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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / September 2007Lights dim at idol speed.
At idol speed my dash lights and headlights dim. (1987 Towncar, 5L engine) When motor speeds up they get bright again. Thought it was the alternator so got a rebuilt one from NAPA. 3 weeks later the problem still exists. Now the alternator does not charge at any speed. Car has new battery cables and coil. I have noticed that the electric door lock on rear door clicks buy itself occasionally. Why would lights dim significantly at idol speed? If there is some current drain or short wouldn't it affect the current at any speed? If there is some current drain, would it cause the alternator to not charge at all? Will a new alternator be more reliable than a rebuilt one? (The NAPA store said they have as many new alternators returned as rebuilt ones) > At idol speed my dash lights and headlights dim. (1987 Towncar, 5L > engine) When motor speeds up they get bright again. Thought it was the [quoted text clipped - 15 lines] > Will a new alternator be more reliable than a rebuilt one? (The NAPA > store said they have as many new alternators returned as rebuilt ones) It's a Ford, I have the same issue with my F-150, I have been told that there's nothing I can do about it. I would far rather have bought a Chevy or Dodge, but this seems to be Ford country, no used and cheap non-Ford pickups to be had. Of course, now that I *bought* the F-150 I have seen both a late 60's Jimmy for sale locally and also a nice Dodge stepside WITH A STICKSHIFT NO LESS that has been at the corner garage for the last two weeks. Grr. nate > > At idol speed my dash lights and headlights dim. (1987 Towncar, 5L > > engine) When motor speeds up they get bright again. Thought it was the [quoted text clipped - 28 lines] > > nate It would seem the problem might be able to be resolved by installing a smaller diameter pulley on the alternator? >At idol speed my dash lights and headlights dim. (1987 Towncar, 5L >engine) When motor speeds up they get bright again. Thought it was the >alternator so got a rebuilt one from NAPA. 3 weeks later the problem >still exists. Now the alternator does not charge at any speed. Car has >new battery cables and coil. Did you do any voltage and current measurement? In this car, is the voltage regulator built into the alternator or is it external? Are all connections to the alternator, battery, and fuse panel all nice and bright? >I have noticed that the electric door lock on rear door clicks buy >itself occasionally. This is probably unrelated. Take the door locking system fuse out anyway, though, just to make sure. >Why would lights dim significantly at idol speed? Because the voltage regulator isn't regulating properly, or because you have too much load to allow it to do so. Measuring the battery draw at idle and running and the voltages at idle and running will tell you a lot. >If there is some current drain or short wouldn't it affect the current >at any speed? Probably, but remember the alternator can produce a lot more current at high RPM. >If there is some current drain, would it cause the alternator to not >charge at all? Possible, but very unlikely. >Will a new alternator be more reliable than a rebuilt one? (The NAPA >store said they have as many new alternators returned as rebuilt ones) Depends where you buy it from and how good their rebuilders are. You may well have a bum replacement part. Voltage and current readings will tell you for sure. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." > At idol speed my dash lights and headlights dim. (1987 Towncar, 5L > engine) When motor speeds up they get bright again. Thought it was the [quoted text clipped - 15 lines] > Will a new alternator be more reliable than a rebuilt one? (The NAPA > store said they have as many new alternators returned as rebuilt ones) One thought came to mind, but first a question or two. You said you have new battery cables and a new coil. What is the condition of the battery? Has this car always displayed this problem, or did it start happening sometime recently? I'm thinking that a worn battery can be drawing more current from the system as it is trying to charge. This extra current draw would be more noticeable at idle that at speed. Of course, this is just a guess. > At idol speed my dash lights and headlights dim. (1987 Towncar, 5L > engine) When motor speeds up they get bright again. Thought it was the [quoted text clipped - 15 lines] > Will a new alternator be more reliable than a rebuilt one? (The NAPA > store said they have as many new alternators returned as rebuilt ones) Is this a recent change or have the lights always dimmed somewhat at an idle. I can notice some dimming at an idle on my 2 Volvo cars and i've seen that happen on others. It's more noticable when there are a lot of components drawing power. > At idol speed my dash lights and headlights dim. (1987 Towncar, 5L > engine) When motor speeds up they get bright again. Thought it was the [quoted text clipped - 15 lines] > Will a new alternator be more reliable than a rebuilt one? (The NAPA > store said they have as many new alternators returned as rebuilt ones) Looks as if you traded a good one for a defective one. If NAPA dealer is a lo-volume store, it may still have your old "core". If so, try to trade back & refund your $. Then have it checked by an alter. rebuilder before re-installing it--chances are good it's ok. Even my wife's '05 Chrysler's lights dim at idle, as well as my '03 GMC, my '02 Camaro with 12000 miles, and my '87 S10 Blazer. BTW: If your lites are actually dimming TOO MUCH, that rear door lock could likely be drawing the extra power which only shows up at idle. It needs to be disconnected regardless, as IIRC, that car's door locks work (and could drain the battery) even with ignition cut off. Luck to you. s some years ago i had an olds toronado where the lights dimmed when I was stopped at a red light with the head lights, heater/defroster, and rear defroster all on. I asked a local alternator rebuild shop to rebuild the alternator so it would put out more amps. So I think they boosted it from about 70 amps to 105. that solved my problem. So if it is just normal accessories that are causing your problem this could be the solution. If there is short circuit draining the battery then you will have to get that fixed. The voltage at the battery should be about 13.2 when the car is shut off. When the car is running it should be higher than that, maybe around 14 or 15 volts. |
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