Author Topic: AC/Heater blower motor does not work on high.  (Read 2761 times)

Offline Rapid Roy

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AC/Heater blower motor does not work on high.
« on: September 05, 2019, 11:33:50 AM »
Searched and did not find this issue. The blower motor shuts off when switched to high. Works on all other speeds. Not sure if it switch, relay, or the motor. Suddenly stopped yesterday while using AC. Changed to Bi-level to use the fan speeds, except high.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
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Offline VileZambonie

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Re: AC/Heater blower motor does not work on high.
« Reply #1 on: September 05, 2019, 05:31:28 PM »
Check the connections at the relay. Very common issue
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Offline Rapid Roy

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Re: AC/Heater blower motor does not work on high.
« Reply #2 on: September 05, 2019, 08:07:15 PM »
Check the connections at the relay. Very common issue

Thanks VZ, I did run a jumper from B+ to blower motor, and motor did run on high. Removed connectors from relay and resistor and reset both and no change. I will remove connectors and clean contacts tomorrow and I am planning to have the resistor tested by a co-worker that is an electronics engineer to see if it is still good. May need a new resistor or relay, not sure yet.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Online bd

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Re: AC/Heater blower motor does not work on high.
« Reply #3 on: September 06, 2019, 03:22:08 PM »
Use a test light to probe the 10-ga orange wire at the relay.  Is it constant 12 volts (B+)?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
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Offline Rapid Roy

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Re: AC/Heater blower motor does not work on high.
« Reply #4 on: September 06, 2019, 05:24:12 PM »
No voltage at orange wire at all in any fan speed positions.

Today removed relay, cleaned connector prongs, took apart and cleaned contacts. No difference. I was tapping on relay and fan came on high for maybe a minute then stopped. Decided to replace relay and still same problem. Also removed resistor, inspected and cleaned contacts and same problem.
« Last Edit: September 06, 2019, 05:28:16 PM by Rapid Roy »
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Online bd

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Re: AC/Heater blower motor does not work on high.
« Reply #5 on: September 06, 2019, 06:28:05 PM »
That's because the fault is probably the inline fuse holder in the 10-ga orange wire where it connects to the firewall junction block.  The high blower fuse is notorious for overheating due to degraded electrical connections, which melts the fuse holder, which degrades electrical connections of the fuse even further until, eventually, no connection remains. 

Temporarily use a 10- or 12-ga jumper wire to bypass the fuse holder and restore power directly to the relay, then try the high blower using the dash fan switch to see what happens.  If high blower function is restored, replace the inline fuse holder and fuse using a 20-amp ATO/ATC style that has a dust cover.  Post the results and we will continue from there, if necessary.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Rapid Roy

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Re: AC/Heater blower motor does not work on high.
« Reply #6 on: September 06, 2019, 07:09:07 PM »
That's because the fault is probably the inline fuse holder in the 10-ga orange wire where it connects to the firewall junction block.  The high blower fuse is notorious for overheating due to degraded electrical connections, which melts the fuse holder, which degrades electrical connections of the fuse even further until, eventually, no connection remains. 

Temporarily use a 10- or 12-ga jumper wire to bypass the fuse holder and restore power directly to the relay, then try the high blower using the dash fan switch to see what happens.  If high blower function is restored, replace the inline fuse holder and fuse using a 20-amp ATO/ATC style that has a dust cover.  Post the results and we will continue from there, if necessary.

I believe you were right on it. The fuse holder looks to be the OE. The long end use to be white, which is now partially brown, and the short end is black. Tried to remove and couldn't cause it was melted (welded sort of) together. Removed from firewall and had to cut the old plastic apart and has a 30 amp fuse. Going to get a new fuse holder tomorrow.

Does the wiring need to be 10 gauge or can I use 12 gauge which I have?

Thank you BD. I probably would not have thought about the fuse holder at all.

I will report back after repair to let you know.
« Last Edit: September 06, 2019, 07:11:03 PM by Rapid Roy »
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline 75gmck25

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Re: AC/Heater blower motor does not work on high.
« Reply #7 on: September 06, 2019, 07:17:38 PM »
That old inline barrel fuse holder on my '75 kept burning up on my truck and finally melted on one end.  I bought an inline fuse holder for a 30 amp blade fuse and spliced it into the power wire as a replacement.   It seems to have the right size wire (not sure of the gauge), and has worked fine without blowing the fuse or burning the holder.

Bruce




Online bd

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Re: AC/Heater blower motor does not work on high.
« Reply #8 on: September 06, 2019, 08:11:54 PM »
Does the wiring need to be 10 gauge or can I use 12 gauge which I have?

The blower motor typically draws ~20 amps.  Will 12-gauge wire work without incident?  Perhaps.  However, under conditions of 20 amps continuous flow, I recommend 10-gauge wire and a similarly gauged ATO/ATC fuse holder.  Install a 20- or 25-amp fuse.  If it melts, there may be voltage losses across the wiring that need to be addressed or the blower motor may be worn out.  A 30-amp fuse is overkill if everything is functioning as designed.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Rapid Roy

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Re: AC/Heater blower motor does not work on high.
« Reply #9 on: September 07, 2019, 06:21:36 PM »
High speed back again. Installed new fuse holder and fuse and works normally now.

Thanks again BD for helping troubleshoot issue.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Online bd

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Re: AC/Heater blower motor does not work on high.
« Reply #10 on: September 07, 2019, 09:11:00 PM »
You're welcome.  Glad it turned out to be a simple fix.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)