Another Squirrelly FJ40 Electrical Thread. . . (1 Viewer)

Joined
Sep 26, 2005
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86
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JP, MA 02130
9/78 FJ40.

I should mention that I have a Mallotte fiberglass tub, and it's 17 years old. If I knew then what I know now, etc. Done before Aluminum tubs hit their stride. So, this may be as simple as a ground connection, but it's only a recent issue. No insurmountable electrical problems in 16+ years. I should also mention that my entire front end is steel. Bib, aprons, hood, and 9 year old fenders. OEM turn signals and connectors into loom. OEM loom.

The issue:

Headlights and driving lights off: turn signals work
Headlights and/or driving lights on: turn signals don't work

1: Pulled the headlight switch out, opened and cleaned up all the contacts. Discontinued, thankfully these appear bullet proof.

2: Pulled the fuse block and noted 35 years of oxidation. Wire brush and emory paper. Reinstall. No change.

3: Swapped the fuse block out for a cleaner spare I had on an extra wiring harness thinking there may have been something rotten in my original block. Reconnected, dash lights are brighter, but the issue persists.

Fuse block has three major connectors on back: Green 8-blade connector feeds juice in to all blades/ fuses.
Black, and Clear connectors split off to send juice to the loom and devices.

Black 5-blade connector feeds Tails, Stops, Lighter, Heads, and Heater.
Clear 3-blade connector feeds Turns, Wiper, and Engine.
Single feed to Radio.

Headlight switch in first or second position, if I pull the black connector of the back of the fuse block, disabling tails, stops, lighter, heads, and heater; signals work. As soon as I plug the black connector back onto the fuse block, turn signal stop, but control cluster signal light is lit as if the turn bulb is burnt out. I can manually actuate the signals as a stop-gap by flipping the signal arm up and neutral for a right turn, down and neutral for left turn.

Headlights, and/or driving lights ON, signals disabled; wipers work.
Note that turn signals and wipers share a fuse block circuit.

So, signal relay works (with head/driving light off), and juice is going where it needs to, since I can force signals manually.

Is this clear? Apologies if this is exhaustive in detail. I try to be thorough.

Any insight? Thoughts? Next steps? My electrical knowledge is functional, not advanced.
 

spotcruiser

Geezer
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Jan 30, 2003
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Western WA
Hard to say. Do you think your truck is wired anywhere close to stock? In the stock 78 wiring, turn signal and wiper circuits are totally independent of the headlights. Out of curiosity, are there individual ground wires run from each light fixture to some common ground, or did the person that modified the truck run some sort of grounding ring bus around the vehicle and then to a ground point? Or perhaps they ran a ground wire to each corner and picked up circuit grounds along the way?

Do the turn signals do anything with the headlights on? Do they light, but not blink?

Sorry, I just read your post again and noticed that the front clip is stock. That makes some of my questions moot. I'll ask again, do the turn signals light but not blink?
 
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Joined
Sep 26, 2005
Messages
86
Location
JP, MA 02130
Entirely stock 78 OEM loom and wiring. No separate aftermarket ground bus.
(I pulled a ground wire to dome light since there was no ground loop with fiberglass tub/cowl).

Tail lights are frame mounted, so neutral ground loop as stock.
Front turn signals/ bayonet connectors are OEM on steel fenders. Same stock neutral ground loop.

Stock fuse block has turns and wipers sharing a fuse (to save space?), but the circuit splits out from there. Wipers work with lights on or off.

Signals don't work when either heads or driving lights are on.
Instrument cluster turn signal lights are lit as if turn signal bulbs are burnt out when lights are on.
Head/ driving lights off, signals relay as normal.
 
Joined
Jul 26, 2014
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Seattle, WA
I had a similar issue recently and found a corroded ground at the rear taillights and park lights. I freshened up both grounds and it all worked well.
 
Joined
Sep 26, 2005
Messages
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JP, MA 02130
interesting. The rears are definitely getting old. I suppose I should inspect them. That might explain why the signals work without the driving lights on. Since they share a common steel housing, they may be shorting with drivers on.

A next test might be to see if signals stop working when stops are applied.
 
Joined
Jun 7, 2003
Messages
138
Location
Southern Oregon
It sounds like a ground issue. I looked at the wiring diagram for 1979 and the turn signal indicator is connected to the front and rear signals, so the issue could be coming from either end. I would start by removing the turn signal bulbs from the front. If the system was functioning normally and you did this the signals should flash fast, simulating a burned out bulb. The point of this test is to determine where the issue is, front or rear. If removing the front turn signal bulbs makes no change, reinstall them and remove the rears and retest. My assumption is that you have a marker light (that comes on with the headlights) that has a bad ground. The bulb socket itself could be where the issue is, or where the socket grounds to the chassis.
 
Joined
Sep 26, 2005
Messages
86
Location
JP, MA 02130
chased it down.

headed forward to the front signals. checking continuity from bayonets to bulb holders. No conn. OK.

hit the signal assembly nuts on the underside of the fenders with PB Blaster and began to rock them to break the rust, and the signals started working.

Ground interference from rust. I knew there was a reason to undercoat the fenders but didn't do it. Live and learn. Next pair.
 

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