Curious Lighting Issue on 79 FJ40 (1 Viewer)

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

CruiserInProgress

CruiserInProgress

Joined
Feb 23, 2024
Threads
12
Messages
225
Location
Idaho
20250116_151004.jpg
After getting my project 79 FJ40 running again after more than a decade of sitting and previous abuse, I've been concentrating lately on getting it road worthy. I hope to have some fun driving it while I get the bugs out and collect the parts to re-do it someday.

To get the lighting done I picked up some aftermarket front and rear turn/parking light assemblies and plugged them into my mostly stock harness, rewiring the assemblies to match the stock connectors and scheme, adding soldered grounds inside each lighting assembly for a solid connection.

(I have condensed my writeup a bit to focus more on the problem identified and steps thereafter)

When installing corner lighting fixtures, using incandescent bulbs, all seemed ok, parking, tail, turn, hazards.

But when upgrading incandescents with LED bulbs, I noticed my turn bulbs are on very dimly when I have my parking lights on and vice-versa. Getting the meter out, I found 7-8v on my turn signal sockets when the parking/tail lights on, but no flasher activated (turn signals off, but tail lights on).

I'm thinking there is a low current bleed from the parking light circuit to the turn circuit, 8v sufficient to turn on the LED bulbs, but not sufficient for higher current incandescents (or I just didn't notice).

OK, so I set about trying to isolate the issue, by unplugging everything related to the turn or park/tail circuits, light assemblies, fuses, etc.

The schematics don't show anything in common between the turn lighting and tail/park/marker circuits, except the corner lighting assemblies and the fuse block, which I have kind of eliminated as l possibilities.

But, maybe there's a common connector in the harness somewhere that may have pins/leads that are slightly touching? I haven't unwrapped the underside wiring to the rear lighting yet. Is there a notorious connector in there that I should look at? (see update below)

Anyway, I'm scratching my head about this and wondering if anyone might have a suggestion for me.

20250202_114607(0).jpg


20250202_114404.jpg
 
Last edited:
Update -

On a second attempt at finding this lighting gremlin I went back to basics and disconnected all corner lighting assemblies (turn, park/tail, brake, reverse) and measured around 8 ohms between the turn signal line and tail/park line in the harness, both directions.

So, it's not a diode issue and the resistance is higher than a incandescent bulb somewhere. I must have some sort of resistive short between the turn and park/tail circuits. Ok, but from where?

Then it occured to me. While cutting off a crude, welded on hitch from the rear bumper a couple weeks ago, I tucked the old, aftermarket trailer wiring out of the way, thinking I will dig into that some other time. In the process I may have disturbed some connector in that taped up mess?

20250116_141048.jpg
20250121_133932.jpg


So, that's my next step, to remove that aftermarket trailer wiring/converter and get back to straight oem. We'll see if that's the culprit!

If not, I'll have to dig deeper into the harness. Fortunately, most of the factory wiring is intact (except smog-related circuits and components), but it has been cobbled in places by previous owners. I've tried to repair to match the 79 schematics when I find problem areas, with the help of @Coolerman and his specialty parts.

Well, I hope the old aftermarket trailer wiring is the problem, but any other ideas would be appreciated.
 
Last edited:
I’m dealing with this on my Cruiser. All lights are LED and I have everything working properly except for the rear lights in the frame.

I’ve found that the Cruiser grounds are inadequate for LED lights and I have had to run new ground wires to each bulb. I am not running the front factory lights anymore but the rear lights are factory. The best that I can tell is that there is a faint power draw through the taillight housing which goes away when I remove the bulbs from the housing and hook them up directly while keeping the bulbs separate from each other. I’ll be watching to see what you end up doing as I am not excited to cut the housing apart to create separation between the light fixtures.

IMG_2235.jpeg
 
Thanks for chiming in Mike. Nice to know I'm not the only one seeing this.

Poor/floating grounds is an interesting idea to investigate. I had to clean up and rewire several grounds around the engine compartment to get my 79 to crank properly.

Because poor grounds (and other connections) are a common problem with FJ40's, especially those that have seen salt, I clean them as I find them and apply dielectric grease to the cleaned connections.

I have read somewhere that there is a ground point between the fuse block and the rear lighting, but I haven't tried to find it yet. I'll look today while removing the old trailer wiring.

About the light fixtures, grounds are an issue with the discount aftermarket assemblies I have found from overseas. I pulled mine apart and made sure grounds are soldered to each socket or housing.

I'm not sure the bad or floating ground theory would explain how I get 8 ohms (resistive short) between my turn and tail lights in the harness with all light fixtures removed. Still thinking on that one.

I'll report back after working on my 79 today.
 
Last edited:
Grayscale said:
Aside from a fried wiring loom it, the short must be in the hazard switch (turn and brake circuits). Does the cowl leak?
Click to expand...
I considered that, but in the 79 schematics I didn't see the brake circuit associated with the hazard or turn switches. The harness is certainly a question mark.

The vehicle has been sitting for many years, but it looks like it's not been out in the open. There is definitely rust in the usual places, which I assume is mostly due to winter driving with salt on the roads up here in the Pacific Northwest. Under the dash is old and dusty, but doesn't appear to be rusty.

Having said that, I have had to take apart and clean every switch I have tried to use.
 
Last edited:
Grayscale said:
I think it is so that the hazards can function without the key in the ignition (two hot sources).
Click to expand...
Yep, makes sense. On my hazard switch that function seems to work fine. But, it could probably use a good cleaning anyway. I'll also check for that resistive short I found while the hazard switch is pulled.
 
Last edited:
Oops, I just realized I got a little sidetracked by the possible connection to my brake lights. My issue involves a possible resistive short between turn and tail/parking lights.
 
