Grrr!!! Heater frustrations - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**truckster** · 11-20-2009, 07:22 PM

Here's the deal: 72 Blazer w/AC. The AC isn't hooked up, but I don't think that's important. The heater fan was working fine until two days ago. It stopped working, so I went through everything I could think of. Checked the fuse, connections, etc. Noticed the heater relay housing was loose, so I replaced the relay. Checked the resistors and they seem to be fine- around 1 ohm on one side and 3.3 on the other. I pulled the switch apart and cleaned it, since it was pretty dirty, and lubed it and reassembled it. It tests out fine too. I also checked the ground on the relay. I ran through the wiring diagram and checked all the wires and everything seems to be where it's supposed to.

Here's the deal. With the ignition on, I get 12 volts at the switch. As soon as I turn the switch on, the voltage drops to near zero. If I disconnect the ground at the relay, the voltage goes back up to 12. If I manually close the solenoid on the relay, the blower comes on. Otherwise I have nothing. I've run out of ideas. Any help would be appreciated.

**john** · 11-20-2009, 07:35 PM

Have you tried a 12V. jumper to the motor to see if it is the problem?

**truckster** · 11-20-2009, 07:43 PM

The motor runs if I close the solenoid in the relay manually, so I'm sure it works.

Critter · 11-20-2009, 08:42 PM

I don't know if it is pertinent at the moment, but have you looked at the resistor on the inside of the evaporator box.

I had similar issues with mine before I tore it down for restoration. I changed everything but that resistor and the anti-dieseling relay (I don't even know where it is) and had similar results to you.

**LockDoc** · 11-20-2009, 09:29 PM

-
On my '71 I had to have a good fuse in the A/C slot in the fuse block before my heater would work. Just a thought...

LockDoc

**truckster** · 11-20-2009, 09:58 PM

Thanks for the suggestions. I've given up for the night, but I'm going to start at the fuse block and work my way through again in the morning. I'll double-check the fuses and the resistor.

I don't have the anti-dieseling relay, either. I looked at that diagram earlier when I was tracing the wiring.

Thanks again for the suggestions.

1970CustomC10350 · 11-20-2009, 11:18 PM

I would try bypassing the relay and see if the motor blows. I recently had a similar problem. I dissassembled the relay to reveal alot of corrosion. I also had a defective switch. The cardboard like backing was warped just enough to make intermitent contact. A new switch and relay and its working fine now.

11-20-2009, 07:22 PM	#1
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,886	Grrr!!! Heater frustrations Here's the deal: 72 Blazer w/AC. The AC isn't hooked up, but I don't think that's important. The heater fan was working fine until two days ago. It stopped working, so I went through everything I could think of. Checked the fuse, connections, etc. Noticed the heater relay housing was loose, so I replaced the relay. Checked the resistors and they seem to be fine- around 1 ohm on one side and 3.3 on the other. I pulled the switch apart and cleaned it, since it was pretty dirty, and lubed it and reassembled it. It tests out fine too. I also checked the ground on the relay. I ran through the wiring diagram and checked all the wires and everything seems to be where it's supposed to. Here's the deal. With the ignition on, I get 12 volts at the switch. As soon as I turn the switch on, the voltage drops to near zero. If I disconnect the ground at the relay, the voltage goes back up to 12. If I manually close the solenoid on the relay, the blower comes on. Otherwise I have nothing. I've run out of ideas. Any help would be appreciated.

11-20-2009, 07:35 PM	#2
john member #16 Join Date: Dec 1999 Location: Yuba City Ca. Posts: 3,618	Re: Grrr!!! Heater frustrations Have you tried a 12V. jumper to the motor to see if it is the problem? __________________ john http://67-72chevytrucks.com/vboard/showgroups.php board vendors http://www.flickr.com/photos/1745471...7602865984234/

11-20-2009, 07:43 PM	#3
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,886	Re: Grrr!!! Heater frustrations The motor runs if I close the solenoid in the relay manually, so I'm sure it works.

11-20-2009, 08:42 PM	#4
Critter Future TOTM Winner Join Date: Aug 2006 Location: Bulverde, TX Posts: 2,692	Re: Grrr!!! Heater frustrations I don't know if it is pertinent at the moment, but have you looked at the resistor on the inside of the evaporator box. I had similar issues with mine before I tore it down for restoration. I changed everything but that resistor and the anti-dieseling relay (I don't even know where it is) and had similar results to you. Attached Images __________________ 72 Chevy CST K5 HIGHLANDER BLAZER Getting a 6.0,NV4500,NP205,14BFF (Currently laying all over the place in little pieces) 72 Chevy swb step-side "LS" 6.0 Here's a build thread of sorts 2002 HD2500 Crew Cab, 8.1L Allison 4x4 Daily Driver Check it out www.lsdyno.com

11-20-2009, 09:29 PM	#5
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,423	Re: Grrr!!! Heater frustrations - On my '71 I had to have a good fuse in the A/C slot in the fuse block before my heater would work. Just a thought... LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

11-20-2009, 09:58 PM	#6
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,886	Re: Grrr!!! Heater frustrations Thanks for the suggestions. I've given up for the night, but I'm going to start at the fuse block and work my way through again in the morning. I'll double-check the fuses and the resistor. I don't have the anti-dieseling relay, either. I looked at that diagram earlier when I was tracing the wiring. Thanks again for the suggestions.

11-20-2009, 11:18 PM	#7
1970CustomC10350 Registered User Join Date: Nov 2008 Location: Kansas Posts: 47	Re: Grrr!!! Heater frustrations I would try bypassing the relay and see if the motor blows. I recently had a similar problem. I dissassembled the relay to reveal alot of corrosion. I also had a defective switch. The cardboard like backing was warped just enough to make intermitent contact. A new switch and relay and its working fine now.



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