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03-10-2011, 12:18 PM
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#1 |
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Registered User
Join Date: Oct 2004
Location: Fruitland, Idaho
Posts: 68
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Heater Switch on 72
Ok. my heater/AC switch just blows on high. I was going to install a new switch from classic truck parts but it fell apart before I got it plugged in. So could this be the heater/AC resister or relay? That glove box is a bugger to get out to get to the resister , the relay is not bad to get at. If anybody has any ideas I sure will listen.I put an ohm meter to both switches before the new fell apart and they both tested the same.
steve |
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03-10-2011, 12:36 PM
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#2 |
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Moderator
 
Join Date: May 2007
Location: Southern Cal
Posts: 20,001
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Re: Heater Switch on 72
sounds like a bad relay or connection. jiggle the connection at the relay with the fan switch at a lower setting and see if it kicks in.
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1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man |
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03-10-2011, 01:42 PM
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#3 |
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Old member
Join Date: Jan 2002
Location: Liberty, & Garden City S.C. , U.S.
Posts: 19,936
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Re: Heater Switch on 72
Look at the wire connections on the switch. If they are melted then it's the connector.
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1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs. |
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03-10-2011, 02:24 PM
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#4 |
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The Older Generation
  
Join Date: Aug 2006
Location: Montezuma, Iowa
Posts: 25,464
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Re: Heater Switch on 72
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You don't really say if you tried another switch or got the new one put back together. Did you test the old switch after you got it out? The resistor cuts the speed down so if it only works on high and you've eliminated the switch I would suspect the resistor.. Beware! I have been wrong before...  LockDoc
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Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 - Last edited by LockDoc; 03-10-2011 at 02:25 PM. |
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03-10-2011, 04:18 PM
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#5 |
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Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,145
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Re: Heater Switch on 72
A usual tell-tale sign on a GM product that the relay is bad is that it only works on high. Could also be the switch or resistor, but if the switch is not melted, I would suspect the relay first.
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project |
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03-10-2011, 04:41 PM
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#6 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
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Re: Heater Switch on 72
the relay is ONLY used on high, so your relay is fine. (this is the norm on all vehicles)
Your new replacement switch broke upon installing it? They're sending you another one right? It could be the switch/plug (as mentioned above, look for melting) or (more likely) the resistor |
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03-11-2011, 12:44 AM
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#7 | |
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DND Trucks
Join Date: Dec 1999
Location: NW Indiana
Posts: 2,382
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Re: Heater Switch on 72
Quote:
Not sure what all you have going there, but the original GM A/C fan switch, 3857992 is still available as well as the resistor, 3929055. Hope this helps. |
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03-11-2011, 08:55 AM
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#8 | |
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Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,145
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Re: Heater Switch on 72
Quote:
"With a switch-controlled motor, power from the fan speed control switch is routed through a “resistor block” on its way to the blower motor. The resistor block usually contains a series of resistors (three or four) that reduce the current and voltage that reaches the blower motor. When the driver chooses low fan speed, power is routed through all the resistors in the resistor block. This creates maximum resistance so the blower motor will turn slowly at its minimum speed. When the driver chooses an intermediate fan speed, power is routed through fewer resistors and the motor spins faster. At high speed, power may bypass the resistor block altogether and go straight to the blower motor causing it to run at maximum speed." On high speed you get straight 12v through the normally open part of the relay, on any other speed you get through the relay and then you get power through the resistor(s) to supply the fan motor I may be wrong, but that is my experience.
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project Last edited by MARKDTN; 03-15-2011 at 11:22 AM. |
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03-11-2011, 10:32 PM
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#9 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
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Re: Heater Switch on 72
if you have an A/C truck, go unplug the relay and tell us what speed(s) don't work. I'll bet everything I own all that happens is high goes out. I'll even throw in my income tax return and my next 3 years pay.
Wrenching on cars, light duty trucks, med duty trucks, heavy duty trucks, heavy duty equipment, army equpment... I have never seen the relay used for lower speeds. I'm sure something out there has a relay for each speed, so I won't say never, but I have yet to see one myself. |
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03-11-2011, 10:39 PM
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#10 |
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Eat, Sleep, Drive a Chevy
Join Date: Apr 2010
Location: the town of the cow, Texas
Posts: 1,152
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Re: Heater Switch on 72
I fully agree with Longhorn Man on this. The relay is for high speed only
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Trey Silence may be Golden, but Duct Tape is Silver! 72 Short Step 57 "big window" short step |
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03-14-2011, 11:10 PM
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#11 |
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Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,145
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Re: Heater Switch on 72
I'll be looking for a large check.
I did try this on my '83 K20 with factory a/c. Everything died. As it will when you remove the only source for power to the fan motor. Look at the wiring diagram for an '83 (Figure 1B-14 on page Air Conditioning 19 of the 1983 Light Duty Truck Shop Manual) or a '72 (page 1A-41 of the 1972 Truck Service Manual). The purple wire that goes to the relay is the power source for the motor. If you unplug the relay, the fan can't work. Non-a/c truck is different but that is not the original question. Did the original poster get his issue worked out?
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project |
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03-15-2011, 08:05 AM
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#12 | |
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Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,145
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Re: Heater Switch on 72
Quote:
Please correct me if I am wrong.
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project Last edited by MARKDTN; 03-15-2011 at 08:11 AM. |
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03-15-2011, 10:49 AM
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#13 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
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Re: Heater Switch on 72
ok... I misspoke.
The relay doesn't control the lower speeds, but acts as a bus bar. The actual relay only controls high. A BAD relay that is plugged in will not prevent the lower speeds from working, but will prevent high from working. |
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03-15-2011, 11:10 AM
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#14 |
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Registered User
Join Date: Oct 2004
Location: Fruitland, Idaho
Posts: 68
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Re: Heater Switch on 72
Thank guys for all the information, I have learned a lot on this forum. You all have some good info . It looks like the relay is the problem, Now ,I have to remove my glove box, not an easy task without ruining it, But what the hey, I like to keep the truck parts suppliers in business, Thanks again guys I appreciate it. FYI the switch I bought from Classic Truck was a generic Taiwan and while I was testing it with my ohm meter one of the wire connecting tabs fell off. I called CT and the gave me credit and told me not to send it back. They treat me ok just wish the quality was better.
steve |
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03-15-2011, 12:34 PM
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#15 | |
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Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,145
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Re: Heater Switch on 72
Quote:
In my experience it will. I don't know why, but from what I have seen they fail and only high speed works. I guess the contacts on the closed side get dirty or burnt, don't know, can't explain it. Maybe it's the higher resistance on lower speeds encourages corrosion on the contact? But I have seen more bad relays than bad resistors. And again from what I have seen, only high speed works then.
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project |
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03-15-2011, 06:31 PM
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#16 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
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Re: Heater Switch on 72
on the two trucks I've had with A/C, they both had rusted out relays that were literally falling apart. The 'can' or body was actually missing on one of them. They both worked on all positions but high.
On late model stuff at work, we almost never replace relays for the blowers, but we replace a resistor at least one a month, usually more than that. |
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