Innocent Fool

Hey guys

I?ve got a bad electrical problem with my 91 4runner and I?m hoping someone can someone help me out

91 4Runner, 3.0, AT, 4wd

Got a bizarre problem in the stop light circuit.

Brake lights work fine until:

- Foot on brake
- transfer case lever in 4wd (Hi or Low doesn't matter)
- Engine running.

When all three events happen I get 32 amps on the stop light circuit.
Blows fuses like an SOB!

What I?ve done:
- disconnected all connectors going to the levers in the console, both in the cab and under the truck.
- Disconnected the 2 vacuum lines to engage front diff under hood
- Checked brake light switch (It?s OK)

Still no luck.
4wd light does not light up and I don?t think 4wd is actually engaged

If ANYONE has ANY insight please let me know!

I don?t understand why the engine has to be running for it to short out...???
Does not short out with key on engine off....

Help!

Innocent Fool

Grog hate electial probs!!!

frodin1

Did this just randomly, start or did something recently happen ie: "Stereo install , aux.lights, etc" and then it started?

Innocent Fool

I just bought the truck so I don't know the history

Didn't know there was a problem till just the other day.
REALLY frustrating! GRRRRRR

scrid

does it blow the fuse when the ignition is in the "ignition" position, and the engine not running, with all other events besides engine running?

Innocent Fool

Nope

It's fine with the key on and engine off.

ewong

Any aftermarket wiring
-- cruise control
-- alarm
-- RADIO?

I *assume* that the fuse that blows is the 15amp tail like fuse?

I have a Gen1 1989 so my elec diagram isnt "exact".

Here's a GUESS

Toyota uses a GROUND SWITCHED configuration.
Most people would wire a 12v circuit as a POSITIVE switched configuration.

So - what gets wired to the stop light?
-- cruise control
-- alarm?

I wired in an aftermarket cruise and had to um "modify" how it gets wired to get it to work.

My guess is someone grabbed the 4WD indicator wire as a random 12v wire to be used for something...

Maybe a stereo amplifier?

I have also seen strange things with radios that have both a BATT connector and a 12v switched connector... where the full current was pulled through the BATT wire and not the 12v switched wire and someone didnt realize that when they wired it and connected the batt side to something that couldnt handle the current draw.

scrid

Ok, I got my service manual out and according to it,... looking at the diagram, since your problem only happens when the engine is running then it got me thinking that there must be a relay that is actuated when the engines actually running. And reporting this to another system.

The diagram shows that a factory cruise control system is integrated with the brake system. At this point I would try and elliminate the cruise control ecu and cruise control main relay. But start with one first,....the easiest being the ecu.
Unplug the cruise ecu connector and retest to see if the fuse still pops.
The cruise control ecu and main relay are located under the dash on the left side by the drivers kick panel.
If the fuse doesn't blow than we need to look at the circuits associated with the cruise control inputs.

Innocent Fool

Dude, Thanks for looking at the service manual for me!

It makes sense that it's a relay or something...

Went back in and unplugged:
- Speed sensor ECU by fuse box
- Speed sensor relay by fuse box
- all relays in fuse box
- power antenna relay
- headlight reminder box in center console
- disconnected Vacuum lines going to cruse control under hood
- connectors going to cruse control under hood

Still no change
Now pulling 28 amps with engine running...

Slowly trails off to 4.5 amps when I shut off the engine

WTF???
I'm running out of things to unplug!

scrid

I wouldn't be messing with vacuum lines right now.
Plug all that stuff back in and get that cruise control relay and the cruise control ecu unplugged under the dash. You have to keep disabling things until the fuse stops blowing. Where are ya located?

ewong

How do you know you are pulling 28 amps?

What wire has 28 amps?

Innocent Fool

Plugged all the Vacuum lines back in as suggested.

I am at a total loss...

I?ve got a multimeter hooked into the blown stop fuse. It?s showing 28 amps with the engine running.

The wire is green with a white stripe. going from the stop switch to the stop lights. I know this is the one cuz it got really hot and crispy!

ARRGGG!!!

Think I might look into building a circut to drain off the extra current and engage the front diff...

drunner4

Could it be a ground wire? They do strange and unusual things when not working properly.

ewong

DOH

Didnt think of doing that.

-----

Hmmm

So how many amps are going through the circuit when
-- when the engine is off and the brake pedal is ON?
(this tells you how many amps the BRAKE light circuit draws - AFAIK brakes lights work even if the ign is off)

What Im trying to figure out is if there is something in the BRAKE like circuit thats shorted or is the short somewhere else.

I dont have a 91 wiring diagram - I have an 89 Gen1 diagram so its not gonna help much except for generalities...

The Stop fuse feeds
-- brake lights
-- clearance lights
-- glove box
-- combi meter (aka dash cluster)
-- rear power window
-- defogger
-- AT indicator
-- cruise control

I dont have ADD so I have no idea what feeds that...

Toyota uses a GROUND switched configuration.
That means all devices that are controlled by the STOP fuse are "hot" - the switch that turns em on is switched between the device that is being turned on and the GROUND (most cars switch the HOT side, not the cold side).

I had asked previously if there were aftermarket devices...
maybe a trailer harness?

One thought is that the short is in the brake circuit, but that there inst enough juice in the battery to pop the fuse (and the resistance is high enough that the voltage drops) so that the circuit doesnt pop until the alt is spinning

Id start by unplugging BOTH stop lights at the rear... you want to see if the short is BEFORE the tail light or after it....

If there is a trailer harness - esp if its a hacked in one - disconnect it...

scrid

Hey ya might try and remove and unplug the taillight clusters, also does it have a trailer light loom back there?

Innocent Fool

Well I gave up finding the short and just spliced in my own wires to cut out the original circuit. Still have to splice a wire into the cruise control so it cuts out when the brakes are applied but other than that it seems to have done the trick.

Next I?ll have to run wire to take over the 4wd engagement but I think I?ll leave that for another day.

Thanks to everyone for their ideas and feedback.
Wow, Electrical probs can be a real PITA!

Thanks again
Cheers!

Innocent Fool

Im starting to think it has something to do with the transfer case pressure switch. The ADD circuit only grounds out when there's pressure in the transmission which only occurs when the engine is running.

Hmmm...

Big wheels keep on turning...

ewong

I dont have ADD - is it an elec motor that actuates the axle disconnect

I vaguely recall that its a fork that pulls a collor off the longer side of the front IFS axle...