71 Cheyenne AC Help

[hammered68]

I have been in the process of bringing my heater and a/c to life on my 71 Cheyenne. A few months back, after replacing the high speed relay, switch ect., I was able to get it to work on only low and medium speeds. I traced down my lack of high speed issue to the anti diesel relay. I was unable to find a new relay at the local parts stores, however I located another relay lying around the shop and tried it out. Everything worked as it should until three weeks ago. While out on an afternoon trip this relay apparently burned out. Not only did I loose high speed on my a/c, but the compressor would not engage. I am able to place a jumper wire in the actual harness that the anti-diesel relay plugs into, and it will allow the blower to blow on high speed, and will also allow the compressor to engage however, the compressor will stay engaged unless this jumper is unplugged. I've sense bought a new anti diesel relay off of ebay, which came in today. It's NOS, but did not fix the issue. I've been hoping to keep everything as original as possible but I'm at my wits end on this a/c issue. I have researched the anti diesel relay and learned the original part number from GM was 3996203, which is the first relay I took out. I also learned that GM replaced it with part 6273301, which is what I found on ebay. Long story short, does anyone have any ideas? I truly dont want to have a jumper wire hanging there on an original un-butchered bb truck.

[Andy4639]

Just a couple of thoughts.

Since the last time it burnt up the relay did you go back and check the main fuse under the dash?

The switches in the dash unit are bad about going out also. Check the plunger on the back side of the vent in the floor which is the on/off switch for the ac compressor.

[hammered68]

The fuses are all ok, pulled and checked them first. I'm trying to think where the plunger is you are talking about, I'll head out and start snooping around see if I cant find it.

[hammered68]

Dont know what I was thinking when you described the switch, guess I had a slight laps in my memory. This was one of the items that I originally replaced. I pulled it out and found that it was fried. Tried two additional switches and still nothing. I noticed that the wiring harness to the switch was melted as well. Thinking I have big issues. What are anyone's thoughts on just changing out the trucks complete wiring harness? I'm thinking if it was all replaced i could simply bypass the anti-diesel switch all together? Am I correct on that thought? Also ifin I did do a complete harness, any recommendations on brands?

[Andy4639]

That's it. Take the dash unit out and check the back side of the fan switch.
Get a test light and start tracking the power to the switch and so on until you find where the juice stops.

[Davidf]

When all wiring is in good shape, everything should work WITHOUT the anti-dieseling relay installed (except the anti-dieseling function, of course). I had similar issues as you did when I purchased my truck and careful understanding of the wiring diagram allowed me to trace all circuits and find the issues.

Have you checked the high speed fan relay. Located on side of heater core box (under glove box, right most side).

[hammered68]

I have checked it, and have replaced it twice to see if the second one solved the issue, all to no avail. thinking seriously about replacing the entire under dash harness with a new one from american auto wire and using an a/c harness from a 70 model without the anti diesel relay.

[Hutch]

hammered68
There is a relay behind the Glove Box up high on the firewall very close to the passenger side cowl (Near the Antennae) that has a short black ground wire with a ring connecter that is supposed to go to frame ground.
When Folk's work on the heater cores them either does not attach it correctly or not at all.
This can be tested with a short (2 Ft.) length of wire.
Turn the Fan on High and attach one end of the wire to ground and then to the Black wire
The black wire is supposed to be under the stud nearby that holds the air box together.
This may not be the issue but this is what I found when my high speed on the fan was not operating on High Speed

[hammered68]

Thank you sir, just closed up the shop for the night, will check it out first thing in the morning and let you know what I find out.

[Davidf]

Originally Posted by Hutch

hammered68
There is a relay behind the Glove Box up high on the firewall very close to the passenger side cowl (Near the Antennae) that has a short black ground wire with a ring connecter that is supposed to go to frame ground.
When Folk's work on the heater cores them either does not attach it correctly or not at all.
This can be tested with a short (2 Ft.) length of wire.
Turn the Fan on High and attach one end of the wire to ground and then to the Black wire
The black wire is supposed to be under the stud nearby that holds the air box together.
This may not be the issue but this is what I found when my high speed on the fan was not operating on High Speed

This is the fan high speed relay. The ground wire is suppose to go under the screw that holds the relay to the heater core box. Without the ground, it won't operate at all.

Again, the OP: Don't do away with the anti-dieseling relay circuiting. If the wiring is in good shape, the anti-dieseling relay can be removed and everything should still work perfectly (except anti-dieseling). But, you WILL want anti-dieseling for the cooler season when you are not using the a/c.

[hammered68]

Ok, so I've searched high and low on everything that we spoke about thus far. Today I began tracing wires and followed the orange 12 ga wire from the anti diesel relay to the high speed relay back to the fuse panel. I found it had no power, anywhere along the route, but still had low and medium speed on my fan. Further searching, I found the fuse just below this terminal, labeled for the glove box & spot lamp burned out. I replaced this fuse and everything came to life. Now, I have constant hot to this 12 ga wire even when the ignition is off. Is this supposed to be a constant hot? I noticed that when I turn off the ignition with the a/c on, it will stay on for a couple of seconds after the ignition is off.

[Hutch]

hammered68
Sounds as if the High Speed is working
That is great because it verifies a lot of the components are good like the Blower\Fan\Switch\Relay etc.
The blower should be switched off with the ignition
It sounds as if you have a bad ground somewhere in the circuit that takes a while to bleed off. Maybe the engine ground strap?
I am unsure if the fuse and hot wire location you are using is correct on the fuse panel

[Andy4639]

Can you get some pictures of the fuse block and which fuse it is?

[Davidf]

The heavy gauge orange wire connects to the terminal labeled "CIG".

Also, I recently discovered (service bulletin) that the brown (field) wire at the voltage regulator should have a blocking diode installed to prevent the anti-dieseling relaying from being damage. GM says this should be used in addition to the updated anti-dieseling relay 6273301 that has a blocking diode built it. Apparently, early trucks/relays did not have either diode and the result was power feedback that damage the relay and caused constant 12v to the a/c compressor clutch thereby resulting in a drained battery.

[Andy4639]

Originally Posted by Davidf

The heavy gauge orange wire connects to the terminal labeled "CIG".

Also, I recently discovered (service bulletin) that the brown (field) wire at the voltage regulator should have a blocking diode installed to prevent the anti-dieseling relaying from being damage. GM says this should be used in addition to the updated anti-dieseling relay 6273301 that has a blocking diode built it. Apparently, early trucks/relays did not have either diode and the result was power feedback that damage the relay and caused constant 12v to the a/c compressor clutch thereby resulting in a drained battery.

That is interesting info. Can you be nice enough to share where this info came from. Where did you find a service bulletin for these old trucks?

[hammered68]

Andy4639 here is a photo of my fuse block. As I look at this photograph it appears that a wire had been cut off of the harness I have labeled a/c, could this have been the original cig lighter? Also, as mentioned by Davidf, if this wire was placed originally in the cig terminal, that would lead me to believe it was Chevrolet's intention to have a constant hot? Hope this makes sense...

[Andy4639]

Here is mine. Ac wiring and cig wires both come off the cig terminal. I will check it tomorrow to see if it's hot all the time.

[hammered68]

Thank you Andy4639!

[Andy4639]

Mine has power all the time as well!
Which I knew it did but I wanted to verify it with a test light.

[hammered68]

Awesome thank you sir! Gonna drive it and enjoy the remaining time of a/c weather hand hopefully it will all hold together!