I keep blowing fuses in my tail circuit. When I replace the fuse the dash and tail lights work correctly with the vehicle off, switch on. When I start the truck the fuse blows and circuit goes dark.

Relay clicked when bench tested. Dimmer is currently bypassed. Same results when I unplug the rear harness behind the passenger seat. There are no visible problems with the harness in the cab. Everything was working fine until last week, then it wasn't. Headlights, brakes, turn signals and hazards all function correctly.

Any ideas?

 

So the fuse blows even with the whole wire harness going to the tail lights is disconnected? My answer for anything tail light related is always check the grounds. But with this I'm not sure...I'll think. Something is shorting and sending a surge.

 

Correct. I repaired the rear harness about a year ago when I had the bed off. Have also replaced the rear tail lights with new units. I'm confident it is something up front. I'm stumped too. There is the ground near the instrument cluster, is there another in the cab? Could it be something upstream from the fuse?

 

These are the diagrams I have been looking at:




I also have the 83 FSM and diagrams. They look pretty similar.

 

The relay might be clicking on a bench test, but the contacts inside may have some pitting or corrosion on the contacts that can cause some excess resistance, causing a higher current draw & then blowing the fuse. I couldn't find one specifically for the '79, but here's one for an '83 that might be correct:
https://www.rockauto.com/en/catalog/...amp+relay,3836

As for grounds, my '78 has 2 underhood. One goes from battery negative the the motor mount (engine ground) and the other is from the cylinder head to the firewall on the driver's side. Since the '78 & '79 both use the 20R I'm pretty sure those two would be there. I do not know for sure if Toyota added any extra ground wires from '79 on. I do know that from 83/84 onwards they added 2 more, an alternator ground and another one at the intake manifold (may be for fuel injected models only). Have a good root around under the hood & you should be able to find them.

 

Thanks, I'll have another look at the relay. It looks like my local autozone has one in stock, I might swap it in and see what happens. Engine grounds look good, I replaced all those recently.

 

Happy 2020!

A problem well-stated is almost solved.
When vehicle is off:
1) Rear marker lights OK?
2) Front corner markers OK?
3) Brake Lights come on when brake pressed?

There is a short somewhere.
Triple check your wiring and connectors to make sure there is no short to ground.

Rear marker lights, front corner markers, turn signals, headlights, hazard, horns work when vehicle off?

By start you mean IGN On, or when you CRANK, or when engine runs?
Fuse blows without you doing anything other than starting engine?
Fuse blows even when you do not step on brakes?
When this happens, reverse lights still work?
Brake lights still work?
You sure the wiring had not been butchered? The tail light fuse (15A) is always hot. I don't see why it would blow when engine is turned on. Turning on IGN or engine will put 12V to other parts of circuit. 12V from IGN IF it goes to tail light fuse will not blow it.
More likely a short that happens when engine vibrates. Quadruple check wiring, especially running from front among the bed to the tail-light assembly.

Do not spend time and money replacing parts unless you are sure they would fix the problem.

Excess resistance will cause LESS, not higher current draw.

 

Hi RADRunner, I was wondering if you might stop by! Let me work through your list and I'll report back. Engine vibration causing a short has got me wondering........

 

I have some thoughts you should consider, when you start the engine your battery voltage goes up from about 12.7 or so to probably 14 volts due to the alternator charging the battery. That will cause the current (amperage) to increase slightly.
I would recommend that you make sure you have the correct light bulbs for all of your marker/tail lights, with the wrong ones you may be right at the limit of a 15 amp fuse and when you start the engine the increased current blows the fuse.
I would tend to agree with RAD though, you probably have a short when things are vibrating.
Another thing you could check is to remove the front marker light bulbs, (the license plate light bulbs and the tail light bulbs are probably not suspect since you disconnected the harness) and see if the fuse still blows. If it doesn't blow put one bulb at a time back in (making sure they are the correct ones for your truck) and see which one blows the fuse or if it is related to how many you have connected (take a different one out and try again).
You could also have a short in the dashboard since the heater illumination and dash lights are on that circuit as well.
If you know how to use a meter for amp measurements you could do all of these tests without having to sacrifice fuse after fuse. Or you could remove ALL of the bulbs (dash and heater too) and with the relay removed check for resistance to ground on the green wire from the relay to the lights (if the diagram attached earlier matches your truck).
Another test to eliminate the control side of the relay would be to remove the relay and make up a jumper wire with a 15 amp fuse and connect it directly from the battery to the terminal with the green wire at the relay thereby powering the lights without the relay.
I know there is a lot here, but my mind wandered. Please point it in the right direction if you see it wandering around unattended.

 

Cool
Also, pls make sure u have the correct schematic.

 

Had time to do a quick check this morning after thinking about akwheeler's comment about voltage increasing. I am getting the following readings at the battery terminals at idle:

16.7V no accessories
14.7V with headlights, fan on high, wipers on

16.7V seems too high, correct? Voltage regulator suspect?

 

I'm not sure what the spec is, but that is really high.
I'll bet your headlights are really bright!
And, yes I would suspect the voltage regulator.

 

Yeah. 16.7V is to high. on first-gen 4Runners, charging voltage range is 13.5V to 15.1V.
Before you totally suspect your voltage regulator, check your wiring. I had overvoltage fault that went all thee way to 17V. I double-checked, cleaned and tightened my "S" wiring and connection to alternator. Fixed it.

"S" pin senses battery voltage. In your case, based on the vague schematic (Toyota had much better mechanical engineers than electrical ones back in those days), I think it's the white wire going to the voltage regulator.

 

Well, small update. I tried cleaning the "S" connector as per RAD4Runner's suggestion. No change. So I picked up a new voltage regulator and swapped it in. Voltage is now around 14.6V at idle.

Unfortunately it did not solve the tail light circuit issue. I put in a new fuse and they worked for a couple of minutes (engine running), then blew the fuse. So I think there is a short somewhere from vibration. I am going to check out the front marker lights first. The passenger side wiring was looking pretty crusty last time I was around there.

 

Problem solved! The passenger front marker light rotted out and the socket was making contact with the fender causing a short when the truck was vibrating. I bought a new one from Mr. Bezos and all is well. Thank you everyone for the help!