heater and heater controls

[wood8835th]

I'm working on a 75 that originally had A/C and a 350. Now has no A/C and a 454. Heater controls don't seem to get any response out of heater box. Fan switch works but blower doesn't. Vacuum lines appear to be in good condition. Seems like vacuum switch is functioning, but I am obviously missing something. I can manually move the doors on heater box with little effort and they return to a default position on their own.

Any suggestions on troubleshooting, or is there an obvious answer I've overlooked.

 

[lone73]

So what's the question? You can't get warm air out the heater vents?

 

[wood8835th]

Thanks for taking a look. Get no response from anything it seems. Vent produces no air flow (all also tested at speed). Heater produces no heat, or air flow. Defroster provides no air flow. No blower either, but I have checked the integrity of the fan switch and blower motor and will work when powered individually.

My question is really that I am at a loss to know what to check next.

 

[lone73]

You definitely need to start with an electrical schematic. I have a Haynes manual with a decent schematic in the back. My old manual skips every other year on the schematics. I believe '75 is in it though. On my '73 the roller wheel that controls ac/vent/heat...etc also controls the fan on/off. If its rolled all the way forward it turns off the fan regardless of fan switch setting. You might want to check that switch also. The best way to track it down though is just gonna be to get a schematic and find the last spot in the wiring that you have 12v with the thing set "on". It could also be a faulty ground as well. If you have 12v at the fan and no movement then the problem is in the return side of the circuit. I'll take a look at my manual when I get home to see if I can tell anything in particular t check...

There is a plug or two under the right fender connected to the heater box, might want to check in that area for something unplugged. That was a problem on mine one time, as I recall the connector was really hard to see. right up on top of the box between the underside of the fender and top of the box - close to the firewall.

If you have no air movement from the vents just due to the forced air effect from driving at speed on the road - count your blessings. The rest of us older C3 owners want to know how you (or the previous owner) stopped the flow. I can't get the heat to STOP on my car. Others have reported the same prob. If you have absolutely no flow at all then somebody may have blocked the system physically with a piece of cardboard or something to stop the age old "heat-at-the-feet" prob that most of us C3'ers have.

I just recently bypassed the core on mine due to yet another core (not to be confused with "warp core") breach. After 30 minutes of driving I still have significant heat coming out the ducts.

good luck with it and keep us posted please...
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[Bob Chadwick]

I hear you saying you have no air volume from the fan. Start by checking the fuse at the panel and the connection on the top of the heater box.

 

[Mr. Chuck]

When trouble shooting older wiring and related connections, it is often better to use a test light that draws current to light the test lamp, this helps to identify defective / corroded connections that would other wise read voltage on a meter but will not drive a fan or other electrical components.
If you can solder, making a test light only requires a spare 12 volt lamp, two lengths (2 - 3 ft ea) of insulted wire stripped 1/4" on each end; two alligator clips (insulated ones are better). Solder one wire to lamp center contact and other to lamp base at the small solder point were the base contacts the glass bulb. Crimp or solder the loose wire ends to the respective alligator clip. Hint, use a small wattage solder iron, other wise you can damage the lamp. You now have your test light that will provide years of service. A trouble tip from a old, ex-navy aircraft maintenance guy, always start a the none operating component and move towards the source of power, unless you recently had part of the wiring disconnected; always check points that were last distrubed for proper connection first. If no, voltage at component, check fuse with continuity meter to be sure it is good. Good Luck in finding the problem!

 

[Mr. Chuck]

A side comment regarding constant hot air from vents: I recently recarpeted my 81 and part of the re-assembly noted heater control cable coming from the heater / vent control panel, out and under the carpet on the passenger side of the center tunnel. The cable has a round, simi-clear plastic sleeve along the exposed cable. While ripping out the old carpet, noticed that the sleeve turned and at the time thought it was loose and so I tightened it (turned it CW). After completing carpeting & reassembly every thing, including the center console carpeted side pieces. After couple days of driving around, noticed that I had heat from the vents in any position of the heat control knob (full left / cold). Finally remembered tightening the cable sleeve; removed center console right side carpeted panel and with car running & at temp, began turning the sleeve CCW until no more heat was felt at the center console vents. I looked every where in my Chevy shop manual, but didn't find any referrence to this sleeve, bottom line is if you have similar problem, check to see if you have a similar adjustment on the cable coming from the heater control panel. Car designer must have put it in to compensate from varations in control cable length & routing position.

Good luck!