Home » Public Forums » GMCnet » Dash air/heat Blower Not Working - Blowing Fuses
| Dash air/heat Blower Not Working - Blowing Fuses [message #371291] |
Sun, 23 April 2023 14:26  |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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On our last trip, I noticed that the dash blower was not working. After looking into it, I discovered the fuse was burned out. I replaced the fuse and the blower came on, but the fuse blew when I switched it to the high position. I think it is supposed to be a 25 amp fuse and the one I put in was a 20 amp, so I tried a 30 amp (i didn't have any 25s), same result. The blower motor is fairly new so I don't think that is the problem.
I'm guessing it is either a bad switch or a bad blower relay. Any ideas? Am I on the right track? I DID check the wiring on the blower relay, and didn't see any shorts, so I don't think it's a wiring issue.
Thanks
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371293 is a reply to message #371292] |
Sun, 23 April 2023 23:37  |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Okay, that's my understanding of how that circuit works as well. The blower also does NOT operate when the switch is in the high position with the engine running. The alternator is charging normally. So, maybe it's NOT the switch, but the relay or the wiring to it, right?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
[Updated on: Sun, 23 April 2023 23:39] Report message to a moderator |
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371294 is a reply to message #371291] |
Mon, 24 April 2023 07:04 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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I would pull the connector from the High Speed relay and measure the coil resistance. It should be something like 50 ohms. If it were a short, it would be Zero ohms. If its about 50 ohms (12 ohms would give 1 Amp of coil current, and I'm sure it must be higher resistance than that), then , with the fan switch set to medium or low and the HVAC set to OFF, measure the coil wire resistance to ground and it should be open (infinity ohms).
From the wiring diagram, the coil wire is Orange and its ground is black.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371296 is a reply to message #371291] |
Mon, 24 April 2023 08:23 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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It’s also easy enough to pull the relay and bench test. With relay out, run your controls through the ranges (engine on or key on) and see if you still blow the fuse. If no then wiring should be good. You can inspect/ bench test the relay while out. If water gets in there they can corrode and chunks short out. Some clip leads and 12V to the relay coil and the NC and NO contacts should alternate with no power/power applied to coil.
Another quick check is to inspect the speed resistor bank module. They, like headlamps, CAN fail in odd ways and bridge weld the windings across the leads. That COULD apply near full 12V to the motor through the low speeds wiring for which it is not designed or fused.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371297 is a reply to message #371296] |
Mon, 24 April 2023 21:28 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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Thanks,
I will do some more diagnostics based on your suggestions. Is that relay a commonly available part?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371330 is a reply to message #371299] |
Wed, 10 May 2023 20:06 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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So, I had a chance to look further into this this afternoon and I think it is the relay. I disconnected the wires from the relay, jumpered the power lead to the fan motor lead, and it ran as it should. I'm thinking the relay has failed and is shorting out when the fan switch is switched to "high".
I'll replace the relay (which does not appear to be the original) and see what happens. Thanks for all the advice.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371331 is a reply to message #371330] |
Wed, 10 May 2023 20:11 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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As a side note, I have been told that the current crop of aftermarket blower motors are not of very good quality. My mechanic friend said he replaced the one in a customer's Jeep XJ five times before giving up and getting an OEM one from the Jeep dealer. So far that has fixed the problem.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371334 is a reply to message #371331] |
Fri, 12 May 2023 19:32 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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I replaced the blower relay with a new one from Standard and repaired a couple of overheated wires/terminals.
NOT the problem. The fuse blew again when I put the switch in the high position. It didn't happen right away, and I noticed a couple of blips in the volt and amp gauges as I was going through the speeds.
I'm thinking that leaves the dash switch (most likely, I think) or the resistor pack. I pulled the resistor pack and didn't see anything obviously wrong with it, although, I really don't know what to look for. No obvious shorts or anything though.
Have the dash fan switches been known to be a problem? It IS 48 years old, after all.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371335 is a reply to message #371291] |
Sat, 13 May 2023 08:30 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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The switch could be “coming apart” with the bent over tabs stretched if it’s the type I think it is. Or the phenolic material cracked? That could cause possible hot to case or full 12 to motor through switch instead of the relay. Best to inspect at this time. Also you could watch exactly which positiin caused the fault so you can observe on bench.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371339 is a reply to message #371336] |
Tue, 16 May 2023 21:10 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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My mechanic friend thought my blower motor might be drawing too much current, so I checked the amp draw with the blower connected directly to 12V +. It was only pulling 15.5 amps, so the problem is not the blower motor.
My latest thought is that the wiring to the relay must have a short to ground somewhere. I have looked over all of it that I can get to or see and am not finding any bare wires or any other problem. I guess the next thing is to unplug the coil energizing connection on the blower relay and see if the fuse still blows. If it does, I'll need to look further.
Is there any chance that the switch might be grounding out somehow? In other words, could the short be in the switch?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371342 is a reply to message #371341] |
Thu, 18 May 2023 23:56 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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The switch action seems a little rough. I checked the orange wire from the switch to the relay and it appears to have a short to ground somewhere. I'll try unplugging the switch end and see if it is still shorted. If ii is, I'll know I have a bad wire.
Like I said earlier, I looked at the resistor pack and didn't see anything obviously wromg with it. It should be out of the circuit when the fan switch is on high anyway, right?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371344 is a reply to message #371343] |
Fri, 19 May 2023 22:14 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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Right, that's why I am going to test the wire while unplugged from the switch end. That way I can rule out the wire if that is not the problem. If it is shorted to ground, then it's not the switch. if it's an open circuit, then the switch is likely the culprit, right?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371346 is a reply to message #371344] |
Sat, 20 May 2023 18:59 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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So I just checked continuity of the orange wire from the switch to the relay from both ends (unplugged) to ground. There appears to NOT be a short in that wire. When I check across the switch itself, I get continuity from the orange wire terminal to the black wire terminal, but only if the switch is in the high speed position. My thinking is that that should not be happening. It seems to me that the switch should complete the circuit to the relay when the switch is on high, but should not short to ground.
Am I right?
Also, it looks like I could take the switch apart. If I do, what are my chances of fixing it and returning it to proper function? Is it even worth it to try, or should I just buy a new one to replace it?
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Dash air/heat Blower Not Working - Blowing Fuses [message #371347 is a reply to message #371346] |
Sun, 21 May 2023 00:53 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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I am looking at the wiring diagram again. From your posting I am assuming that the orange wire terminal and black wire terminals you are referencing are on the relay. If this is true, then you are reading the orange connection to ground THROUGH the relay coil.
I do not know what you are using to read continuity but there is a resistance connection between the orange wire THROUGH the relay coil to the black wire which is connected to ground.
Ken B.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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