Last edited:
Something to note and that I have personally experienced, especially with the factory front turn signals and rear light housings, is that because LED lights require a much lower amount of current to illuminate, they can sometimes draw small amounts of power through the ground circuit. It is normal to see a half of a volt or higher through the ground circuit, especially in a light bulb circuit (and also in the horn circuit, but that is a separate matter). Power travels through the light that is intended to be illuminated and ends when the power is used or wasted, which normally terminates at the ground connection. If the shortest and least resistant path is through the light bulb, then you will often get a dim glow from that bulb, which is an indication, not a sure sign, that the bulb may be receiving power through the ground circuit.

I am not an electrician though I consulted one to help trouble shoot and this information was very helpful for me. I don’t know if this is the problem that you are facing but I do believe that it is my problem with the rear assemblies because the lights work fine when hooked to the same system without the housing. I’d be willing to bet that this is why auto manufacturers isolate the light bulb housings on newer cars, and that seemed to start a long time ago.
 
Mike Shull said:
Something to note and that I have personally experienced, especially with the factory front turn signals and rear light housings, is that because LED lights require a much lower amount of current to illuminate, they can sometimes draw small amounts of power through the ground circuit. It is normal to see a half of a volt or higher through the ground circuit, especially in a light bulb circuit (and also in the horn circuit, but that is a separate matter). Power travels through the light that is intended to be illuminated and ends when the power is used or wasted, which normally terminates at the ground connection. If the shortest and least resistant path is through the light bulb, then you will often get a dim glow from that bulb, which is an indication, not a sure sign, that the bulb may be receiving power through the ground circuit.

I am not an electrician though I consulted one to help trouble shoot and this information was very helpful for me. I don’t know if this is the problem that you are facing but I do believe that it is my problem with the rear assemblies because the lights work fine when hooked to the same system without the housing. I’d be willing to bet that this is why auto manufacturers isolate the light bulb housings on newer cars, and that seemed to start a long time ago.
Click to expand...
Interesting theory. I'm still trying to see if it applies.

Here's my take. For a bulb, incandescent or led to illuminate, even dimly, it has to be part of a complete circuit, somehow. Current must flow. So, if a bulb is being powered by a raised ground (reverse current) through its ground pin (base), then in order to light, it must have a path from the power pin back to power or real ground for current to flow and the bulb to illuminate, even dimly.

In my case, for example, my turn switch is open while my tailight switch is closed. So, unless I have some type of short (which sounds likely) in my harness connecting the turn wiring to the tailight wiring the turn circuit should not be complete.

In the process of troubleshooting, I disconnected all my lighting fixtures and all but my tailight fuse and still found 8v on my turn pin in the harness connectors.

All that kinda leads me to the conclusion that I have a resistive short (8 ohms) in my harness somewhere.

My first step is to open up and rip out the aftermarket trailer wiring and converter which needs to go anyway.

My second step is to try and locate that mid-chassis ground I recall reading about to make sure that's clean and solid. I should also try to verify the ground circuit from the rear all the way back to the battery.

The more I can open things up down there, the better I can isolate the fault...hopefully.

I'll report back later.
 
Update -

It was a little cold in Northern Idaho today, working in the unheated barn, under my FJ40 to try and figure out my lighting wiring so I didn't get as much done as I had hoped. Still, better than working on your back, under a vehicle in the snow.

However, I did untape the trailer wiring mess and removed it. I found and repaired a questionable 3-wire ground junction (factory Toyota brass crimped connection).

I also may have discovered a mismatch between the rear lighting fixture connectors and the harness connectors, specifically the position of the ground pin. Strange, because both seemed oem before I started, and I matched the new rear fixtures to the old. A mis-wired ground could cause some strange effects.

To complicate things a bit, the harness wiring colors in the rear (solid colors) don't really match my 79 schematic (that should be striped?). I can guess, but I'd better make sure. So, tomorrow I'll verify signals.

20250115_153103.jpg


Does anyone know the oem pinout of the rear lighting fixtures for the 79 FJ40, or where I might find that info? This is the connector I took off my old tailights and fitted to my new ones.

Found the correct pinout here:

Screenshot_20250205_182537.jpg
 
Last edited:
A continuity/test light is real helpful for ID'ing wires. Bad grounds make weird unexplainable things happen.
 
Success!

20250206_151125.jpg


That resistive short between turn and tail lights is now gone. I assume it was caused by the cobbled trailer wiring or the trailer wiring adapter box.


20250206_172141b.jpg


When I opened up the rear wiring I found the colors didn't match the 79 schematics, so I had to verify each pin of the rear lighting connectors for accuracy and function, including ground. A little tedious, but necessary in this case.

Now on to my reverse lights that don't work yet. My next stop is the switch, supposedly located near the transmission somewhere.

And next my turn and hazards that are slow to get going and blink slowly when they do. I may need to do a bit more cleaning on the turn and hazard switches.
 
Last edited:
Well done. That's the value of persistence and assuming nothing. Plus it never hurts to walk away for a while so you can come back with a clear mind to check what is actually there. And you didn't even let out the magic smoke :)
 
You must log in or register to reply here.

Similar threads

E
Help Identifying Brackets Behind Front Seats on 12/79 FJ40
Replies
2
Views
332
ElephantRebel
E
CruiserInProgress
Curious 79 FJ40 Project Runs Very Rough - Ideas?
2
Replies
36
Views
3K
charliemeyer007
charliemeyer007
dmcdon17
Help with diagnosis - ‘79 FJ40 with 2F
Replies
13
Views
531
Engineer8000
Engineer8000
B
Using late closed chamber head (81) on earlier (79-) open port motor.
Replies
0
Views
255
BullHab
B

Users who are viewing this thread

Total: 2 (members: 0, guests: 2)

Share this page

Back
Top Bottom