Hi everyone!
Just picked up my 1989 Pickup back at the beginning of July and have been reading through and catching up on all the great posts in here. Figured it's time I create my own account and contribute

When I bought the truck only the speedo worked in the Gauge Cluster, figured it was just a blown fuse so started by replacing the fuse.
All the gauges worked for about 30sec-1min before it blew the fuse again.
I'm assuming it's a bad ground or a short in the wiring harness or back of the gauge cluster somewhere. Whats got me confused is that when the Gauge Cluster fuse blows, the blower for the AC also stops working.
From all the wiring diagrams i've found these two shouldn't be related?? Could be a crossed wire between the two circuits causing the short?
Figured i'd ask here to see if anybody has any ideas to help narrow down my troubleshooting before I tear apart the dash and start checking all the wiring w/ a multimeter.

In addition to this, the blower only works at its highest fan speed. Speeds 1-3 don't do anything (although you can hear the solenoid clicking when you change speeds)
I checked the fuses for the blower/AC in the passenger foot well and they were all fine.
Next steps was going to be heading down to the junkyard and pick up a spare blower and/or blower switch to see if those work.

 

Sounds like the fan speed is the blower motor resistor, not sure if being bad will cause the cluster fuse to blow but either way the resistor needs replaced, pipe a search on the resistor you will get lots of results!!

post some pics of your truck too, we like pics

 

Welcome!

Which fuse, exactly, keeps blowing?

If it is the GAUGE fuse that keeps blowing, than you either have an intermittent direct short, or something in one of the gauge circuits is drawing too much current and is starting to short out.

Here are the two Wiring Diagrams I have for the Combination Meter (gauge cluster) and the Heater & A/C:









There is actually one place in the diagrams that connect the GAUGE fuse to the Heater and A/C system. While the power source for the blower comes from a different fuse, the relay that controls the power going to the blower gets it's coil power from the GAUGE fuse. If you look really closely right above the "Heater Relay" you can see that the power source for the control side of the relay comes from the GAUGE fuse (noted: From "GAUGE" Fuse, I boxed it in red). If the relay does not have control power, the blower will not work.

Finding a short can be tricky, and there are several related methods you can try. Most people start removing loads from the circuit and see if removing the loads has removed the short. Once you know what load circuit caused the short, you can start looking in a more specific area. If all loads are removed but there is still a short, there is a wiring problem between the loads that you removed and the fuse.

For your blower fan speed problem, let's look at what we know. The blower motor is fed a 12 Volt supply but gets grounded through different paths, depending on the fan speed. The fan speeds are controlled by selecting the speed you want on the fan speed switch. The switch then routes the ground side of the blower motor through different paths where the first three speeds go through different points in the blower motor resistor and high speed gets grounded directly without going through the blower motor resistor at all.

Did you catch all that? It does look a little complicated.

Here is what that means. Since your blower fan works on HI speed, this tells you that your blower motor works, and you do not need a new one. This also means that the power going to the blower motor is good and the (direct) ground for the blower motor is good. This leaves the fan speed switch, the wires & connections going from the switch to the blower motor resistor, and the blower motor resistor itself as your potential problems. The blower motor resistor is probably your most likely suspect, followed by the switch and (sometimes the switch and its connectors melt due to poor connection).

I think the 1989 Trucks used the resistance wire coil type of resistor which is usually obvious if it is bad because one (or more) of the wire coils has broken.

The resistor is right by the blower motor near the firewall side and should have 4 wires coming out of it.

Here is a short video from South Main Auto showing the blower motor resistor problem:

 

So it is the GAUGE fuse #5 that keeps blowing.
After reading up on it, that resister definitely sounds like the culprit.

Guessing that whatever caused the resistor to fail might still causing the fuse to blow.
Going to pull it apart this weekend and take a look, have a feeling I might find signs of a short inside the blower switch or along the line running to the resistor.

Thanks for the information guys, appreciate it! (Hopefully) saved me a bunch of time playing wack-a-mole with the multi-meter

 

I went ahead and pulled the resistor for the blower last night and 2 of 3 coils were broken, have a new one on the way and that should resolve that part of the problem.

Did some more troubleshooting w/ the blowing fuse problem. Started to go through things one at a time and seeing what behavior was causing the fuse to blow.
Turns out it's putting it into reverse that blows the fuse every time. (and why I thought it would only work for 30sec before I backed out of the garage)
The reverse lights don't work at all as it turns out. All the wiring in the tail lights looks healthy, and the rest of the lights work properly. The reverse lights turn on when you put 12v directly to them in the tail light housing.
So suspecting it might be the "back-up light switch" in the transmission that's failed/shorting?
The reverse lights have to be on the GAUGE circuit and not the TAIL circuit w/ the rest of the taillights.

Both of the FSM chassis wiring diagrams I have talk about everything except for the reverse/backup lights. http://htftp.offroadsz.com/marinhake.../4lighting.pdf
Does anybody have the applicable section of the FSM for this handy? or do you know if this backup light switch is a common part to fail?

EDIT: Found this thread from surf4life, going to give these steps a try and see what happens
https://www.yotatech.com/forums/f116...e-fuse-307744/

 

Was able to track down the source of the short to a faulty back-up light switch in the trans. When connected, shorts every time it's in reverse.

Pulled the connector and bridged it with a short wire and BINGO we've got reverse lights and no more blown fuse
Have a new back-up light switch on the way.

Thanks again for the info guys

 

I am glad you found the source of your problems!

 

Yup, excellent news