1980 K20 Heater Issue

[ChevyFreak60]

What's up guys. I have a 1980 K20 I bought a few months ago that I am having heater issues with. I have replaced the fan motor and the switch. I also checked the fuses and I am unable to get power to the fan motor. The truck warms up inside after a while once the trucks warmed up. Anyone have any idea what might be going on? Thanks for your help.

[tucsonjwt]

I would trace the wire to the fan motor back to the source. Also, blower motor resistor is a usual suspect.

[old Rusty C10]

could be a contact in the harness just keep tracing wires

[ChevyFreak60]

Thanks guys! What does the blower motor resistor look like?

[tucsonjwt]

In the engine compartment on the HVAC box near the firewall:

http://www.rockauto.com/catalog/more...102&cc=1051849

[Bigstevex4]

there easy enough to bypass

[Your41Plague12]

Originally Posted by Bigstevex4

there easy enough to bypass

And also easy enough to replace. Two screws, and about $7 at a parts store. Do it the right way.

[motornut]

look here

[hatzie]

The resistor pack is not in the fan circuit on High so a bad resistor will still allow the blower on High.
If you have A/C and it doesn't work on High then the fan relay ( AC Delco 89057581 ) is likely kaputski or you have a fan power wiring problem. The relay is 1/4 to 1/2 way down the passenger side of the A/C box.

Do you have volts, with the ignition switched on, at the Brown wire on the fan switch plug?
...Note.... If it's a "non A/C" cab there are two Brown wires in one terminal at the fan switch and one Brown wire continues from the switch power terminal to the LOW resistor terminal in the engine compartment. On A/C the Brown wire shares the fan power terminal with a Yellow wire that goes out to the Relay on the LH side of the AC box on the firewall.

There should be an ATC fuse marked HTR at the top center of the fuse block. This is what feeds the Brown fan power wire.

There is a Packard 56 disconnect in the Brown fan power wire shown in the wiring diagrams. It's probably pretty close to the heater controls and it's present on A/C and Non A/C Cabs making the heater-A/C a sub-harness.

[motornut]

Look for the brown plug to the left of the AC vent hole in the metal dash.
That's where I noticed the plug is on my non- ACs

[ChevyFreak60]

This is what my truck looks like.

[hatzie]

Marked it up with circles.
Left Top RFI Motor Noise Condenser
Left Bottom Fan Motor Power
Right Top Resistor Pak
Right Bottom A/C Fan Relay

If you have no fan on High you have No power at the switch, Dead A/C Fan Relay, or maybe a bad condenser. The resistor could still be dead but the fan will still run, on High only, with a bad resistor.


The 1980 GM wiring manual is in my signature. RPO C60 A/C has its' own sub harness diagram.

Check your Brown fan switch power wire at the switch with a meter. Alternatively the Yellow wire on the A/C Relay is the same circuit. It should have power when the ignition is on.

Does the fan have a ground strap? It looks like there is one to the left of the condenser. Is it hooked to sheet metal? No ground will keep the fan dead.

[85sierraclassic]

So is the rfi resistor inline with the purple wire? Purple doesn't look hooked up

[hatzie]

It's not a resistor it's an RF noise suppression capacitor.

[85sierraclassic]

Yeah was watching tv and not paying attention lol, how does that wire with the purple wire, I don't think mine has the noise suppressor on it

[hatzie]

Heater A/C wiring diagram. Purple is the fan power....

[ChevyFreak60]

Thank you for your help Hatzie! I really appreciate it. So I replaced the ac fan relay and still nothing. I remembered you mentioning the ground wire so I checked it out and there were two wires going to the firewall. There was black tar or something and lots of build up from the years. I scraped it off and made a contact. I tried the wires together and nothing. Then I tried the one from the condensor and the fan turned on. I tried the other wire coming from the lil harness and nothing. I have no idea what that wires for. I left the wire screwed in that worked the fan but there is no power to the switch still......what else could be causing this?

[hatzie]

Throwing parts at it isn't going to work. Hail Mary repairs are expensive unless you're damn lucky. The AC Relay was a reasonable WAG. At least it's cheap in comparison to the fan.
Troubleshoot it before you throw any more parts at it...

BYPASS the RFI condenser by plugging the purple wire from the fan relay directly to the fan terminal. You probably don't listen to AM radio so it doesn't matter for the moment.
No fan? Leave it hooked up this way.

Check what could've been disturbed in the motor replacement next.

Break out the multimeter and set it for resistance or the continuity buzzer.
You should have a ground strap or wire from the motor case or one of the motor retaining screws to the firewall. Test it with the meter. If it doesn't test OK you will not have a fan on any speed.

Unplug the Relay socket and test the relay socket connections.

Is the Black Fan relay Ground wire connected from the fan relay socket to the firewall? test it with the meter. No ground means no fan on high.

Set your multimeter to Volts DC and follow the electrons to find out where they stop.

Test the RED wire that runs from the firewall junction stud to the AC relay socket. Do you read volts with the meter ground on the black wire terminal and the meter + on the red wire terminal? Is it powered? Is it there? NOTE this is battery power. No power on the Red wire means no fan on High.

Are there volts on the orange wire in the relay socket with the fan switch on high and the ignition on? No volts here means you don't have volts inside the cab at the fan switch or you have a damaged harness between the switches and the AC box.

