| Simplifying Something From The Factory [message #221654] |
Sun, 08 September 2013 15:18  |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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When I had to troubleshoot the heater and AC blower on my 73 Sequoia, I cleaned up that section of the drawing from the MM and posted some aids for the next guy with that problem. I have since pointed a few people with similar problems at the pictures and writeups, so I hope there is some benefit to the community.
When it came up again yesterday, I started looking at the system and came to a conclusion that may or may not be correct.
If battery voltage (0.7VDC lower than alternator voltage) will spin the blower fast enough do we really need the blower relay?
I ask partly because I suspect that no one has tried running the fan on high, and the AC compressor clutch BOTH off the 25A HTR A/C fuse. Given that a lot of places have a 30A fuse and 14AWG wire, is 30A going to create an electrical hazard on the 14AWG wire feeding the fan speed control, or is that too much current for the switch contacts? Is that why GM installed the blower relay?
Here is what I am talking about:
If you disconnect the orange wire, the tan wire and the purple wire from the blower relay and connect them all together, you bypass the blower relay, and you have three fan speeds. HI is a little slower than it was before, and I don't know if even a 30A fuse in the HTR A/C position will be big enough for the fan on HI and the compressor BOTH.
If 30A is enough for both the HI speed blower and the AC compressor, and it does not create an electrical fire hazard, what does the collective brain trust think of removing the blower relay from the circuit?
[Updated on: Sun, 08 September 2013 15:20] Report message to a moderator |
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| Re: [GMCnet] Simplifying Something From The Factory [message #221659 is a reply to message #221654] |
Sun, 08 September 2013 15:29  |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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I have seen several feed wires to the heater partially melted so it does draw a lot of current on high. Especially if the terminals are not clean.
I believe that the relay is there to take the heavy current flow from the small switch built in to the heater controls in the dash. The current could easily burn up the small contacts.
It also does help to have the alternator voltage when running the blower on high. Every little bit helps as the blower really should have been larger for the motorhome.
Emery Stora
77 Kingsley
Frederick, CO
On Sep 8, 2013, at 2:18 PM, A. wrote:
>
>
> When I had to troubleshoot the heater and AC blower on my 73 Sequoia, I cleaned up that section of the drawing from the MM and posted some aids for the next guy with that problem. I have since pointed a few people with similar problems at the pictures and writeups, so I hope there is some benefit to the community.
>
> When it came up again yesterday, I started looking at the system and came to a conclusion that may or may not be correct.
>
> If battery voltage (0.7VDC lower than alternator voltage) will spin the blower fast enough do we really need the blower relay?
>
> I ask partly because I suspect that no one has tried running the fan on high, and the AC compressor clutch BOTH off the 25A HTR A/C fuse. Given that a lot of places have a 30A fuse and 14AWG wire, is 30A going to create an electrical hazard on the 14AWG wire feeding the fan speed control, or is that too much current for the switch contacts? Is that why GM installed the blower relay?
>
> Here is what I am talking about:
>
> If you disconnect the yellow wire, the blue wire, the tan wire and the purple wire from the blower relay and connect them all together, you bypass the blower relay, and you have three fan speeds. HI is a little slower than it was before, and I don't know if even a 30A fuse in the HTR A/C position will be big enough for the fan on HI and the compressor BOTH.
>
> If 30A is enough for both the HI speed blower and the AC compressor, and it does not create an electrical fire hazard, what does the collective brain trust think of removing the blower relay from the circuit?
> --
> '73 23' Sequoia For Camping
> '73 23' CanyonLands For Sale
> UA (Upper Alabama)
> "Time is money. If you use YOUR time, you get to keep YOUR money."
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| Re: Simplifying Something From The Factory [message #221707 is a reply to message #221654] |
Sun, 08 September 2013 20:27 |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
Karma: 0
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As a former long term GM employee, I'm absolutely sure they wouldn't spend the money for a relay if they thought the switch and associated wiring could handle the load. They tend to go a little cheap on wire sizes anyway, as unless it was an extremely short run, most would use a 10 gauge for a 30 amp load.
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: Simplifying Something From The Factory [message #221710 is a reply to message #221654] |
Sun, 08 September 2013 20:56 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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GM has pretty much always done it this way. Don't have the figures ( now I need to get out the amprobe!) but I don't think its a linear increase with each speed selected and current is pretty high on high. Those resistors on the first 3 speeds get hot and blower is used to cool them. Dash switch whould have been massive and heavy wire run into the cabin w/o the relay and longer runs would give voltage drop.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Simplifying Something From The Factory [message #221713 is a reply to message #221710] |
Sun, 08 September 2013 21:13 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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To pull the kind of current required for the fan on high, everything in the electrical wiring would have to up graded including the ignition switch, the writing to the fuse box, fusible link, the heater control switch, etc. Using a relay moves all of that high current off of those components and puts it on a short wire between the isolator and the fan.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Simplifying Something From The Factory [message #221725 is a reply to message #221713] |
Sun, 08 September 2013 22:27 |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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Senior Member |
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| Ken Burton wrote on Sun, 08 September 2013 21:13 |
To pull the kind of current required for the fan on high, everything in the electrical wiring would have to up graded including the ignition switch, the writing to the fuse box, fusible link, the heater control switch, etc. Using a relay moves all of that high current off of those components and puts it on a short wire between the isolator and the fan.
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Good point.
On high, running off alternator current, the blower is on its own 30A fuse. I don't know how much of that 30A it actually pulls. At the lower fan speeds, the AC compressor and the blower share a 25A fuse. At HI speed, the AC compressor and the blower relay solenoid share the 25A fuse. Taking that solenoid out of the equation would lower the current on the ACC bus. If HI was battery voltage instead of alternator voltage, it would be about 5% less current than on alternator.
I can see the dashboard fan speed switch contacts being undersized for the load, but not sure that the ignition ACC bus wouldn't be able to handle it. I defer to your knowledge of the ACC bus capacity.
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