A while back I posted I think in my build thread about this.
Back ground at O-dark thirty going to work the dash turn signal lights would dimly blink / flash.
At the same time the motor felt link it sputtered and would hear something like a electric zap.

At first I thought it was the ICM as it was leaking it's guts running down the inner fender and onto the garage floor. I replaced it with top of the line NAPA one.
A little while later it did the same thing so it was not the ICM but did not know what it could be?

Well today again at O-dark thirty going to work the dash turn signal lights started dimly blink / flash again.
Unlike before when I got off the high way, the only place it would happen, it happened on a road I could go slower.

Well I am sure I found what was happening and but know why the lights would blink?
The AC compressor was kicking on and off. This would cause the motor to sputter as it turned on & off, look at how much power it pulls from the motor just to run it.
The same goes for the noise as the compressor makes, the zap noise.
What I cant figure out is why the the dash turn signal lights blink / flash dimly?

Now I think why it stopped for a bit was 2 reasons.
1 it was hotter out, the AC was on full so it did not turn on / off.
2 I think I added a little more charge to the system.

Since then I had a leak and when recharged it did not get a full charge, in the 40's on the low side where it should be mid 50's low side.
Also the OAT was cool but humid so I had the AC on normal but the temp up about half waybecause it was blowing to cold, so the compressor I could see turning on / off.

So why the dash turn lights blinking / flashing dimly?
Dave ----

 

Is the problem repeatable? If so, switch off the AC and see if that helps.

I wonder if the noise could be from the headlight switch. It has an automatic reset circuit breaker. If there was an overload in the headlight circuit, this breaker could be cycling. After repeated cycling, the breaker contacts could get pitted and you might hear arcing. One telltale is the switch body gets warm.

On your drive, are there plenty of streetlights or are you way out in the boonies? If well-lit (talking about the road, not you), it's easy to miss the headlights cutting out.

I can't remember if the marker lamps are controlled by this same automatic breaker. I'd have to dig a bit deeper tomorrow. But I do know the dash lighting circuit is part of the taillight circuit. The logic is to get your attention if the taillight circuit blows a fuse or otherwise loses power.

 

grounds, thats where I start.

 

The head lights are on a relay kit so the head light switch is only a control for on / off.
It is just the dash turn lights that dimly blink / flash, you can not see this in the day light.
Vary few street lights on my way to work and I see it mostly when I am close to my exit to get off the high way for the last mile or so.
I will see if I can repeat but the OAT would need to be cooler so the compressor is not working as hard and turn up the temp a little and fan on low.
Coming home it was mid 80*'s so the AC was on cold but fan on low and I never heard the compressor cycling

The "zapping" noise I hear is from the AC compressor but the only way to describe it was electrical short, yes the compressor is noisy JY used and that way since day 1.
Because it was short cycling and me trying to figure what all was going on I did not know when it first happened that's what the noise was.

I am sure why the AC compressor is short cycling, out side temp being cool as shown by the temp control turned up half way to hot, fan on low and it could be a little low on charge?

What has me is why the dash turn lights dimly blink / flash when the compressor is doing its thing.

I would think that but I have all the factory grounds in place and they were cleaned of paint before I put them in place when rebuilding the truck.
Battery cable to frame rail then to top starter bolt. there is a 10 ga wire from the block to the fire wall. Think I put the wire from the fire wall to the hood like factory and the new battery cable had a wire off it I ran to the fender just in case. Every thing on the truck electrical wise works as it should from day 1.
Being a flare side and a early model it dose not have a ground wire for the tail lights up to the fire wall like the later ones do so the tail lights ground to the frame and have also worked since day 1, even the back up lights.

The compressor has its own ground I think to the motor, have to look as I dont remember now but the clutch has 2 wires, power & ground.
It has a new clutch as the old one stopped working and the gap is set right as per the book.

I will have to see if turning on & off the AC makes the dash lights dimly blink / flash now that I know 1 of the causes.
Also being I now know I may not rush in to this farther, put off what you can till it breaks

I just thought of something when filling in something above.
All my dash lights have been replaced with LED bulbs, wonder if that plays into this?
No I am not going to swap in normal bulbs to test, to much work and I am lazy
Dave ----

 

Are they cycling in concert with the AC compressor? You haven't really specified.

If so, run this test:

https://www.ford-trucks.com/forums/1...on-switch.html

Note how the same pole of the ignition switch feeds the heater/AC and also the turn signals. If the contacts for that pole were starting to get pitted, once you add the heavy draw of the AC clutch, voltage would dip to the turn signals. I haven't worked out the mental aerobics for how this would make the little arrows light up dimly, but this could be part of the equation. So spend five minutes with a meter and test the ignition switch as described at the link. It's a very simple test.

