Guys, I've been consuming the threads here that have to do with turn indicators and I can't seem to find information that will help resolve my issue. On my 1978 F-100 Lariat, the left indicator began blinking slowly a few weeks ago. Then the other day, it just quit. The dash light and the bulb up front and in the rear still illuminate if I push the lever down, but won't blink. I replaced the flasher (small cylindrical unit under the dash to the left of the steering column), but that didn't change the situation. I checked the bulb up front and it's good. I can't get to ground wire at that socket because it's sealed. Same with the other two. Anyway, my sense is that the relay must be bad or some connection there is loose or corroded. The right turn signal works fine, and I can hear it clicking inside the steering column. Is it safe to assume that the left and right indicators are working off the same relay? Anyone had experience with this issue? Thanks in advance.

 

For starters, you need to take a ground jumper wire, or your 12v test light and find a way to ground the light bulb in its socket. It sounds like you have a ground issue in your left hand turn lights.

 

Yes, the turn relay is used for both lights.

To see if the light is truly functioning,
test it by turning on the hazard switch.
If both sides work, then bulbs & wiring should be solid.
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Your lighting issue sounds like a bad bulb grounding itself out,
"Or" you have open ground.

Get a new bulb, & if that don't solve it.
Check the socket's ground tab inside the socket, it is on the side of the socket.
They ether corrode or break.
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The 78-79 front LEFT turn-signal
(@ the socket) wire color is white with black-stripe.
It turns into Green with white-stripe @ it's connector.

Brown wire is running/marker lights

Black is the ground wire.
{The ground is shared between the headlight, marker, turn & that ground is usually bolted to body.}

 

Thanks guys. Hazards work perfectly, front and back on both sides.
I'll check out the ground issue today and post what I find.
The right hand indicator has begun to blink slowly now (same thing that happened with the left.) That's why I was thinking faulty relay.
Crazed Fox, sounds like you know the wiring on these trucks pretty well because your color description was spot on.

 

OK. I replaced the front left bulb, and that's not the issue. The ground connection looks good inside the socket. All the wires going into the socket are sealed (some kind of heavy plastic), so I can't really get at them. I'm going to put a tester on the wires later today.
I think I may have created this problem by unexpectedly dragging my trailer pigtail connection a few weeks ago. Now, when I connect the trailer and trailer light hookup, the right hand blinker on the trailer works, but not regularly and not very brightly. The left hand one doesn't illuminate at all. I'm going to replace the pigtail and see if that sorts out the whole issue. I'm assuming that a faulty trailer hookup could be what's causing ground issues for the indicators. Does that sound sensible?

 

Definitely one of two things.

Grounding issue (bad-bulb, lack of ground, or short.)
Or
you've got a bad turn-signal/hazard switch assembly.
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Of course it can't hurt to order brand-new turn/hazard switch & check everything else (bulbs & wiring) while you wait for it to arrive.
———
Got to remember that the design of these trucks was only to last 10-Yrs.
So yeah, gotta love build quality of the decade these came from…

 

probably a bulb or socket giving you grief. My truck's doing that on the left side every so often. couple nights ago i got out and looked around and its the tail light giving me issues. give it a thwack and it worked for awhile lol

 

To check the quality of the ground of your bulb socket, remove the light housing from the grill, take out the plastic bulb socket, take your piece of wire to use as a ground and stick one end in beside your light bulb in the socket, ground the other end of your jumper wire. Have the lights on or turns signal on of coarse, and see how much brighter your light becomes or starts blinking if your ground is messed up inside the sealed plastic socket assembly.

Yes, this is probably your problem...

 

Have you ordered a new turn-signal/hazard switch assembly yet?
Remember these switch assy. can bare issues rather sudden.
But when they do, they can be rather bothersome.

 

Crazed Fox,
No. I haven't ordered a new switch assembly just yet. I've got another issue that's been occupying my energy, replacing the heater core. It's been a bit of a bear so far even though most guys write on this site that it's a piece of cake. I don't think this unit has ever been replaced, so the fasteners are all a little resistant. Anyway, once I get that tackled, I'll be back on the turn signal issue.
The only thing I've done yet is replace the trailer pigtail connection, but that didn't resolve my issue with the left hand signal.

 

1978/79 F100/350, Bronco & Econoline: THREE different T/S switches.

1) D8TZ-13341-A .. T/S Switch-Use with Tilt Steering Wheel.

2) D8TZ-13341-B .. T/S Switch-Use with A/T without Tilt Wheel.

3) D8TZ-13341-C .. T/S Switch-Use with M/T without Tilt Wheel.

There are TWO different flashers:

T/S flasher plugs into the back of the instrument cluster.

Heavy Duty 4 Way Emergency Flasher (has a louder CLICK) nestles into a bracket that is retained to the backside of the dash w/a screw.

Heavy Duty flasher also used as T/S flasher with Trailer Tow.

 

NumberDummy !
It's grand to see the master of parts here.
Always good to hear your input.

 

Number Dummy, thanks for that diagram and parts info. Very helpful. I haven't yet finished the heater core replacement project, but when that's done, I'll be back on the indicator issue and use this stuff.

 

I'm having kind of similar issues with my turn signals and rear marker lights. Hoping I can explain this properly to get a resolving answer!

When I put turn signal right on, no issues. When I put turn signal left on, rear light is weak but it still lights and flashes. When I put lever back into cancel position (from turning left position - changed steering wheel and don't have auto cancelling any more) I can hear a clicking sound of the flasher going and on checking the rear lights they flash alternating between sides. To stop it I have to put the lever into the turning right position and then back to cancel to get it to stop.

Also when indicating with the lights on, the opposite light goes out but when I go back to cancel position they both light up again. Am I right in thinking that the lights should be lit when the turn signal is on so that turning left the right light stays on etc?

Also with the rear marker lights I had both working solidly but now I noticed tonight I had passenger rear working until I got to the store and then neither when I drove home. I'm guessing that's a bad ground and hopefully an easy fix. And possibly the weak driver's side rear light too.

But for the awkward flashing turn signals, has anyone else had this problem and how did you fix it? Could this also be a grounding issue or a faulty flasher unit? I must add that I replaced the turn signal switch assembly because the previous one was broken but this started before that I believe.

Hoping someone has experience this or can otherwise help me on this because electrical stuff on the truck isn't my strongest area...

 

Kierobi:
Your going to have to start with a bulb check.
You do a meter continuity test on the bulbs, or use a 12v batt. to test.

#1156 bulbs are single filament, barrel is ground & contact on bottom is positive.

#1157 bulbs have a duel filament, the barrel is ground + the two bottom contacts are individual positives.
One for turn (bright), one for park/running (low).

Sometimes a filament will blow & land crosswise onto the other,
Causing it to ground/short through other contact.
So a visual comparison of it with a new bulb will add in seeing if bulb is bad.

I just recommend changing to all new bulbs to save time & frustration.
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After bulb check, you'll need to check socket contacts & grounds.
Make sure they are clean, no broken contacts, loose wires, Ect,Ect.

Especially the grounds behind the tail lamp lenses.
The tail lamps grounds to the bed & the bed grounds to the frame.

I've been known to take the driver-side tail lamp ground & extend/run it down to the frame.
One just needs to clean to bare metal & attach the new extended ground with a self tapping machine screw.
You just tap onto the current ground that screws to the bed behind the lens.
So the bed is still grounded, it just will now have a second "wired" ground.

You can "coat" your bare grounds with "hair-spray" to seal them.
Good luck !