1987 gmc jimmy full size runs great until I turn the lights on, then it chugs like it is not getting enough gas?
by 
Billyg
ANSWERS: 2
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It could be a bad alternator. Have it checked by a garage or parts store. If it isn't putting out enough energy, the headlights will rob the engine of the power it needs to produce spark, etc. That's the first thing I'd check.
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It is probably the altenator. To test it, you will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.
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