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2003 Ford Taurus speedometer fluctuates on idling, car sometimes stalls out

my 2003 ford taurus SES radomly has problems with the speedometer fluctuating from 0 to 10 mph upon idling. Sometimes the car will eventually stall out but always restarts without a problem. You can feel it “jerk” when changing gears or what I describe as a “hesitation” once in awhile. The car is an automatic and we have already replaced the relay and the voltage regulator/alternator - that has not helped. I can’t afford to keep throwing money into it replacing parts that don’t fix the problem. Any ideas on what could be wrong?

I have the exact same problem in my 2003, but solved the stall issue with a new ignition module. What gives with the speedometer? The odometer also clicks tenths off while waiting at the stoplight! Also noticed a high pitch sound when the problem occurs. Help!

Is the Check Engine light on?



No, the check engine light is not on, and when I plug in the OBDII reader, there are no faults. I did notice the lights flicker as the speedo needle bounces while the engine is at idle. As a test, I pulled into a parking space this morning and the needle fluctuated wildly and I heard a whine like the alternator was at max load. When I restarted the car a moment later, all was normal, like restarting reset the voltage regulator. I checked the temperature of the wires at the battery and alternator, they were not even warm so there doesn’t seem to be an overcharge condition. I’m suspecting a bad diode in the alternator…your input solicited!


Have the alternator tested. The regulator is contained within the alternator, so they’ll be tested together.

Yes, a blown diode is a good candidate.

It was in-fact a blown diode. The new alternator fixed the problem!