I have started to notice that at idle my truck (1996 2500 with 220k) bounces between 700 and 500 RPM. Same things happen at night when the lights are on, as the RPM bounces the lights either get brighter or dimmer. Anyone else experience this problem or am I just imagining it????

 

Just updated my sig

 

probally your grid heaters coming on. if thats it it should quit when its warmed up.

 

It sounds like your idle is too low. It should be about 850 in drive with the AC on and the engine warm. The alternator cannot put out much current at really low rpm which could explain the dimming lights. I would start by adjusting the idle up and seeing if the problem goes away. Once that is out of the way, if it is still bouncing, you can look for other problems.

 

Check the 90* plugs that go into your grid heater relays - I'll guarantee one is loose.

This same thing happened to me and drove me crazy for at least a year.

Search through my old posts...