Headlights flicker and radio cuts out when going over bumps
 

Hi all, I wanted to stop by and ask about an electrical gremlin I'm having in my truck. It's an 87 model with the Tbi 350 and 700r4 transmission, 2WD. Every time I go over a bump, the headlights seem to flicker a bit, and the radio cuts out.

It's a modern radio so when it loses power it restarts, which takes a few seconds. And that should help with finding the problem. Cause you'd just wiggle stuff until you hear the radio cut out.

But despite all the wiggling and jiggling of wires under the hood, I can't make the radio cut out while the truck is in my garage. For some reason, it wants to go over a bump in the road before it will reproduce the problem. So I must be doing it wrong.

Is there someplace else I should check?

I've recently checked all the grounds because I had to replace one to get the starter to work. So I know the main ground to the frame is good, and so is the one that is on top of the engine. If anyone knows of other more obscure ground wire connections, I'll check those too. Thank you

Re: Headlights flicker and radio cuts out when going over bumps
 

Re: Headlights flicker and radio cuts out when going over bumps
 

Re: Headlights flicker and radio cuts out when going over bumps
 

But why would the radio ground make the headlights flicker? I could understand if only the radio cut out when going over a bump. But the headlights flicker at the same time.

Re: Headlights flicker and radio cuts out when going over bumps
 

It sounds like one of the grounds on the core support

Re: Headlights flicker and radio cuts out when going over bumps
 

Also check for corrosion where the harness connects to the fuse block on the firewall.

Re: Headlights flicker and radio cuts out when going over bumps
 

Pull and reset the fuses for both radio and headlights. Simple to do and there might be bump sensitive corrosion on the press fit connectors.

Possible the headlight wires have a damaged spot in the wiring harness caused by years of rubbing on something sharp during bumps. Hit a bump, brief short, quick voltage drop, radio resets.

Might not even be the headlight wires. A similar type short in the rear lighting wires might cause it. Any trailer hitch light harnesses? Those often involve cheap crimp connectors that corrode over time. Or the tape comes loose.

Re: Headlights flicker and radio cuts out when going over bumps
 

Thanks to all, I will check these suggestions tonight.

I did recently replace the starter. Is there any possibility that the starter wiring could have an effect on the radio and headlights?

Re: Headlights flicker and radio cuts out when going over bumps
 

Yes on starter wiring. The main supply for the junction box often comes off the starter. If the nut has come loose, could cause your 'bump' problem.

Re: Headlights flicker and radio cuts out when going over bumps
 

Re: Headlights flicker and radio cuts out when going over bumps
 

Quote:

Yes on starter wiring. The main supply for the junction box often comes off the starter. If the nut has come loose, could cause your 'bump' problem.

I would bet you starer would be running hotter then or would be cutting out not starting now and then. If the starter working easy seeing sparks when starting or it not heating up starter or wires hot to touch start up for 10-15sec. It most likely ok. After turning off a extra hands eyes will help there. At this point you check clean nut and the wire rings.

Re: Headlights flicker and radio cuts out when going over bumps
 

theres two fusable links going up from the starter wire. they are in white or black plastic usually. I've had one of those start losing connection and caused my headlights to be intermittent.

Re: Headlights flicker and radio cuts out when going over bumps
 

Yes I only just now learned not to use harbor freight fuses. You need to use the name brands like Littlefuse or Bussmann. So I'll be replacing those first. Trailer plug wiring all looks fine. All the headlight wiring is new because I did the relay mod and went with LED headlights. And for a long time it all worked fine. These flickering problems started after I replaced my starter.

So I'm going to redo the starter wiring because it looks pretty bad. There's some holes in the insulation, and the factory fusible links (those in-line plastic cylinders) look like they're ready to fall off.

So does anyone know why the starter wires are 12 gauge, but the fusible links they connect to are only 14 gauge? I'm kind of an electrical newb but that doesn't seem to make sense. Seems like your fusible links would also need to be 12 gauge. Here's some:

https://www.ebay.com/itm/313506742421

Re: Headlights flicker and radio cuts out when going over bumps
 

GM fusible links are normally 4 sizes smaller(larger number) then the wire they protect. 12ga wire = 16ga fusible link. The newer OEM links will have the size number stamped into the plastic cylinder. The smaller link wire size lets the link get hot and melt before the main wire. They are a fuse of last resort if something goes wrong with the electrical system.

I have found several good OEM links on later model trucks in the salvage yards. While the wires are routed a bit different, the lengths are pretty a pretty good fit for the earlier trucks.

Re: Headlights flicker and radio cuts out when going over bumps
 

Is your cab sagging heavily in the front?

My 91 s10 is rusted bad to the point the cab actually was pressing the harness against the back of the block.

So I would hit a bump and it would cut the lights, radio etc. Not really a huge deal as they came right back on and it would only do it every now and then. Eventually it cut the truck off then it would go right back to normal a second later with only a "check engine" light illuminating for a second. It would only happen once every 20 days or so though and I figured it was a ground somewhere.

I replaced the main bat to engine and ran a new engine to chassis ground and called it good. I kept driving it for short runs etc. That little 2.5 would just go and the thing handled great. Did nothing for like 60 days so I thought I fixed it. I hit a bump and the thing straight up shut off. It would crank but that's it.

Towed it back to the house. Upon further inspection I fried the harness back by the head as well as fuse able links off the alternator and pretty sure the ecm or at very least distributor pickup is toast. The truck sits now waiting to be turned into something cool later.

So yea if your cab is sagging forward with like 1/2" fender gap at the bottom of the doors maybe you should check the back of the block to see if your harness is being crushed. I went through everything on that truck mechanical wise. But the body was not worth saving and the electrical worked so I left those alone. It will make a great chassis for a hotrod at the very least. But simply running my hand behind the head to do a basic inspection may have had that truck running today. Live and learn.

Re: Headlights flicker and radio cuts out when going over bumps
 

Naw not in this case, rust isn't the issue. Texas truck so very little rust, if any.

In TX our interiors get consumed by the sun, but we don't have rust.