Mechanics

MISFIRES WHEN A/C COMPRESSOR ENGAGES

1993 Chrysler Le Baron • 112,000 miles

The car is a 3.0L v6. I've had it for the last 30k miles with no major problems. Within the last few weeks now running it with the A/C on causes it to misfire. I've changed plugs, wires, cap and rotor air filter, PCV valve and fuel filter. I've checked the vacumm lines and all the hoses still feel soft. I believe they were replaced by the dealer I bought the car from. I didn't see any cracks or anything loose. Car runs great with A/C off. I'm in phoenix and that is not an option this time of year. Any Ideas?
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Kpieper
July 8, 2012.




Check the charging system. In particular, have it load tested for maximum output and especially ripple. If ripple is high and all you can get is one third of its rated output, typically 90 amps, the alternator has a bad diode.

Caradiodoc
Jul 8, 2012.
Have checked fuel pressure? Check engine light on?

Docfixit
Jul 8, 2012.
No check engine light. And now it is constantly misfiring a/c or not. Checked plug wires and none are crossed. Will check alternator. Thanks

Tiny
Kpieper
Jul 8, 2012.
Constant misfire at idle and on up. Also doesn't accelerate at all. I punch it to floor and it revs to 4000 before it shifts real lathargic

Tiny
Kpieper
Jul 8, 2012.
Ok went from 13.69v at battery under idle to 11.19 with a/c on. Is it a bad diode in alternator possibly? Also saw smoke from that area after drive around block.

Tiny
Kpieper
Jul 8, 2012.
13.69 volts is just a fuzz below the minimum acceptable voltage. 11 volts is definitely too low. Now the thing to do is to see if the voltage will come back up when you turn off all the loads possible, meaning radio, heater fan, dome lights, etc. If the voltage does go up to 13.75 or higher, suspect a bad diode. It's not very practical to replace them; replace the entire alternator instead.

If it has failed completely and the voltage no longer goes up when the engine is running, suspect worn brushes in the alternator. They can often be replaced without even removing the alternator from the engine. To verify they're worn, use an ohm meter to measure the resistance between the two small terminals in the back. If you get an open circuit reading, irritate the belt a little while measuring. It's common for good brushes to not make real good contact when the rotor isn't spinning, and bumping the pulley a little will create a better contact intermittently so you can take the reading.
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Caradiodoc
Jul 8, 2012.
Ok I replaced the bosch 90 amp alternator with a new one and not any difference. Voltage stays between 13.71 unloaded a idle and 13.64 under load at the battery. Still backfires bad out the tail pipe. Any other ideas?

Tiny
Kpieper
Jul 12, 2012.
Sorry to take so long to get back to you. Just skidded in from the nation's second largest old car show swap meet.

The voltage is still too low but that may not be the reason for the running problem. My first concern is getting that voltage up to where it should be, between 13.75 and 14.75 volts. With the engine running, measure the voltage on the two small wires on the back of the alternator. One will have full battery voltage. The other one will have less but not 0 volts. Use a jumper wire to momentarily ground that wire with the lower voltage. You should see output and battery voltage rise considerably. Don't raise engine speed above idle and don't ground that wire longer than necessary to get a reading. That procedure is called "full-fielding" and is done to see what the alternator is capable of.

If there's little change in voltage when you full-field it, have it professionally load-tested to see how much current it can produce. You could still have a defective alternator with one bad diode.

Caradiodoc
Jul 16, 2012.

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