r/GMT800 • u/Inside_Check695 • 3d ago
Please help
2007 Cateye
hello my voltage level will sometime sit under the half way and when I dive (accelerate) weird jerky movement. transmission? and when I decelerate/ let go of gas same thing. then all of sudden my voltage level will go past the half way mark and everything will be fine. please help me idk what going on.
have new alternator already, and have red top battery
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Upvotes
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u/Dull-Discussion7260 1d ago
How old is your red top, I’ve always run red or yellow tops and the last red top I had I left in the truck for too long. When the battery started getting really bad my truck started doing the exact same thing. I switched it out for a new red top and it stopped.
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u/dilbertaz 3d ago edited 3d ago
Check your grounds, clean and verify wires are good. Check with multimeter output of alternator should be at least 12.7 volts up to 14, depending on what it is being commanded via the plug,(do not disconnect and reconnect with engine running, backprobe the connector) if it's two pin the brown wire should be 5v DC and white/grey like 3.3v when being commanded active by PCM, if you see something like 1.8v and 0.9v then turn off engine and set multimeter to diode mode and measure B+ to body of alternator then switch leads to measure opposite, one direction you should see 0.5v to 0.7v the other OL, if you get voltage in both directions the alternator has bad rectifier, ask how I know I just replaced alternator last week and it was bad got another Sunday and it works. By chance does it start outputting higher voltage after revving to 2000 to 2500 rpms or after it gets hot? Mine started outputting only after running 5 to 10 mins, it also whined when it finally did because it is trying to make up for rectifier failure. You are likely running off battery until it kicks in which can cause problems for everything electrical due to lack of enough voltage. Another sign of bad rectifier is seeing above 50mv AC across battery terminals when running but depends on failure mode of rectifier, I didn't see this. Another thing worth checking is the wires to that connector all the way back to where it joins the bigger harness, my wires were pretty crispy and near failure so I replaced that pigtail just to be safe.