blctalon

Hi all, I'm knew here and I'm going to start with a question. This may be a little long but I try to be as thorough as possible.

I bought a 1978 corvette a few months ago and I've been working on her in bits a pieces. Right now I'm trying to figure out why my ac/heater fan motor won't work. I can put the selector on MAX A/C - Defrost, and the fan speed from low-high and it doesn't blow on any combination of settings.

The first thing I did was make sure the fan motor actually works. I did this by running a jumper wire to one of the connectors on the blower motor itself, and it turned on. So the motor isn't frozen. I spent a couple of hours looking around at the wires and they seem alright. I found my way to the high speed relay where it looks like the lower connector (3prong) was replaced once or removed for whatever reason. The connections look fine here. With my knowledge falling somewhat short on troubleshooting relays, I decided to replace the high speed fan relay. I first plugged in the 3 prong connector to the relay, then to 2 prong. I noticed when I plugged in the 3 prong connector and moved the head around a little the fan motor started blowing wide open (with key on or off). Move a little more and it'd stop, little more and it'd start..etc. Also, I tried setting the fan speed to a different level when I wiggled, but it would still blow the same. I noticed when I had the selector on an A/C setting it would click the compressor clutch on while I moved the 3 prong head on the relay switch. When I had it on vent it would not do this as it was not trying to engage the compressor, this much seems normal.

I really don't know what's going on here. I traced the wires as best as I could and everything looks like it has a solid connection. I even looked at the connections on the control box inside the car, underneathe the console.

So I can get the fan motor to only blow when I wiggle the pronged connection on the new relay switch, but not when I just plug it in and leave it alone. Another thing that is odd is I can touch all 3 contacts from the connector to the relay before actually sliding the hookup on all the way and nothing happens. It's almost like once I plug the connector all the way in and move it around, something shorts the contacts inside the plug and makes the fan work, but work improper. Does anyone have any ideas I can try to solve this problem?

hunt4cleanair

Can't really help you with all that wiggle stuff but let's review how it should work and that may give you some clues. Probably about five years ago I replaced my heater core and as I tore into the dash did a check on all the seals, doors, etc on my 78.

The blower always blows even when in off...note on blower switch that off is not an option. Didn't know that when set at HIGH...turns compressor clutch on. I'll have to check that out.

The blower switch work independently from climate control switch (VENT, A/C, HEAT, etc). This switch is run by vacuum and simply modulates various doors underneath the dash to open and close. A/C has two options, normal and MAX. At MAX, it shuts the water shutoff valve that allows hot coolant to circulate through the heater core many don't work and those that do don't work well but that's beside the point. It also closes outside air doors to permit recirculation of cabin cooled air.

Hope this helps.

s car go

I can assure you that the one of the problems is where you are wiggling the wires. Its a bad connection. And remember there are always at least two problems.

blctalon

My father has an '80 and his fan runs at a low speed with just the key turned on whether or not the ac/vent/heater is selected. This sounds normal, but mine doesn't work at all. As far as the clutch turning on, I meant I could hear it click if I put the control selector on MAX A/c, but when I put it on vent it would not initiate the clutch switch.

The connector does look a little used, like maybe it got hot once and sorted melted a little. I was thinking about cutting the connector and attaching the wires directly to the relay to see what happens, but I haven't been able to find that 3 prong connector any where and I don't want to destroy it just yet.

Thanks for the input guys. I also noticed that the large orange wire on the 3 pronger has constant 12 volts, even with the car off. I would assume this is normal as I read on another post it may be picking up power from the horn relay which is a constant 12v.

BBShark

Not realy sure about this (my info is for a 79) but I think you should have a dark blue, purple and red wire running to the 3 prong connector. The 2 prong should have a orange and black. Maybe someone rewired it in the past. Also I think you can buy the connectors from Rockauto.com

blctalon

You're right. There is a blue, purple and red. It does look like someone has rewired a new 3 pronger on, and the color of it's hot wire is orange, which goes to the red. It still however has the dark blue and purple.

I'll check the rockauto site and see if I can find it.

I also have somewhat good news. This morning after I made my first reply I went out to mess around with it some more. I plugged the new relay into both connection heads carefully, making sure not to wiggle. Turned the key on and low and behold I had a low speed fan running! :o I then turned it the unit on and variable speeds all work perfectly. I started the car and tried the a/c and it works as well. I'm very happy it's working, but I still think I need to replace that one connector that looks kinda ragged.

Also, I must have a vacuum leak somewhere as the A/C, vent, and heater all seem to blow out the floor ports only. I checked the connections on the actual control panel and they look good and snug. I think the problem is somewhere under the hood. But hey, once I get this one figured out I'll have a working heater and a/c.

Thanks for the help guys.

blctalon

To sum things up a bit, I got my new 3 prong connector that plugs into the lower relay a few days ago, and I just got around to putting it on. I was suprised to find once I had installed the new connector the fan would not run on high speed, but all the others. I was also suprised to find that the old relay I had replaced in the beginning worked with the new connection too.

The fact that the hi speed was now not working at all kind of inflamed me so I started investigating a little more. I dunno how tracing wires ends up taking hours at a time (maybe it's because I find something else to fix) but they do. I traced the hot all the way from the relay to the fuse box. The final fix to my problems lay within 1 foot of the relay.

There is a connector that comes off the main harness which has a hot and a ground. On this side of the connector the hot (red) wire was loose and hanging out just enough to not make contact when the female end of the plug when connected. I cleaned the copper end of contact, spun it 180 degrees and inserted it into the connection end where it will now stay and not come loose again. It looks like someone pulled it out once (before I owned the car) and got it twisted 180 degrees off then tried to stick it back in the wrong way which it didn't stay snug.

I assume this is the wire that the "HI" setting uses that bypasses all the resistor stuff I read about in the other posts on the forum. On my 78 this wire is red and runs from the relay connection, down the firewall to the drivers side, and directly into the fuse box upper right that reads "heater a/c". I did not find an inline fuse anywhere on this wire, but I didn't exactly search every inch of the wire wrap either. The harness runs behind the wiper motor assembly and is very hard to get to.

Anyhow, I thought I'd post my final find that solved my particular problem.