Need help here... Have two 1978 F150's.. One wit hthe 351M and the other with a 400. Have no problems with the 400 which Ipretty much drive daily.. The other (351) I use for hunting, bogging, etc.. Late last year I noticed that the lights would flicker and go off an on now and then.. I don't use this truck much, maybe 1-2 months out of the year. This year it got worse, I was told that it could be a ground wire corroded out, or bare live wire touching some where on the frame. I pretty much changed the whole harness(under the hood to the back lights).. checked all wires.. did find one wire that wasn't grounded properly and fixed it.. Thought I had the problem fixed.. Then it burnt out my headlights. first the high beams went, then the low beams. Inoticed that the lights would go bright when i accelrated and then dim out when the truck ideled down.. was told it was probably the voltage regulator. I replaced that.. didn't solve the problem... then I replaced the aternator and solenoid and all seemed well til this evening... went for a ride and again both headlights burnt in a matter of minutes.. I noticed that they were flickering real bright when the motor was revving up.
Is it possible that the real problem is the Voltage Regualator, a friend of mine told me that if I don't use the proper V.R., it will either send too much or not enough voltage through the system and the problems I have, flickering bright to low, burning headlights, etc... is because of not using the proper Voltage Regualtor.. If this is true, is there specific VR that nees to be used.
Anyone out there that may have this problem and know ... Plse help.

 

It could be a bad voltage regulator, or a bad ground (or other faulty wiring) which fools the voltage regulator into telling the alternator to put out too much voltage. Get a cheap digital multimeter and use that to check the voltage (cheaper than replacing light bulbs). You should see not more than about 14.4 volts maximum; that should at least warn you before blowing more bulbs. Plus if the voltage is high enough to blow headlights, it'll be overcharging the battery which will kill the battery.

 

Thanks... but i tried that.. borrowed a tester from a friend and was getting about 11.3.. I replaced the alternator yesterday and it went up to 12.6 -7 .... thought that would have fixed the problem, then the head lights blew out... Ken

 

Are you revving the engine while checking the voltage? If the voltage is out of control, it will only rise when the rpms are up. Make sure the "A" terminal on the regulator has a good connection to the large solenoid terminal where the battery + hooks too.

 

Dave, I picked up a digital multimeter... I took apart and looked over the wiring from the alt - VR - Solenoid, didn;t find any wear, it looked oaky. I was getting roughly around 13.5 -14.2 on the tester, but when I rev the engine up, it jumped and would go as high as 18+ and then settle back to around 14 - 15 when I let off the gas.
Any suggestions... new VR perhaps.. wiring looks okay... took it apart and didn't find any wear, etc... also removed and cleaned the main ground.. All the other lights are okay... The signals, tail, clearance and dash lights are okay, none of them blew...just the headlights....

Thanks and any help/advice is appreciated... Ken

 

Forgot to mention that while I was testing it, I removed the yellow wire,( one that runs from the VR to the Solenoid) and the reading would drop to 12.5 and with the lights on it would slowly drop(not charging?)... but when I would make contact to the Solenoid (+), it would jump to 13-14 and then slowly rise, going as high as 15-16... and hitting 18(+) when I would rev it up....Ken

 

Sounds like a bad ground to the regulator. It is seeing a different voltage than the rest of the vehicle, it notices only 8v at idle and at speed it is allowing what it sees as 14 volts which is 18 at the battery. View the voltage at the regulator ground and power output to verify it.

 

If it is the regulator ground, you can tell by taking your voltage readings with your meter negative lead on the case of the regulator. If the readings are a lot different than if you have your meter negative lead on the battery negative terminal, then it probably is a regulator ground problem.

Make sure you have small ground wires from the engine to the firewall and the radiator support. The large ground lead from the battery hooks to the engine block, but the engine and tranny are mounted in rubber mounts.