I just wanted a blowing fan motor...This is a mess!

[BrentBTK]

Guy who sold me the truck said he couldn't find a switch for the heater, so he just wired on in under the dash. Ha. Turns out, said switch just went to the battery...yep....the battery. Nowhere else. Well, I see 3 wires hanging down that are taped up, Yellow, Orange, and Light Blue. I assume these must be for the switch. See Pic #1

I then find a plug with a burnt red wire, a green, and yellow going to a small gray box screwed under the dash. See Pic #2

I then find this plug with 1 Yellow and 2 Green Wires attached to what seems to be the diverter (?) box? See Pic#3

I then find this plug inside the blower box that goes to a couple coils. They are Yellow (x2), Light Blue, and Dark Blue See Pic #4

I then find 2 plugs on the outside of the far passenger side of the blower box. One is a single Orange plug, the other is Orange (x2), Dark Blue, and Purple. See Pic #5

Standby for the next post with it all in one pic and what I've determined goes where...

[BrentBTK]

So I grab my meter to see what goes where. Here is what I've determined, although I'm not sure what it means, lol.

The Yellow wires from Pic #1 (Switch?) and #3 both go to the plug inside the box in Pic #4.
The Light Blue Wire in Pic #1 also goes into the box in Pic #4
The Orange Wire in Pic #1 goes to the single orange plug in Pic #5

The Red and Yellow wires in Pic #2 both go to the Orange wires in the large plug in Pic #5.
The Green wire in Pic #2 goes to the plug in Pic #3 on the "diverter" box.

I have no idea yet where the other green wire in Pic #3 or the Purple wire in Pic #5 go, but before I started unplugging things, I didn't have voltage anywhere. I new wiring harness is getting more and more appealing... I hope my crude drawing and descriptions of things I have no idea about aren't too confusing, lol. I know I am!

[pjmoreland]

That's a fun looking puzzle you've got there. There's a schematic for the A/C harness in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=813743

[Accelo]

Persistence and a diagram will get you there. I noticed this is a air truck, a little more complicated.
Start with two things;
1) Does the orange wire have battery voltage? It should terminate at the relay. with the ORN, PPL,& DK BLUE

2) Does the brown wire on the fan switch (part of the control panel) have battery voltage

Typically one expects a single power source. This usually isn't the case. Just a warning.

[tdangle]

I will also add on a A/C cab, high fan bypasses the relay completely and pulls power straight from the fuse block.

[BrentBTK]

I do love a good puzzle! This one is gonna keep me busy for a while. That thread with the schematic is very helpful, thanks! I quit for the day and the spousal unit is taking me to the mountains for the weekend tomorrow, so I'll be standing in a trout stream thinking about wiring instead of wiring and thinking about fishing, lol.

[57taskforce]

For what it’s worth I fought mine for a day or so when I put it back together. Tested and had power to all the places that should have it in all the Fan switch positions but couldn’t get the fan to work. Ran 12v from the battery to the fan, fan would run. Wtf…
So after checking a little more, the power to the purple fan motor wire would drop out completely when plugged into the fan. In my case I pulled the silver relay bolted to the side of the inside ac box in OP’s last pic. Took it apart, cleaned the contacts in the relay, put it back together, and bam. Fan motor alive again. They had a light amount of corrosion from sitting for several years that wasn’t making good contact. Also I cleaned up the fuse and the fuse holder for the HVAC in the fuse box and that helped get a little more speed out of the fan.

[Accelo]

This should help.
Sadly it leaves some details out like the internal functions of the switch.

[BrentBTK]

Just broke down and ordered the AAW complete wiring harness. I'm not gonna chase 50 years of bad decisions.

[Accelo]

Originally Posted by BrentBTK

Just broke down and ordered the AAW complete wiring harness. I'm not gonna chase 50 years of bad decisions.

Good move. Great start to getting it running again. However, don't be surprised if it still doesn't work. At least it will be easier to figure out.
Old switches and relays tend to corrode up especially if they are not moved for an extended length of time. Hopefully you will get lucky.
God Speed.

[BrentBTK]

Originally Posted by Accelo

Good move. Great start to getting it running again. However, don't be surprised if it still doesn't work. At least it will be easier to figure out.
Old switches and relays tend to corrode up especially if they are not moved for an extended length of time. Hopefully you will get lucky.
God Speed.

Thanks. Yeah, I am going on no faith that this will fix the issue, but as you said, I know I'll be starting from a good base for everything from here on out.

[68 TT]

Make sure you have the short ground wire in the engine compartment that goes from the blower motor face plate to the adjacent blower housing mount screw in place.

Also make sure that it is making good contact with bare metal at the mount screw and at the blower motor face plate. Mine had some corrosion on the mount screw surface of the wire terminal and it wasn't making good contact. It had just enough contact to read as having continuity on a volt meter but it did not have a good enough connection to carry the amp load needed to run the blower motor. A good wire brushing was all it needed.

[BigBird05]

I just took my relay apart, cleaned and filled it and the points inside. I thought it would be better that buying a "new" one.

[toolboxchev]

Part of the problem, IMHO, is that too many amps flow through the ignition switch with GM's standard wiring. It cooks the under dash harness in many ways.

Then of course when you upgrade the alternator things just get worse. Do your electrical studies. GM used smaller gauge wiring in many instances to limit amps flowing throughout the dash circuit. Many have switched the light wiring to relays under the hood.

[Accelo]

I just checked two of the dash fan switches from Air Vehicles.
Neither worked as found. Both had not been used for years.
Here are some pictures of the internals.
I cleaned up the switch internals. Still no joy.
I guess a replacement is in order.

[truckdude239]

one of the relays in teh second pciture looks like the horm relay