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Trying to tidy up some wiring in my 78 project before I put the dash all back in place. I can only get the fan to blow 3 speeds, one is on all the time that the sign is on, then med, then another speed but when I put the lever up to hi it shuts off. I DO NOT have the A/c hooked up and was thinking that the hi speed only works if the A/c is on. I hope that is the case. This is in a circuit with a new blower switch, and fan from a C4, and a new relay on the firewall. Thanks.
Generally GM's high blower speed relay has a constantly hot red 10ga wire going to the relay. Power for it comes off of the starter battery terminal and is ether fuseable link or 30amp inline glass fuse is used for circuit protection. Check to see if you power there.
On your 78 the only time the High Speed Blower relay energizes is when the function control switch is in the MAX A/C position.
78 circuits numbers are the same as Late 1977 circuit numbers except 988 on the 77 changes to 52 on the 78...Same wire color (Orange) same function control switch position...
To me is sounds like the High Speed Blower relay is energizing but like skids says, see if you have voltage on the large RED wire at the High Speed Blower Relay.
"IF" energized (contacts 1 & 3 are closed) and if no voltage at 3 your blower will not run...
It will be HOT at all times the battery is connected...
I've blown 3 of the high speed relays on the fender and finally bought a late model relay, I think they are solid state? and havent had a problem since.
EHDIS Car Relay 4 Pin 12v 40amp Spst Model No.: JD2912-1H-12VDC 40A 14VDC, Auto Switches & Starters,Pack of 2
on Amazon. $9.00 for 2 of them. Better than the $30-40ea the Corvette stores rip you for. They wire right up.
OK. Time to man up. The only GM harness in my car is the Starter Extension, which I replaced because the original melted over the years. All else is custom wiring. I was doing something with the starter and disconnected everything. When I put it back, I forgot the that little(big) red wire with the eye end on it was tucked away someplace safe, where I wouldn't forget it. Well I forgot it and once you all explained that the red was constant hot with the starter I knew where my issue was. So now I have the blower going at all speeds. All is good. Thanks.
OK. Time to man up. The only GM harness in my car is the Starter Extension, which I replaced because the original melted over the years. All else is custom wiring. I was doing something with the starter and disconnected everything. When I put it back, I forgot the that little(big) red wire with the eye end on it was tucked away someplace safe, where I wouldn't forget it. Well I forgot it and once you all explained that the red was constant hot with the starter I knew where my issue was. So now I have the blower going at all speeds. All is good. Thanks.
You may have already changed this when you installed a custom harness.
1977-1978 the function control switch was wired so that the High Speed Blower relay "ONLY" energized when MAX A/C was selected on the control switch.
You had "NO" fan speed control in MAX A/C position.
If you look at the drawings above in 1979 they ran Circuit #52 directly from the fan speed switch to the coil of the High Speed Blower relay.
Doing this gives you "FULL" fan speed control when MAX A/C is selected.
All that needs to be done is remove the terminal from the function control switch connector Cell 2 (Double Orange Wires) and wrap some electrical tape or a small piece of heat shrink around the now exposed terminal.
Actually when I did my wiring ( Ron Francis 24/7) there were plenty of circuits to use but when I got to the Starter Extension I thought it would be easier to just drop another couple of bucks ( I think it was close to $200) for a repop of the oem. RF had a circuit for Heater/A/C and for the compressor. When I got done all that was left was that red wire on the extension. . Everything powered up fine. Or so I thought. I had fan speeds but did NOT check the MAX. Who knew that if the first 3 speeds worked, the 4th wouldn't. At least not me! I had no idea of the GM thought process. So I taped the end off and put it in a safe place near the starter JUST IN CASE. It reminded me of when I was a young teenager and the first car I played with had spaghetti wiring under the hood. I decided to clear it all out, including the ballast resister for the ignition. Took me awhile to figure out why the car wouldn't start! IF only there was an internet AND a 57 chevy forum back then....
Fan Hi Speed
