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Need some serious electrical help with the new truck...

Stomis

Stomis

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Copy and pasted from gmt400.

When I bought the truck it had no heat hooked up and was told the wiper motor was fried. So I bring it home and run some heater lines to the core. Let it warm up and go to test the new heat and Ive got no blower. So I start poking through the motor, resistor, and switch with a test light. Come to find out the fuse is missing, der stupid oversight. Plug in fuse and Ive still got nothing. So now Im really stumped. Out of shear curiosity I decide to try the wiper with said fuse replaced. Turn wipers on and on goes the blower...

Now Ive got the blower on, and the speedo is bugging out to the tune of 60mph with the blower on "high" (its not getting full juice I can already tell its running slow). So I fiddle with the wiper switch some more and the motor starts making terrible noises like a buzzer almost. My first though was theres a circuit crossed in the wiper motor and its back feeding the ground causing weird to happen. So I replace the motor but much to my disappointment the issue persists. AND I can get a brand new wiper motor to make this buzzer noise...

This is a 94 GMC originally had a 4.3/5spd but is now a carbd 355/5spd heat only truck. All the original gauges are hooked up and function and everything else is working normal. Ive thumbed through the wiring diagrams but cant seem to come up with any shared circuits between the ac/heat and the wipers. I dug around in the dash and dont see any crushed or botched wires. I inherited this project from someone else and already cleaned up some of his wiring. its not terrible but not great but it all pertains to the dizzy, fans, and engine peripherals...


Ugh help?
 
So I need some help guys. Ive tried unplugging the wipers, blower ground, wiper switch, blower resistor all one by one to see if it makes a change. Ive got nothing here.

From my trouble shooting the long and the short of it is the blower circuit is not getting any power w/o the wipers being turned on. Im assuming the funky wiper stuff will solve itself when I figure out where in the hell the two circuits are crossing.

Im also leaning towards a ground because the speedo is picking up a pulse signal from the blower when it is on...
 
Probably the quickest is to follow the blower motor power wire from the switch. As incredible as it sounds, it seems the wiper and blower motor are in series.

Quicker thing to try, is to get a fairly heavy wire, run it from the ground at the battery, and try touching it to the motors as they are running. If its a ground problem, something should change.
 
Ok fordum as suggested I isolated some grounds and got the wipers working, mostly...

Something was backfeeding hot through the wiper ground. Sometimes it was full voltage, sometimes less. So I clipped the wiper ground and gave it a dedicated ground stud on the body. That solved all my problems wiper wise except my wipers wont turn off now, they seem to like to go to the slowest intermittent speed when turned off. I suspect this is the switch though.

Now on wards to the blower and my new problem that came from fixing the wipers w/o actually finding the source of the fubar.

Blower is still dead. It has juice at the fuse but no where else. Not at the switch, motor, resistor nada. Still gotta dig into this one. It took me a while with the wipers before I said screw it and just grounded it.


My new issue is that my speedo is dead. Every once and a while it will work when on the brakes (again with **** finding grounds in all the wrong places...) Im guessing I better just trace the wiper ground back and find where its pinched into a hot wire...
 
At that point, I'd almost be considering finding a not hacked harness. Not sure how hard thst would be out there tho.

If nothing else you could upgrade the gauges to fix it.
 
Do a good visual before you dive in. Might save you some time. Look at any and all obvious pinch points and wear points, bulkhead or support pass throughs.

Also, I would concentrate on that fan motor hot wire. If you are sure you have power at the fuse, and none at the switch, then you have narrowed the problem's location to the wire between those two points.

Odds are all the problems are related, so if you find that spot, you will find all the troubles.
 
Focus on the ground wires. This is a classic grounding problem. Don't think it is just the blower and wipers that may have a failed ground either. Check the cab to engine and frame connections as well as the main harness grounds behind the instrument panel
 
Grounds where the first thing I went for. Engine to battery I'll double check but instrument panel to column is good, body to frame is good too.

I'm no electrical genius but if my wiper motor ground in the harness is still back feeding hot after being cut off from the motor don't I have to have a wire pinched into that somewhere to make it hot?
 
Very possibly. Electricity follows the path of least resistance, so a bad or missing ground somewhere else can cause electricity to flow where it shouldn't. That is always the risk when taking over a truck someone has worked on. Often its the simple things. As others have said, make sure the grounds are good, and trace the circuit back. When I got my truck the blower motor didn't work, all because of a loose ground wire, yet it was the last thing I checked.:doah:
 
Still searching for this. I'm gonna tear into it tonight. I wire a fan controller and redid all the PO's wiring. The issue wasn't hiding in there. I started stripping the engine harness back of all unused wires and gonna strip back the body harness next to figure out where that wiper ground is grabbing juice.
 
