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Why Does Headlight On 1990 V1500 350 V8 Dims When Stepping On Accelerator?

Blue85 said:
Wait, the problem is only when accelerating? Or the voltage always dips when headlights are on? Lights are bright at idle, then you speed up the truck and the lights get dimmer?
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Voltage dips on meter a bit when lights are on and more when accelerating along with the gauage cluster lights flickiring going "in and out"
 
Blue85 said:
Wait, the problem is only when accelerating? Or the voltage always dips when headlights are on? Lights are bright at idle, then you speed up the truck and the lights get dimmer?
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Also like you said when at idle the lights along with headlights dont dim but when accelerating/speading up they dim.
 
illmanners said:
As Justin V said earlier, "All of they negative battery cables, backside of the passenger side cylinder head should be a ground strap to the body, on the headlight wiring there should be some ground wires attached to the body, look for corrosion and rust." Take a quick look at both the grounding straps on the head to block location, (could actually be on the either side, or both sides,) and pay special attention to the ground to the headlight wiring. Probably wouldn't hurt to run a new one in that location. Just make sure you are grounding to and from a good, clean, unpainted metal source. Typically, when I do an engine rebuild or install, I install all new ground straps and always run one or two extra ones. Bear in mind that the more options and electrical equipment a vehicle has on it, the more important ground is and sinec in most cases, external grounding is a bolt to strap situation, they are always subject to vibration; then can vibrate loose. Hope that helps..
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I have a similar problem but mine is a 1988 Chevrolet k5 blazer it had a rod knock so I pulled the motor and found out it was a 307 out of a 69 Camaro bored .30 over so I sold that and got a 350 out of an 86 Chevy truck, cleaned it up put it in everything is good and working good except now my headlights/daylights and interior lights don't work they did before the swap. I replaced the switch and the 2-30amp circuit breakers and cleaned up all the grounds I could find and nothing. My turn signals and hazards work fine but no dash no dome no headlights/daylights. Any help would be greatly appreciated also i only have 1 ground strap to the firewall from the passenger head and then next to that is 2 more grounds that come out of the harness
 
My stereo amps dim the headlights on each bass beat in my suburban and Blazer. Installed yellow top deep cycle ones and no more dimming. Guess they can handle those instant load demands better.

Now, long life on these.....another subject, entirely!!
 
It could all be explained by a goofy regulator, but before that assumption, make sure all the ground connections are good and the other connections to the alternator are good.
 
I've seen a few serpentine belts that slide over a seized idler pulley and not make a peep,one on a Lincoln had the idler pulley seized up so long the belt wore off the metal grooves on one side of the pulley--guy drove it a few thousand miles before his mechanic,a good friend of mine, noticed the pulley wasn't moving one day after he did an oil change!.

He said "hey--look at this",and started the engine,and I was surprised the belt could skid over a seized up pulley so long,without burning up--but it had polished it so smooth it glided over it with little resistance..

The customer said his battery "died" one night out of town,so he put a new one in a few weeks before,and said he felt it wasn't recharging properly--it was possible at highway speeds the belt slipped enough to not turn the alternator fast enough to keep up with the A/C and other accessories..

If you have no power getting to some items at the fuse box though,I'd say that is probably the source of your troubles..
I have seen fuse boxes corrode internally where the two halves bolt together at the firewall,or get water intrusion on under the hood mounted ones..
Your truck should have at least two fusible links--one at the red wire on the starter solenoid,that goes under the positive battery cable,some trucks have two red wires and fuse links there--also another one near the power brake booster at the firewall,and some had one in the thick red wire coming off the rear of the alternator..if any of those fail,it can cause these troubles..

Another long shot possibility is the alternator has some rotor winding that broke and they "fly out" at higher rpms and touch the case or stator and short out..but I would think it would have failed completely by now if that were the case..
 
diesel4me said:
I've seen a few serpentine belts that slide over a seized idler pulley and not make a peep,one on a Lincoln had the idler pulley seized up so long the belt wore off the metal grooves on one side of the pulley--guy drove it a few thousand miles before his mechanic,a good friend of mine, noticed the pulley wasn't moving one day after he did an oil change!.

He said "hey--look at this",and started the engine,and I was surprised the belt could skid over a seized up pulley so long,without burning up--but it had polished it so smooth it glided over it with little resistance..

The customer said his battery "died" one night out of town,so he put a new one in a few weeks before,and said he felt it wasn't recharging properly--it was possible at highway speeds the belt slipped enough to not turn the alternator fast enough to keep up with the A/C and other accessories..

If you have no power getting to some items at the fuse box though,I'd say that is probably the source of your troubles..
I have seen fuse boxes corrode internally where the two halves bolt together at the firewall,or get water intrusion on under the hood mounted ones..
Your truck should have at least two fusible links--one at the red wire on the starter solenoid,that goes under the positive battery cable,some trucks have two red wires and fuse links there--also another one near the power brake booster at the firewall,and some had one in the thick red wire coming off the rear of the alternator..if any of those fail,it can cause these troubles..

Another long shot possibility is the alternator has some rotor winding that broke and they "fly out" at higher rpms and touch the case or stator and short out..but I would think it would have failed completely by now if that were the case..
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Ya my alternator is crap it was spray painted yellow(PO) and it squeals at certain rpms now unless i let off the throttle then get back on. But the truck has been running and starting fine for 3 weeks now. I want to replace the alternator I just hope it fixes the headlights. At this point Im half tempted to get a new wiring harness and rewire the whole truck so I know everything is good and solid and if I ever have a problem in the future I will know where to look.
 
It sounds like the squealing is due to an extreme load being put on the alternator ,and the belt is slipping--you may have a bad direct short circuit in one on the main charging harness wires,one might be chafed against something and grounding out when the engine moves slightly when you get on the throttle..which dims the headlamps..

It could also be your alternator isn't getting the proper voltage signals to "sense" how much amperage to send to the battery,and it assumes the battery is low on charge and maxes out the output..--if some of your wires are "dead" at the fuse box this is a possibility ..
 
diesel4me said:
It sounds like the squealing is due to an extreme load being put on the alternator ,and the belt is slipping--you may have a bad direct short circuit in one on the main charging harness wires,one might be chafed against something and grounding out when the engine moves slightly when you get on the throttle..which dims the headlamps..

It could also be your alternator isn't getting the proper voltage signals to "sense" how much amperage to send to the battery,and it assumes the battery is low on charge and maxes out the output..--if some of your wires are "dead" at the fuse box this is a possibility ..
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Ok I have a fusible link that goes to the starter for the alternator and the headlights are completely off don't work not dim the alternator didn't sqeel before the swap but that was a 307 sbc and the headlights worked before the swap
 
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