Next, look at the wiring to the AC clutch. Check out the letter T inside a triangle, near the clutch:




That "T" leads you to page 33, near the bottom right corner:



Even if the computer has been removed, that clamping diode may still be present, with one side still connected to ground G756. If the diode is starting to short across, it could be drawing down available voltage in the AC clutch circuit. That could be causing something funky with the power to the turn signal indicators in the dash, as they all share the same suspect pole of the ignition switch.

The diagram makes it look like the clamping diode is right next to the computer, but it's closer to the battery. See page 9, figure 1, right in the middle:

 

You stepped up and did a lot of thinking! Good on you.

The triangles: that wire usually lands on pole #10 (of a 60pin ECM) on the computer. It signals that the AC compressor is engaged. The computer then sends voltage to the idle solenoid for a kick to offset the AC drag.

Don't remember if this truck is running a computer or not. If I were to bet, I bet not.

My only thought was pull out the Hazard flasher to see if that changes the situation. I have experienced an intermittent dim dash light once in my life. It was a bad diode link in my 1977 Harley Davidson XLCR.

 

If you did not have a computer, but did have A/C, most engines had a idle kicker solenoid wire in with the compressor to kick up the idle speed.

And if you are running the original old style alternator, the lights will vary in brightness with engine speed at low rpms. The older alternators did not have the output of the newer style alternators. That along with the increase mechanical load like you said lowering the rpms of the engine aggravates the situation.

 

It just occurred to me, IIRC, this truck had no AC originally. So it would have no provision of any kind to compensate for AC drag.

 

Some more questions about this tantalizing little clue.

1) When the turn signal arrows illuminate, is this when the compressor clutch is on or off? Just trying to get an idea if a marginal ground somewhere (clutch coil?) could be at fault.

2) Can you duplicate the fault while parked? If so, do any of the exterior lights also flash dimly? You will probably have to wait until dark. Just the front? Just the back? All of them? This might help narrow down the root cause.

3) Look very closely at any exterior bulb that illuminates. It will likely be a dual filament bulb. Is the filament closer to the base, or the one further away?

 

From what I just found out the the dash turn lights do blink as the compressor is kicking on & off.
Because it was cycling so fast I could not tell if they blink on when the compressor come on or off?

The truck is a 81 so it never had a ECM.
The wire that would go to the ECM I think is the wire to the carb solenoid kicker to up theidle speed when the AC is on.
I dont have this solenoid so the wire is just in the area safe.
The turn signals & 4 way still worked as I did try them when it was happening when on the high way.

Yes on the kicker but I dont have one at this time but not really looking for one either. The RPM dose not really change much when the AC is on or off.
I am running the head light relay kit so the head lights are brighter. the only time I see the head lights dim a little at idle is when I first start the truck and pull it out of the garage before work.
After the 45 min. drive to work I did not see them dim or get bright with RPM as the battery is now charged from the drive in to work.

You are part right the cab did not come with AC. My parts truck did have AC and everything, dash, HVAC, wiring harness, AC firewall came from the parts truck so it has factory AC now.

1) because the compressor cycles so fast I cannot tell when they blink on. The clutch / coil is new and IIRC 1 of the 2 wires goes to ground but need to check this to be sure.

2) I have not had it happen, compressor cycling, when the truck is sitting at idle. It is a fine line of when it dose, right cool outside temp, fan on low and temp setting to the warm side.
If I can get it to happen I will have to check on that.

3) If I can get it to happen I will check into it.
I think I would need to turn off the lights to see as I do not see any of the dash gauge lights or radio light doing anything strange.
Also the other warning lights dont do anything either.

Now that I know the AC compressor cycling is a sign I can check more when it happens again and run the test Karl posted.
Thanks guys
Dave ----

 

Any progress?

Looking at the wiring diagrams above, it shows a single power wire to the AC clutch coil. The ground is not real clear. It just shows a ground symbol, presumably to the engine.

On my ‘84, the harness to the clutch coil has two wires, power and ground. I’m not sure of the actual ground point, as these two wires go into a larger harness near the receiver/dryer.

My latest hunch is you have a marginal ground for the clutch coil. Does it have a separate ground wire, with two wires at the connector? Or is it a single power wire and grounding is via the mount bolts and brackets? Whichever version you have, I’d suggest checking this ground. If there’s no separate ground wire, try adding one and see if that helps.

 

I have not had time to look into this farther with yard work yesterday I been unable to get to for weeks and today a fund raiser for a pet foster group we are members of.
I do think the compressor was cycling today as it sounded like it was but being day light I could not see if the turn lights were blinking dimly.

I will have to check that compressor clutch ground as I dont remember where I have it going to ATM.
IIRC the 81 compressor harness only had 1 wire to the clutch but the later compressor I am running have the 2 like yours does so I had to add that ground to make the clutch work.

I dont think I will be taking the truck to work this week, gave it a quick wash before the fund raiser and dont want it to get to dirty before a show next weekend.
Dave ----