originally Posted by rfmaster

When measured with a DVM between engine ground TPS reference voltage (circuit 416, Gray wire) should be 4.75 to 5.25 Vdc. At Idle, TPS signal (circuit 417, Dark Blue wire) back to ECM should be 0.20 to 0.95 Vdc, but typically 0.50 to 0.60 is considered a norm.

ECM supplies 450 mV bias voltage - this may be different were you place you ground lead. This value maybe as low as 320 mV.

//RF

Thanks for the information.

It has been over 10 years since I have worked on a computer controlled carbureted system. It’s getting hard for me to recall this information. The manuals I have for these old systems are training manuals with very general specifications. I don’t have access to service manuals that old any longer.

[Rc 415]

Hey, I reset the computer and the code 55 went away, now I have a #13, #14, #34, and #41. I fixed numbers 13 and 14, but I dont know how to fix 34 and 41, any help?

[ChevyTech]

The 13, 14 and 34 are all low signal voltage issues. There is most likely one problem causing these codes. Test the charging system voltage and the power and grounds for the ECM. One wire with rubbed through insulation could be pulling the voltage down causing multiple codes. It is possible that the problem could be internal in the ECM.

The 41 could be the ECM, distributor module or the wiring in between the ECM and the distributor module.

Look carefully for corrosion on connections at the distributor module terminals, ECM terminals and all engine ground wires anywhere on the engine.

NOTE: ECM grounds usually terminate at the engine intake manifold area.

[Rc 415]

Well, heres the thing, my coolant temp sensor really was broken, it got cracked in have by accident, so I replaced it, and the o2 sensor was 10 yrs old, I wasn't sure if it was bad, but I replaced it as well and both codes 13 and 14 stopped appearing.

[ChevyTech]

34 - Manifold Absolute Pressure (MAP) Sensor - signal voltage is low during ignition on.

Being replacing parts has been curing problems you may want the replace the MAP sensor.

On MAP sensors, high voltage readings are obtained at low vacuum levels, and low voltage readings are obtained at high vacuum levels.

A low voltage on the MAP sensor reading would indicate to the ECM that the engine has high vacuum.

[Rc 415]

is high vacuum a bad thing?

[ChevyTech]

High vacuum is not bad, but the system is most likely sensing a reading that it determines to be in error (to high to believe for the other existing criteria), like if you had a hand vacuum pump on the MAP sensor and pump vacuum under open throttle conditions when it expects to see low vacuum.