does not work: climate fan for vent or heat; a/c won't engage

[hplacecf]

hi.

1979 chevy pickup.
350 engine

just got it and trying to get things going.

the climate fan switch when switched on (any of the speeds) does not work, not does the a/c come on when the slide is set to a/c.

when I start the truck i hear the blower motor come on (I think it runs continuously maybe?), so that works.

I found a fuse on the fuse box and swapped it out with a fuse I know works, but still no good.

the fuse i found was 'b' on the 1980 chart at this link:

http://www.chuckschevytruckpages.com/77fusebox.html

any ideas?

I searched and searched but could not find anything.

thanks in advance

[Liberty]

Could be a bad switch or loose contact.

[tucsonjwt]

We had a related thread here recently which started with a blower resistor question, so you might look at that.

My similar issue was due to no power to the controls, which was all because of a bad power harness at the heater control. I would remove the heater control and start wiggling the harness connector while operating the controls and see if it makes any difference. You can check for power at the harness using a 12 volt continuity tester and see if your heater control is the problem or if power supply is your problem.

You will likely have to remove the dash pad to get at the harness, unless you are a very small and agile person.

[hatzie]

On the 79 AC trucks the fan runs on low as long as the ignition is on. Even if you unplug the mode switch the fan will run on LO because the power runs directly from the fuse panel to the LO contact on the blower resistor. The LO position on the blower switch is not hooked to any wires.
Med - HI positions on the blower fan switch get power through the mode slider in the AC to DEF positions. If the Mode slider is set to OFF no power goes to the blower switch so it defaults to LO.
The blower relay switches when the blower switch is set on the HI position. The relay routes power directly from the firewall J-block to the blower on HI. The HI power doesn't run through the blower switch.
The LO & MED positions run through the resistor to the blower through the normally closed relay contacts.
I believe the AC compressor should get power in AC Bi-Lev & DEF positions.

[hplacecf]

Thanks for the detailed info on what to check.

So far, here is what I found so far:

(1)
the blower fan does come on at low speed once the truck is stared and stays on low speed no matter what the setting of the climate control is set at. so medium and high do not work. a/c never clicks on even if set to the a/c setting.

(2)
the climate control works as far as where the air should come out. if I slide it to 'heat' it comes out the bottom; if I slide it to vent I comes out the vents; if I slide it to defrost I think it comes out there as well - hard to tell as the air flow is so low/limited at low speed.

(3)
the climate control unit lights come on once the lights are turned on (so you can see the settings) , so at least some power is getting there.

given these facts,
(1) shows that the fan is working and
(2) shows that at least some of the function of the climate control unit is working, and
(3) the lights are on so at least some power is getting there.

I still need to get behind the climate control unit and check the wiring, but that (as suggested) does not look easy to do.

any ideas given the above?

[hatzie]

The mode slider is a cable actuator and an electrical switch.

The wiring up to the heater controls is intact and OK. We know this because the blower works on LO.
Look at the wiring diagram and you'll see why the blower runs on LO with the ignition on. There's always power to the LO connection on the blower resistor.

You can download the whole 1979 wiring diagram and both service manuals from the RTFM links below.

[hplacecf]

Originally Posted by hatzie

The mode slider is a cable actuator and an electrical switch.

The wiring up to the heater controls is intact and OK. We know this because the blower works on LO.
Look at the wiring diagram and you'll see why the blower runs on LO with the ignition on. There's always power to the LO connection on the blower resistor.

You can download the whole 1979 wiring diagram and both service manuals from the RTFM links below.

thanks for the links. I am at a loss on how to proceed. I know enough to be dangerous , but not much more.

what should I do next?

thanks, and sorry for my ignorance...

[hplacecf]

ISSUE RESOLVED !

it was this:

Originally Posted by tucsonjwt

My similar issue was due to no power to the controls, which was all because of a bad power harness at the heater control. I would remove the heater control and start wiggling the harness connector while operating the controls and see if it makes any difference. You can check for power at the harness using a 12 volt continuity tester and see if your heater control is the problem or if power supply is your problem.

