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
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
700 to 1000.The normal operating engine runs higher than 1200 rpm.
The tests at Oreilly also did the same thing.
I learn every day and I learned about cheap and quickie test equipment.IT IS
YOUR ALTERNATOR FOR SURE!!! Good luck.
Check your fan belt for wear, mine was riding to far down in the alternater
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!
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.