High Speed Setting on Heater

[eoseitz]

I know there are a lot of discussions about the heaters on here, but I can't seem to find any detailed steps to take to test what has failed in my car. I just bought a blower motor resistor. I'm not sure if that's what's wrong but I thought it may be a good start.

The blower works in off, low and medium. When I switch to high speed it shuts off again. How do you test the high speed setting on the switch? Also how do you test the relay to see if it's working? I also heard there is an inline fuse and a fuse in the fuse box. That seems redundant but if there is, where is the inline fuse located.

Thanks for any help.

 

[djsroknrol]

Check the hot (red) lead from the blower relay to 12V power. That's how high speed gets high speed, by bypassing the resistors through that wire for full speed from the blower.

I just went through the same thing with my heater/blower. it seems that when they pulled the engine, they broke that wire behind the bell housing.

 

[SVO]

The fan motor power is routed thru the resistor pack which drops the voltage to give you low and medium motor speed. For high speed, power is fed direct to the fan motor. If you have low & medium fan speed, then your existing resistor pack is o.k. To test your fan's high speed, you need to apply 12 volts directly to the fan. If the fan runs, (which it should, since it runs on low & medium speeds) trace the fan power lead back to it's power source. There should be a white plastic inline fuse holder in the fan power lead. I believe the fuse holder is located in the wiring harness on the engine firewall in the area of the wiper motor. HTH

 

[eoseitz]

Thanks. I'll check that when I get home. I wish I hadn't bought the new resistor now. oh well. I might as well put a new one in since I have it anyway. Will the red hot wire go all the way from the blower motor, through the firewall and connect to the high speed on the motor speed switch? I may just run a new wire.

 

[eoseitz]

Well I moved some wires so high speed works now I think. Must be a short. However there are 4 setting on my control switch. Far left appears to be low. Second setting is the same speed as far left. Then medium, high off. Is this correct? I thought far left was off and far right was off.

 

[JohnZ]

Well I moved some wires so high speed works now I think. Must be a short. However there are 4 setting on my control switch. Far left appears to be low. Second setting is the same speed as far left. Then medium, high off. Is this correct? I thought far left was off and far right was off.

Nope. Far left is off, far right is High. The high blower relay gets full 12V power directly from the horn relay through a red wire with an in-line white plastic fuseholder. Only the lower speeds are powered through the resistor, from the fuse block.

 

[ButchN]

Well I moved some wires so high speed works now I think. Must be a short.

It's actually an open since the fuse isn't blown & there's no power.
You have a bad connection it seems.

 

[petes74ttop]

Here's a schematic for a 74, just scroll down to the sideways drawing called Air Conditioning & Heater diagram.
http://www.nrjvette.com/johdotukset/Chevy Corvette 1974.pdf

The junction box is near the front of the left front frender and the inline 30 Amp fuse is closer to the master cylinder.

 

[eoseitz]

So the horn, amperage gauge and high speed blower is all connected to the horn circuit. I think I may know what the problem is. The horn is rusted apart under my back passenger fender. I was told this is why my amp gauge doesn't work. I guess it senses voltage by the horn and the battery. The horn components in the steering column also appear to be missing. Would this affect the high speed on the heater?

Anyone know where I can find the under the hood part of a wiring harness. My father is a mechanic. He said I should just be able to unplug the harness at the firewall and run a new one. As chopped up and dry rotted as my current harness is, I think this is the best way to go. The rest of my harness appears to be ok still. The previous owner just liked to rig things. There's wire splices and different colors of wires everywhere under the hood. right now the blower motor and windshield wiper motor are grounded with a red wire to the linkage on my carb. lol. I know this is wrong.

 

[petes74ttop]

There is an alarm horn under the rear drivers fender, you have a horn on the passenger side?

The ground for the wiper and fan motor should be a black wire attached to a starter mount bolt or a bell housing bolt.

Parts missing from the steering wheel will keep the horn under the hood from working but should have nothing to do with the heater control.

 

[eoseitz]

sorry. I meant under the drivers side. It's been a long day at work. lol. You said an alarm horn? I don't think my vehicle has a car alarm. Would this also be just the regular horn?

 

[petes74ttop]

Here's the regular horns.

 

[petes74ttop]

Here's the parts at the horn button, some of the vendors have color pic that is clearer.

While I've got the AIM open here's the junction box that has the wire that goes to the 30amp fuse.

 

[Bob Heine]

Anyone know where I can find the under the hood part of a wiring harness. My father is a mechanic. He said I should just be able to unplug the harness at the firewall and run a new one. As chopped up and dry rotted as my current harness is, I think this is the best way to go.

There are two under the hood wiring harnesses. One harness includes the wiper motor, blower motor, ignition, starter and several other items. The second harness includes the alternator, all the front lights, temperature sender and horn relay.
The Corvette parts suppliers carry the harnesses. Here are Corvette Central's listings:
Engine Harness (350 with auto trans):
http://www.parts123.com/parts123/yb.dll?parta~dyndetail~Z5Z5Z50000050g~Z5Z5Z513914~P177.00~~~~S3BQ1F2B1R76109113024Q~Z5Z5Z5~Z5Z5Z50000050G#

Forward Lighting Harness:
http://www.parts123.com/parts123/yb.dll?parta~dyndetail~Z5Z5Z50000050g~Z5Z5Z514275~P215.00~~~~S3BQ1F2B1R76109113024Q~Z5Z5Z5~Z5Z5Z50000050G

If you aren't comfortable with wiring schematics, DocRebuild sells color diagrams with photos of all the connectors. It's big and in laminated plastic and the whole car is shown on two sheets (they offer three but you only need two of them). The first is the dash harness (one is for A/C cars, another for non-A/C) and the second is for everything else.
DocRebuild's 1974 OOSOEZ Wiring Guides

 

[eoseitz]

I just bought the schematic. When I go to the link for the harness, it says I may need the Distributor to Coil and Lead Wires. What are these? I changed my ignition from points to HEI. Will I still need these wires? Can I use the one's I already have if they are in good shape?

Thanks

 

[TimAT]

The distributor to coil is a single wire about 12 inches long, used on points ignition only.

Probably one of the more common heater blower faults is exactly the one you have- the hi speed quits. Usually it's an easy fix, but it drives guys crazy trying to figure it out. The high blow relay is basically a switch that's controlled by the switch on the heater control in the car. When you select "HI" the power goes out to the hi-blow relay, it closes and feeds 12v from a 10 gauge wire directly thru the relay to the fan motor.
Because of the current draw, the power feed to the relay gets hot- heat means higher resistance which means more heat. Weak link in the system is the fuse- be it the inline fuse holder or a fusible link. Once that fuse pops- no hi speed blower.

 

[eoseitz]

Is the blower relay located right new to the blower motor/ac? There is a small box there with 3 connectors on it. Two of the connectos are attached, but the thrid one isn't. The connector that isn't attached would be the one facing the drivers side of the car. The other two face the passenger side. If I had to guess I'd say the connector from my heater switch to the relay isn't connected. I've traced the other two. One is a ground and the other the 10 gauge 12 volt wire I believe.