I took off the face plate to the instrument panel and controls. I then loosened the 4 bolts that held in the climate control unit to remove it. As I was wiggling it to get it out of its hole in the dash the fan came on. So, root casue is found.


Thanks to all for the advice, links, and ideas.

Gene

[hatzie]

Look at the diagram.

Some rules of thumb that assume the wiring is OK.

1. If the blower doesn't work on any speed the heater fuse or wiring from the fuse panel is suspect.
2. If the blower only works on LO the problem is in the heater control switches.
3. If the blower runs on HI and not on MED or LO the resistor is suspect.
4. If the blower runs on LO and MED 1&2 but not HI the blower relay or relay power is suspect.

Check the battery voltage. This is your target test voltage. 12.5V here means you should see between 12.5vdc and 12vdc.

Check the mode switch.

1. With the Mode lever in any position other than OFF the BRN/WHT (Brown with White stripe) wire from the mode control to the blower switch should show battery voltage at the blower switch.
2. Light Green (GRN LT) runs from the mode switch to the AC compressor through the LPCO (discharge pressure cutout switch) When AC is selected with temp slider on Cold this wire should have 12v to the first safety switch.
3. .....If one or both wires don't have battery voltage the mode switch is borked. You may be able to clean it up with De-Oxit D5 but don't count on it.

If the mode switch tests OK then test the blower switch.
With the ignition switched on... check Blower resistor voltages in the engine bay with a digital volt meter.
You know the Yellow wire on the resistor has 12v with the ignition switched on because the blower is on. This is your sanity-check to make sure the meter is connected to a good ground. No matter what happens with the heater controls this wire will always be hot with the ignition on because it is directly connected to the heater wire from the fuse panel.

1. Flip the blower switch up one position to MED1 and check voltage on the TAN wire.
2. Flip the blower switch to MED2 and check the voltage on the BLU LT (Light Blue) wire.
3. No or low volts in steps 1 & 2 replace the blower switch.
4. If the blower works on LO & MED now and not on HI check the Orange wire on the blower relay with the switch set on HI.
5. If the blower works on LO & MED and not on HI with a new switch and you have volts in the last step... check your volts at the red wire in the blower relay socket. There's a fusible link protecting this wire at the J-Block stud on the firewall next to the brake booster.

[hatzie]

I see you found it the easy way when I was writing up the above. Good.

[FarmerDale]

Hattie
Thanks for the write up on diagnosing fan speed issues. I used it while reinstalling the climate control on my 87. I ran into a scenario that your diagnosis did not address:

Key on - switch in LO - will not run. I can move lever to another speed and the fan will run at that speed. I can return the switch to LO and the fan motor will run in LO speed.

Key on - switch anywhere but LO - fan will run at that speed and each of the four speeds.

In other words, fan works in all four speeds as it should, but the motor will not start at LO

Motor was bought new from NAPA Nov 2014 and this is the first time power has been run to it.

Thanks

[hatzie]

Hmmm. That's unusual.

How does the connection look on the LOW terminal of the resistor? Is the blade terminal clean on the resistor and there's no corrosion in the female connector? Is the female Packard 56 terminal tight on the LOW terminal blade?

Maybe the brand new blower is too tight to start on low till it warms up a little? I can't remember if the bushings are accessible to lube them...

The 87 mode doors are at least partially vacuum controlled too but this doesn't affect the blower.

[FarmerDale]

Update on my issue. Connections looked cleaned. Battery needed a good trickle charge so did that. Had same issue but ran fan a little longer on Med. Now it seems that fan wants to start on LO. I think it needs exercise and lubricatiom to function properly.

[1985-GMC]

Originally Posted by hatzie

Look at the diagram.

Some rules of thumb that assume the wiring is OK.

1. If the blower doesn't work on any speed the heater fuse or wiring from the fuse panel is suspect.
2. If the blower only works on LO the problem is in the heater control switches.
3. If the blower runs on HI and not on MED or LO the resistor is suspect.
4. If the blower runs on LO and MED 1&2 but not HI the blower relay or relay power is suspect.

