electrical/engine (?) question

[rubba19]

I have a 93 4X2 with 145,000 miles on it.

Anyway, when I am sitting at a stoplight with the AC/heater on ,the car starts to idle wildly and I notice that the electrical guage starts to fall to about the middle of the dial, the car revs and the guage returns to normal. When the AC/heater is off, the car idles without a problem?

Anyone seen this before? Or is this a case of need of a tune-up?

Thanks,
Scott

 

[jgilbs]

does it happen with HEAT as well as AC? The AC system will increase your idle when it is operating to compensate for the extra load. If you have an overloaded elecrical system, this could cause the gauge jump as you describe, since higher idle will increase the current output by the alternator. does your ammeter jump around during normal driving? Mine did, and I replaced/cleaned/tightened my battery cables and the problem went away for me. you might want to check the condition of your battery cables and terminals, but other than that the high idle is normal with AC usage.

 

[rubba19]

It does happen with the defroster (haven't needed heater yet), but I will check the cables.

thanks.

 

[jgilbs]

if that fails, check all your vacuum lines. an open vacuum line can cause a weird idle as well.