OK so I fixed it, sorta. I stripped my harness all the way down and found that the PO never hooked up the engine harness ground on the back of the pas side head. So I go Aha I got it now, except it didn't change anything. So I jump juice into the back to the Power switch and everything works great. So I trace the hot lead since its dead and find no issues or breaks. Turns out I've got no power in the fuse block feeding the blower switch. Since all the wiring diagrams start at the fuse block I'm assuming my ignition switch has a bad contact in it. I said screw it for the time being a rigged up a fused jumper from aux ign hot over to the blower circuit. Wallah.

As far as the wipers not wanting to turn off I replace the pulse board on the new motor and it still does it. So I busted out the good test ,meter and for what ever reason my switch never actually goes to off. In off it thinks its on long delay. So I gotta get a new switch.

Only thing left not working is the speedo now. Which is odd cause it worked when I bought it. And when the gremlins were in there it was working when I was on the brakes... Guess I'll have to start poking around with the vss box?
 
Sounds like you are still not getting power where you should. Since I suspect the brake lights were powering the speedo box.
I had a GM schematic book around here somewhere that was 90 something. And it had all the power layout.

I'll look around some and see if I can find it again.
 
Fordum said:
Sounds like you are still not getting power where you should. Since I suspect the brake lights were powering the speedo box.
I had a GM schematic book around here somewhere that was 90 something. And it had all the power layout.

I'll look around some and see if I can find it again.
Click to expand...

Well the break light switch also breaks 2 or 3 connections when the brakes were applied. Im assuming that the speedo box was finding a ground through one of these circuits when it was no longer being used. The ground I found missing on the back of the head was a big deal I would think. It was the splice to every ground in the engine harness. The speedo box however has its own ground Im going to check today and then check its receiving 12v. Also there is the possibility the VSS has gone out on my since it was a used one from the used 2nd trans but I doubt that.

I really dont think the lack of blower power and lack of speedo power are related, at least I hope not lol. It could be the same leg in the ignition switch not working. Im going to jump power around to it and check the ground today and go from there.
 
Well forum you mofo. You were right. The speedo isn't working for the same reason the blower circuit isn't. The brake fuse is dead at the fuse panel. I jumped power into it and Wahl ah the speedo works no problem. I'm still going with there's a dead leg on the ignition switch that feeds those two. Circuits on the panel. I need to find a wiring diagram from the ign to the fuse panel and I bet they both come from one splice....
 
Man, you are looking this backwards.
A fuse is a protection device.Some fuses might be switched, but the normal progression is: Power source>fuse>switch>powered device.

You want the fuse to protect as much of the circuit as it can, so it is almost always the first thing after the power source.
Plus, how many cars have you ever seen where the brake lights don't work with the key off??

The brake light fuse should be hot at all times. If the speedo gets its power from that fuse, and the speedo is only hot when the key is on, then the key switch is after the fuse.
 
I better elaborate a little....

Yes, lots of times fuses are switched. But those are almost always secondary fuses. There is usually a primary fuse before them.

For instance in the fuse block, you will have hot all the time fuses, ignition hot fuses, and accessory hot fuses.
But the ignition switch which controls the last two types will its self have power at all times through a fuse of some kind.

We really need to find a schematic. I can't find my book, but having the brake fuse power both the speedo and the fan does not seem right.

I think when you put power to the brake fuse, you are back powering a whole section of the fuse block that is supposed to be hot all the time.

And that is usually powered by either a fuse link, or in the newer cars, one of those big honking fuses.

If the PO left the main ground off, he may have left off a fuse link too......

If you have those big fuses, take the hot wire back off the brake fuse, turn the blower motor and the ignition on, and measure across all the big fuses with your voltmeter set on DC volts.

If you get battery voltage across one, its blown. Always measure, don't go by looks when things get strange.
 
Hmmm I understand what you're saying but oddly enough the brake lights still work and always have (once I fixed the switch). Guess Im gonna check for a bad fuseable link. Theres no maxi fuses in this truck its still the old interior.

I feel stupid for checking all this stuff backwards BUT I coulda sworn when I poked the fuse panel when I first got the truck everything was fine...

:doah:
 
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