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No brake lights with headlights on, 1991 Trooper

1401 Views 19 Replies 6 Participants Last post by  paradise445
When I press my brake pedal with my headlights off, the brake lights come on normally. When I turn on my headlights, the tail lights come on but the brake lights are extremely faint. The car came with incandescent dual filament bulbs originally. Size 1194 and 1157. Both were tested, both work, the other filament lights up, it's just not bright at all. I'd be worried about getting a ticket or rear ended with how little they light up when the pedal is depressed. I even replaced them with LED bulbs and there is still basically no change.

Am I missing something super obvious or should I be questioning my intelligence? I guess I should say I know next to nothing about electrical systems... Might as well be magic
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Use a multimeter to test how much voltage you are getting at the back in the brake light portion of the socket with the headlights on and the brake pedal depressed. Also, does your Trooper have a rear trailer hitch and the 4-way flat plug for trailer lights? With the info you gave it sounds like a grounding issue to me.
Are the bulbs wired correctly? Sounds like the brighten filament is working with the running lights instead of the brakes.
I second checking the voltage there. I would say there's more resistance in the circuit other than just the light bulb. Either wired wrong or a high resistance (corrosion) some where cutting way down on the available voltage for the filament.
Alright, headlights off, voltage was 12.6v. Headlights on, voltage came out to 12.28v. It does have the hitch and the flat plug for the trailer lights. I have seen a lot of wiring that looks like it doesn't belong. It has a couple of ring terminals that look like they were crimped with pliers and some grounds held in place with self tapping screws. They're pretty rusted of course. I'm not sure where the actual grounding points for the brake lights are though. I tried looking at a wiring diagram but it was for a 95. That shows it in the luggage area but not the case for my Trooper. Maybe there is something causing it to run "backwards" so to speak like mud had said?
Can you post a picture of the light comparison-- brake vs. rear running lights? I think mudoilngears is onto something with the wiring being bass-ackwards. You can stick a test lead into one of the wires itself to make sure the brake light wire is getting 12v when the brake is depressed. The harness is accessible under the truck between the fuel tank and the rear frame member. I think brake is green/red and running is green/brown, but I can look at the FSM to confirm. You can also remove the taillight housing entirely but I can't recall how to do this.
Depending on how the bulb gets it's ground I suppose it's possible to have 12 volts to the lamp base but a high resistance ground would cause the bulb to be dim, meaning your meter is not using the same ground as the bulb is.
A dual filament bulb (1157) that lights either filament brightly, but dim when both are lit is probably a bad ground (two different power sources.
Other possibilities: the contacts in the bulb sockets, wrong bulb sockets ( I saw an 1156 - single pole contact put into an 1157 socket. The result was that the single contact shorted between the brake and tail filaments and backfed each circuit causing parking lights to come on when brake depressed and brake lights on when parking lights on)
My guess would be the ground. Dennis
Okay, was able to get under there and take a picture of the difference in lighting when the pedal was depressed vs not. Managed to really take a look at the extra bits added in by whoever put in this towing stuff. The wiring is "spliced" with electrical tape and wrapped around bits of the stock harness.

I'll have more time to get under there tomorrow and take the housing out. Seems like it's just 3 bolts with nuts behind those interior panels. The constant rain and a few errands kept me from getting anything done. Hoping it holds off tomorrow since I have all day to myself. Also hoping it's something as simple as taking out a few of those hack job splices and swapping some wiring back around to its original place.
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For simplicity, I would proceed by removing all the additional crap. I'd take out the 5-way to 4-way box and the wires going to it. I'd remove all the little plastic splice guys and cover the wires good in electrical tape where they were. Make sure that the wires going through the splice things are indeed the same color on each end. That way you just eliminate the hackjob someone else has done.
Also make sure the factory wires aren't corroded or broken where the quick connects were. I've seen corrosion set in at those connectors and when you remove it, the host wire is all but in two.

Got some of that hack job wiring out. Removed pretty much every bit of wiring that went with the trailer package and all those wonderful splices. All that's left is unconnected bits that shouldn't be affecting anything anymore. The wiring on the housing seems fine. Everything looks to be untouched. The ground behind the fuel tank had continuity through it from the housing plug. Low resistance. I'm totally stumped here and unsure what to do. The brake lights are so dim while the running lights are so bright. Would it be possible to just swap the wires around on the plug? I can solder just fine if need be..

All wires are connected from splice to plug. There's no corrosion that's splitting them apart. Even got under the dash to see if anything under there was messed with. Only found that nothing had been messed with there and plenty of dirt falling out of the HVAC plumbing through that little vent down there. Here's some pictures of everything though.

I did find these wires by the transmission and t-case. Bullet connector and a broken wire with solder... Doubt these are connected to the brake light issue though. Figured I'd just include them just in case.

Not trying to drive anyone crazy here, just no clue what to think since I have no experience with electrical problems. No time like the present to learn I guess. I'm definitely appreciating all the help I'm getting here.
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Did this happen suddenly, or did you acquire it this way?
Does anyone know how the rear lights are supposed to ground? I can look in the FSM tomorrow, but that would be my next step.
I had bought it this way. I was unaware at the time I bought it. Not something I thought to check since it's entirely new to me that this could even happen. I had looked for all the other stuff like rust, oil leaks, and what general wear items will I need to replace. I'd greatly appreciate that turnerd. Hate to have a vehicle I can't drive at night or during some rain. Yall are a great help here
The FSM could not be any less helpful. It appears all the lights share the same ground as well as the horn, radio, and rear defrost. It does not say where this is. However, I do know that when I replaced my radio, having it on would blow the taillight fuse if I turned the lights on. I made a separate ground for the radio and all was good. I'm really not sure how to help at this point, but you can figure it out with enough time and patience. If you need any wiring diagrams or harness locations, just let me know and I can pull them from the FSM,
Oh well. I'm sure I can figure something out. If I can rebuild an engine, I can figure out some wiring. Maybe... Hahah. I really appreciate all the suggestions and discussion on this. Every bit is invaluable even if a solution wasn't reached directly.
Let us know what you figure out. One idea to determine if its a grounding issue is to splice into the ground wire on coming from the taillight (like right at it) and make a connector to a known, strong grounding point and see if it makes any difference.
Use another power source to verify which f
Oh well. I'm sure I can figure something out. If I can rebuild an engine, I can figure out some wiring. Maybe... Hahah. I really appreciate all the suggestions and discussion on this. Every bit is invaluable even if a solution wasn't reached directly.
You can visually determine which fillament is the brighter one. The filament coil is thicker for the brake. Check which one is thicker, then turn on the running lights and see if the thinner filament iluminates. If the thicker/brighter filament iluminates with the running lights instead of the brake, then someone wired it backward.
Well, forgot to update the thread when I fixed my issue. That's what I get for going on vacation after making this thread lol. Seems like one of the POs put in an aftermarket light so I just swapped the brake and running light wire around. They work perfectly now and I can drive at night with confidence.

Now on to the other issues..
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