Contour Enthusiasts Group Archives Forums General Troubleshooting Irregular idle

This might be hard to diagnose. I've got a 98 V6 Contour, ~155,000 miles. I've noticed recently while idling it sort of surges up and down, goes +/- 100 RPM constantly. The dash lights also get brighter and dimmer with the RPM.

It starts and runs fine, but I did notice that when I accelerate quickly the battery light will come on briefly.

I don't think my battery would be at fault, I just got it last year IIRC. My guesses are alternator, voltage regulator, or maybe serpentine belt? But would that stuff cause the irregular idle?

Guess I didn't search well enough, but this might be the answer to my problem:

http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1043318&page=&view=&sb=5&o=&fpart=2&vc=1

Although I have not totally straightened out my idling issue in my 96 Zetec Contour, part of my idle surges and noticable power surges I was noticing was due to my weak air conditioning compressor kicking on and off. The surges were most noticable when the selector was in the def or ac positions. Much smoother in heat or panel positions.

My idle is still erratic, but the AC makes it much worse.

Besides electrical issues, a dirty MAF can cause this kind of marginal idling.

Well, I think it's electrical. Last night coming home the RPM gauge started jumping up and down, and the dash lights and headlights got dim. Thought I was gonna get stranded but it made it another 20 miles or so home. It got back to normal for a while but when I was almost home everything got dim again.

Tested my battery voltage today, 12.6. Tested the voltage with the car running, 9.6. That can't be good.

I went ahead and pulled the terminals off the battery and cleaned everything, they were a little dirty but not terrible. I also removed the battery and checked the ground under there, it looked fine. Put everything back together shiny and clean, but still 9.6 volts when the car is running.

IIRC I read somewhere that a failing alternator would cause the running voltage to read high? What would a low running voltage indicate? Something pulling voltage somewhere? Is the alternator re-wire something good to try, or should I just replace it?

Voltage running high with engine running can happen if the voltage regulator looses it's ability ro regulate. That can be a regulator failure or sometimes a bad connection in the wiring to the alt/regulator can cause it to charge too high.

If you're showing <10 volts while running, the alternator is not charging at all. Again, bad wiring could cause such. I believe there is a fusable link in the output wire from the alternator. I had a Subaru that the output wire broke off the back of the alternator. I';d start out checking all connections and fuseable links. A schematic and voltmeter would be very handy, provided one is familiar with both.

I have found it rather common for the brushes to wear out in older alternators and that would certainly stop any charging from happening.

"IIRC I read somewhere that a failing alternator would cause the running voltage to read high?"
Failing voltage regulator.

"What would a low running voltage indicate?"
Wiring or alternator. More probable alternator.

Bought a new Motorcraft alternator online last week and got it installed at a local shop (not a dealer), $123 labor. They said the alternator was failing the test before replacement and passed afterwards. Seems to run great now, glad to have her back after 9 days or so! Very handy to have a spare vehicle for times like this.