On a 75 chevy 3/4 ton ,350, auto,2bbl,2wd I have headlight problems.I have had the alternator checked by AutoZone and it checked out ok especially the diode check.While I am driving with the headlights on they are dimmer than normal,as I accelerate the rpm's the lights become brighter and keep pulsating from the dimmer than normal to the brighter seemingly normal. The battery terminals are cleaned and tight. All the ground connections have been cleaned to the bare metal and are tight.The battery shows 12.4 to 12.6 consistently.and the output at the alternator is around 14 to 14.2 volts. Visibly I do not see any frayed wires.I do not know what else to do and welcome any ideas that may solve my problem.I will definitely appreciate your help. THANKS
Good truck, You might try a few things to try to pin down the problem. Recheck the small ground strap from the back of the engine block to the firewall, make certain it is sound and that the bolts are not corroded. That grounds your driveline and alternator to the body.
Your battery may have a bad cell, low voltage, make sure the battery is @
12.6 to 12.8 volts after resting overnight. A battery with a bad cell will be about 2.2 volts lower, and depending on the problem with the cell, may start the truck just fine. A battery putting out as little as 9 volts may start the truck as long as it still has sufficient current (Amperage) capacity. The lower voltage will however be very apparent in headlight illumination, if in doubt, test with another battery.
I assume that you have kept the truck in good condition and the wiring harness is sound. and that you are using stock headlights, not high outputs that require a relay to shunt the headlight current directly to the lights instead of into the cab, through the light switch and dimmer switch, the wiring is just not heavy enough for the extra current load, a $ 3.00 relay, and a fused shunt wire to the main BATT terminal on the firewall may just solve your problem, but lets go on.
With the headlight switch OFF, I would run a jumper wire from the battery to the headlight wiring harness connector block at the radiator surround. Is it any brighter? then run the jumper from the headlight ground to the battery ground. Is it any brighter.
Headlight switches and dimmer switches do wear out and can become the current resistive load you are looking for, a partially crushed headlight feed wire or a corroded terminal can be that load as well. An overheated or "cooked" wiring loom terminal block can break down and become a partial conductor, have a good look at the ones on the firewall and the fuseblock.
An automotive lazer non contact thermometer can be very useful to quickly find wiring hot spots in: wiring harnesses, switches, wheel bearings, universal joints, brakes, it will even identify a bad battery cell (hot) under charging conditions, scanning the exhaust manifold can sometimes find that transient high speed engine miss, if a catalytic converter is working the outlet will be much hotter than the intake.
I have experienced exactly the same thing.Except that mine is a GMC and has a 4 bbl.I also had mine checked at AutoZone for diode malfunction and it passed on the 3 tests they put it through.It is a coincidence that we both went through precisely the same steps.I argued with not only AutoZone but also Oreilly that the alternator was bad.I bought a remanufactured one and kept my core.Everything is working beautifully.I furthermore took it across town to a place that repairs and remanufactures alternators.I asked the guy to test it for me(I did not mention how the lights were pulsating or anything else)When the guy came back from the testing area he asked me if my lights were flickering. He even explained to me that the AutoZone tests are done at low RPM around
Check your fan belt for wear, mine was riding to far down in the alternater pulley..It wasnot thick enought. It was tight! During the day ,it was charging ok,when at night with lights on, blower heater, air cond,lights would go dim! I lifted the hood at night and shined a flashlight on pulley and it was actually slowing down, from the load! Replaced the belt, belt was then riding high in the pulley groove, and turning the alternater at the speed it should be going! Problem solved. This was a 1977 Chey Impala, back in the days of single belts. My 2 cents worth! Andy
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