Electrical problem going to alternator

JONATHAN KETCHUM

MEMBER
2000 CHEVROLET SUBURBAN
5.4L
V8
2WD
AUTOMATIC
170,000 MILES

Battery light comes on, and it will drain the battery until dead. It has a new battery, new alternator and new PCM. It is sporadic. Sometimes when tested, the charging system test good, sometimes not. It was hard to track down because this, but I finally figured out that the wire that energizes the alternator is going in and out. It test good sometimes, has no voltage sometimes and is low on voltage sometimes. I checked that wire for continuity and it’s good as far as from the PCM to the alternator. Not sure what to do next?

Saturday, October 6th, 2018 AT 1:13 PM

3 Replies

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi and thanks for using 2CarPros. Com.

From experience, I have found that existing continuity can provide a false positive. Basically, when checking for continuity, the power that goes through the wire being tested is so minimal, it checks good. However, when a load is placed on the wire, it fails.

I have a suggestion. Purchase a generic light bulb socket at the parts store, something like a back up light socket, and make sure to get a good bulb. Disconnect the wire in question and basically, using the car's battery, make a circuit which requires power to go through the wire in question and the newly purchased socket. By doing this, you are placing a load on the wire. If the bulb fails to light or is extremely dim, replace the wire.

Try this and let me know what you find.

Take care,
Joe

Was this

answer

helpful?

Yes

No

Monday, April 19th, 2021 AT 10:45 AM

JONATHAN KETCHUM

MEMBER
2 POSTS

I did as you suggested and the wire in question did light up the bulb with no problem. What should I check next?

Was this

answer

helpful?

Yes

No

Monday, April 19th, 2021 AT 10:45 AM

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Disconnect the connector at the PCM and confirm it is not corroded, damaged, or the pin itself is not moved in the harness or PCM. Basically, if the wire between the PCM and alt is good, I have to question the PCM itself. Make sure the engine ground/alternator ground is clean and tight.

Let me know what you find.

Joe

Image (Click to make bigger)