Are there volts on the yellow wire at the fan resistor plug with the ignition on? NOTE Fan switch position is irrelevant.
No volts here means you don't have volts at the OFF AC VENT HEAT BI-LVL DEF "mode selector" switch (Selector Switch on the wiring diagram). This could be a blown HTR fuse or an unplugged brown wire at the sub harness disconnect or a break in the brown wire between the fuse panel and the selector switch or you have a damaged harness between the switches and the AC box.

Volts on the Yellow wire and no volts on the Orange wire means the Mode Selector is not sending volts to the fan switch.

Make up a 4" 16ga jumper wire with male 1/4 blade disconnects on the ends.
Unplug the electrical plug on the mode selector switch and plug your shiny new test jumper from the Brown wire terminal to the Brown/White wire terminal. Check the fan. Fan works? The mode selector switch is bad... You don't need the electrical part of this mode selector to work if you don't intend to make the AC functional. As long as the mechanical and vacuum portion works and you can jumper power to the fan switch you're golden. It's a valid repair to secure the jumper wire in the plug and tie the plug and wire up so it doesn't short on stuff.

[79K20]

[QUOTE=hatzie;6987330]
Heater A/C wiring diagram. Purple is the fan power....[/QUOTE

Hatzie, I have a 79 k20 Cheyenne and am having the issue of the battery draining. I have tracked it down to the purple wire you are talking about on this post that goes from the blower motor relay to the blower motor? So if I leave the purple wire connected the battery will drain completely over a couple days.. with the keys out, if I disconnect the purple wire the battery does not drain... but when unhooked the blower motor has no power. I replaced the relay and it solved nothing. The purple wire has constant power all the time on the relay side of things. I have Spent days looking at wiring diagrams and can't figure it out. Any ideas on where to look next? Would the resistor have anything to do with it?? I'm lost and am tired of popping the hood to hook up the purple wire every morning
Posted via Mobile Device

[wilkin250r]

Hatzie has the wiring diagrams, he's the expert, but I'm thinking the issue is NOT the purple wire. I'm thinking the issue is back at your ignition switch.

How many other functional accessories do you have on your truck? Any radio, lights, anything like that?

I suspect that everything hooked to ignition/accessory power on your car is still "ON", even after you turn the key off. But you don't notice them because your radio is gone (an educated guess), and any courtesy lights are off. The only thing left that you would notice is the fan power.

That's my first guess.

[79K20]

Originally Posted by wilkin250r

Hatzie has the wiring diagrams, he's the expert, but I'm thinking the issue is NOT the purple wire. I'm thinking the issue is back at your ignition switch.

How many other functional accessories do you have on your truck? Any radio, lights, anything like that?

I suspect that everything hooked to ignition/accessory power on your car is still "ON", even after you turn the key off. But you don't notice them because your radio is gone (an educated guess), and any courtesy lights are off. The only thing left that you would notice is the fan power.

That's my first guess.

All my other accessories work just fine, radio and dash lights included. I was thinking it could be the ignition switch but just super lost because when that purple wire is not connected the battery doesn't drain
Posted via Mobile Device

[hatzie]

The purple wire is the common terminal from the SPDT blower relay switch contacts.

Output is switched as follows...
1. Power from the blower switch through the resistor pack with no power to relay coil.
2. Raw battery power from the J-Block stud on the firewall with the ignition on and the blower switch set on HI which energizes the relay. It's unlikely the root cause of your electron leak. Easy enough to rule out. Disconnect the ring terminal at the J-Block that runs to this relay leg and re-test. If this fixes the electron leak you have a bum relay.

You probably have a leak into the HEATER fuse wiring. The heater fuse should only be powered with the ignition on.

I'd test the fuses that are supposed to be off with the ignition off to rule out the ignition switch or back bleeding from non factory wiring.

Rich and VileZambonie should not to be ignored on wiring. Both of them are much better teachers than myself.

[79K20]

Originally Posted by hatzie

The purple wire is the common terminal from the SPDT blower relay switch contacts.

Output is switched as follows...
1. Power from the blower switch through the resistor pack with no power to relay coil.
2. Raw battery power from the J-Block stud on the firewall with the ignition on and the blower switch set on HI which energizes the relay. It's unlikely the root cause of your electron leak. Easy enough to rule out. Disconnect the ring terminal at the J-Block that runs to this relay leg and re-test. If this fixes the electron leak you have a bum relay.

You probably have a leak into the HEATER fuse wiring. The heater fuse should only be powered with the ignition on.

I'd test the fuses that are supposed to be off with the ignition off to rule out the ignition switch or back bleeding from non factory wiring.

Rich and VileZambonie should not to be ignored on wiring. Both of them are much better teachers than myself.

Greatly appreciated the help! I'll be checking everything out this weekend. I'll let you know if I get the issue resolved!
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[chuychevy82lwb]

Update ? Im having the same issure but my ac compressor doesnt kick on either

[iriereggaeman]

Hi Guys. Just wanted to drop in here to say 'thanks' for the tips. I had a similar problem that I was able to resolve with this info. I ended up posting some details about my experience here: http://67-72chevytrucks.com/vboard/s...64#post8073264
Awesome community!