Heater Fan: A/C truck

[1972ChevyCheyenne]

My fan will run in the Low and Medium, but will not run on high, fast. It will run on low speed in HI. So I tried a different switch and a different relay. Same thing. Looking at the wireing diagram I have Here off the side of the relay is 20 gauge black wire that appear that is is suppose to head to ground. So I ground is and the relay will click when the fan is at high speed only. Unground at HI speed the fan will run in the low setting (by sound). Ground The fan doesn't run at all. Are both my relays screwed? Is that black wire not suppose to go to ground? Have I made a mistake Any where else that could easily be done? Live with it as is? Any other Ideas. The A/C is off a 70 truck.

Thanks

Jamie

[jhow66]

Relay must be grounded. Check the orange wire on relay for power with switch on and heater switch on high.If no power trace orange wire back to fuse panel. Where it plugs into pigtail at panel will get loose over time and cause it to heat up. Take apart and clean and tighten this and where it plugs into panel. If thi does not work let me know.

[Hutch]

1972ChevyCheyenne
I had the very same problem on my 71, It turned out to be a broken ground wire that came off the relay that is on the inside of the cab behind the glove box on the upper right side of the a/c box,
It is very close to the right side of the truck.
The wire was pretty short maybe a couple of inches long and had broken because of the tight fit.
There is also a ground wire that is attached to the blower motor inside the engine compartment.

[chickenwing]

The relay only engages in high speed. The fan will operate in med speed in high speed switch selection (a/c must be selected also) ONLY when the relay is not engaged/bad. Power is feeding from a splice at the resistor connector. The splice gets power in every fan switch position but off (duh) and feeds power to the compressor switch. I prefer this relay:
ground wire is a pin rather than a case ground relying on a tight relay mount. Part # is a/c-12.
My diagram is a bit differant it's out of the 72 service manual.

[1972ChevyCheyenne]

Thanks Guys

I never realized that the High speed needed the A/C on. Tried it. Same thing. Off on Off, Low on Low, Medium on Medium, and Off on High. Now I say this assuming that the blowermotor on high should sound the same when it is connected directly to the battery. The relay is grounded as I just have a jumper wire going off the housing straigth to ground. When the Relay is ungrounded the High posistion will run the Heater on low. I also tried shorting the AC on switch, the one on the cable, and shorted them. Same thing. I dont have and a/c compressor on If this would make any difference.

On the plug in on the relay on all setting the 12 gauge orange wire is 12V.
On the plug on the relay, the little orange wire does have 12 volts on high only.

The little blue wire has 0V on off, 6V on low, 9V on Medium and 12V on High.

Anyone have any ideas?

Thanks for the help guys

Jamie

PS I'll try the compressor on yet tonight but the clutch is probably seized.

[chickenwing]

Was hoping to help you understand how the curcuit works. Looks like I ran you around in circles.

Read your new post. Suspect jhow66 is on the money with the 12 ga orange wire having a bad connection at the fuse block. If the wire is frayed, corroded or loose you may very well get 12 volts to the motor but won;t be able to draw enough amperage to power it. The wire WILL get warm/hot also. The reason you get the 12 volts on the blue wire in high is the relay IS working and it will jump to 12V sice there is no ground for the resistor to drop voltage.

If you care:

The high speed does not need the ac on to work. It is just a pecularity of the circuit that the fan will operate in low speed when the A: the fan switch is in any position other than off and B: the relay is for whatever reason stuck open.

The fan switch explanation on the wring diagram shows the "L" pin energized in every on position. Follow the "14 yel" wire from the switch to the resistor connector, through the resistor to the "14 DK Blu" wire through the open relay to the "12 PPL" fan wire. That is why the fan will work in low speed when in the switch is in high and the relay is not energized.

[mrein3]

I ran into a similar problem when converting my non-AC cab to AC. I had 6 volts on low, 9 on medium, and 6 on high. I bought a relay in a bubble pack at the local auto parts store. I used the 6 volts from the high position on the rocker switch to control a new, orange 12 gauge wire from the junction block on the fender. I ran the new orange wire through an in-line 20 amp fuse just incase.

Now I can get high speed no matter what else is happening on the control head.

[1972ChevyCheyenne]

Thanks For The Help Guys. It runs on high speed now. Change the relay back to the other one and it works. I still don't know what i did different but anyway. Thanks again
Jamie