P1637 Generator L-Terminal Circuit - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

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Old 01-03-2018, 04:44 PM
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Turborat Turborat is offline
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2004 GMC Envoy SLT XUV
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P1637 Generator L-Terminal Circuit

Hi all, Iím new here. So Iíll share a recent problem that was finally resolved. But, need to resolve the root cause.

2004 Envoy XUV 5.3L 2WD

Started getting battery light and text message ďBattery Not ChargingĒ. It was an intermittent problem. When functioning properly, generator would run car sans battery. When not functioning, battery would drain rapidly. Scan tool revealed code P1637, but no ďcheck EngineĒ.

I threw an Autozone generator on and got same condition. Removed and replaced with original and started into more diagnosis.

While checking the continuity of wires, I found what appeared to be motor oil in C2 of the PCM. After some consternation, I orderded a refurbished PCM. This replacement has solved the intermittent charging problem. Iím assuming the oil has something to do with the PCM failure.

So on to the oil issue...... My immediate thought was that itís impossible to get oil into that connector unless itís put there on purpose. An internet search did not turn up a specific GM issue, but Jeep owners and some Asian brand owners were reporting oil in the PCM. It seems that oil leaking from sensor on the engine can make it into the wire harness and eventually wick into the PCM connectors. I saw a YouTube where the oil made its way to OBDII link under the dash.

The engine does not leave a signs of leaking oil anywhere. I did notice that the oil pressure gauge is not working correctly. It reads 80PSI and never changes. The sender does not appear to be leaking, but.......... I read that itís a pain to replace, but maybe thatís the source of the oil in the PCM harness.

For background in diagnosis, I monitored the terminal b of the generator connector for 10+ volts while engine running. That voltage comes from the PCM and supplies the current for the field which enables charging. If voltage is present and no charging, the generator is bad. If voltage is missing, the wiring from PCM or PCM itself is bad. Note that itís best to monitor the voltage white itís hooked to the generator. In my case and during failure mode, I would see 10v on the wire when I unplugged the connector and this fooled me. After replacing the pigtail, I left the splice exposed so that I could monitor the voltage while connected the generator. When in failure mode, the voltage would dissapear.
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Old 01-03-2018, 07:10 PM
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It is never a good idea to run the alternator without having the battery connected. This will cause the alternator to develop a higher output voltage and could damage other electrical components.
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