Check the battery voltage. This is your target test voltage. 12.5V here means you should see between 12.5vdc and 12vdc.

Check the mode switch.

1. With the Mode lever in any position other than OFF the BRN/WHT (Brown with White stripe) wire from the mode control to the blower switch should show battery voltage at the blower switch.
2. Light Green (GRN LT) runs from the mode switch to the AC compressor through the LPCO (discharge pressure cutout switch) When AC is selected with temp slider on Cold this wire should have 12v to the first safety switch.
3. .....If one or both wires don't have battery voltage the mode switch is borked. You may be able to clean it up with De-Oxit D5 but don't count on it.

If the mode switch tests OK then test the blower switch.
With the ignition switched on... check Blower resistor voltages in the engine bay with a digital volt meter.
You know the Yellow wire on the resistor has 12v with the ignition switched on because the blower is on. This is your sanity-check to make sure the meter is connected to a good ground. No matter what happens with the heater controls this wire will always be hot with the ignition on because it is directly connected to the heater wire from the fuse panel.

1. Flip the blower switch up one position to MED1 and check voltage on the TAN wire.
2. Flip the blower switch to MED2 and check the voltage on the BLU LT (Light Blue) wire.
3. No or low volts in steps 1 & 2 replace the blower switch.
4. If the blower works on LO & MED now and not on HI check the Orange wire on the blower relay with the switch set on HI.
5. If the blower works on LO & MED and not on HI with a new switch and you have volts in the last step... check your volts at the red wire in the blower relay socket. There's a fusible link protecting this wire at the J-Block stud on the firewall next to the brake booster.

I figured I'd wake this thread back up as I'm having similar problems.
A couple months back my a/c, heater, blower motor completey stopped working. I couldn't figure it out so I gave up on it. Now that it's getting pretty cold I want my heater working again. Anyway.. the blower motor won't come on at all now. I have power from the red 10 gauge wire coming from the junction block on the firewall to the relay on the box and I can get the blower motor to kick on high by running a jumper to the orange wire at the relay but otherwise it doesn't come on. I don't see any fuse for the blower and it seems like the climate control harness was independent from the rest of the harnesses and not tied into the fuse box. I've cleaned and greased the connections at the b/m relay, b/m resistor, fan switch, heater/ a/c switch, taken apart the heater/ a/c switch and cleaned & greased it. Any ideas would be greatly appreciated.

[gmachinz]

Did you inspect the grey connector behind the climate control panel? The brown w/wht wire supply all power to the blower motor thru the selector switch and this voltage goes through the relay as well. Over time this connectorngets really hot/brittle and should be considered a failure point in these trucks-on my climate control harness this is one aspect I upgrade-make sure you inspect it for damage.

[1985-GMC]

No not yet but I will tomorrow. I just looked at another harness I saved from the same year truck(78) and it does have a single connector that splits off from the inside portion of the harness separate from the fan switch and climate control switch. It's a black wire and black connector but that does give me something to go on. Thanks

Edit: I unrapped the black wire and it loops straight back to the climate control switch so it does make sense that that would be my power wire coming from the fuse box.

[1985-GMC]

Ok so I feel like an idiot but I figure I should update this. It turns out that it was a fuse. Top right fuse in the fuse box (last year for glass fuses), it looked good but inside one of the ends the solder apparently got hot and deteriorated then broke connection. It's hard to read what that fuse is labeled as but I think it's either HTR or MTR.

And the black (or dark brown) wire I was talking about above does go back to the fuse box to feed everything power.

[Custom06]

I know this thread is old but I just bought a 2nd Square Body(86) and getting it ready to daily drive it. Apparently the previous owner was having the same issue before. Blower Motor will not come on, New one installed by previous owner. AC/Heater controls will not work either. Will test the motor this weekend to see if it works.

[RotaryRocketeer]

The blower should run anytime the key is in the 'On' position. Start there and then watch this video.

https://www.youtube.com/watch?v=IEqx0g2LAUo