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Old 10-28-2020, 10:13 PM   #1
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Dashboard A/C Blower Motor?

My 1999 Allegro pusher has had dash fan problems since I've owned it, sometimes all speeds worked but usually just high fan. Now no fan function at all.

The schematics show it has a relay/rheostat but as of yet, I have not found it. A mechanic suggested it is a fan speed control issue so I replaced it to no avail. The resistor is under the front access panel and tests OK, but the line from the control is not sending it any 12 volt power to it. Thus my feeling it's the relay. The function selector control works as it should. The controls are GM.
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Old 10-29-2020, 07:17 AM   #2
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My 2000 A Bus has like problems and I have traced it to the condenser fan under the bus between the frame rails. There is a fuse block under the front hood center which has two fuses. I continually both blew fuses. One of the fuses controlled the fan for the condenser fan and the dash blower fan but not the compressor clutch. I don't understand the second fuse but when replaced the blower fan will run on low or high but no where else. I have not been able to understand or trace the wires. since the compressor clutch works and the blower will run on high I don't worry about the condenser fan as while moving there is enough are going through the condenser to allow cooling. I will follow your post to see what you find. My control Panel is not GM.
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Old 10-30-2020, 11:52 AM   #3
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I will follow your post to see what you find. My control Panel is not GM.
I'm familiar with those two fuses, I'll check them. The condenser fan works as it should on mine, actually everything does, only the dash fan is now out. It all worked, then intermittently worked, then only on high, now not at all. Going to search more today to find that relay. I may need to remove the radio to get to it and that's OK too. We never turn it on.
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Old 10-30-2020, 08:53 PM   #4
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Update

Today I ran down circuits via the schematic I posted. As near as I can tell, the power loss is between the switch and the resistor per the sketch.

The orange/black line between the switch and fan hi-speed relay appear to be OK as I now have power on high and the fan runs. I checked the supply fuses inside the front hood and despite the fact that I had cleaned them some time back and applied anti corrosion paste to the lugs, one of them built up resistance and blew. The contacts for the prongs which hold the fuse legs became loose and created the resistance.

So I need to trace the problem between the switch and the resistor shown on the schematic.
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Old 10-30-2020, 09:05 PM   #5
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Pictures show the inside of the covers for the Freightliner boxes buried under the dashboard and almost impossible to access. The relay and circuit breaker fuse are also shown.
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Old 10-30-2020, 11:31 PM   #6
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RK, as to your compressor clutch. The one feed on my block feeds the high speed fan relay for the heater and that is working now. The other runs via a relay to the condenser fan. Have you found that relay?
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Old 11-01-2020, 11:31 AM   #7
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Update 2

Yesterday I put the dashboard mostly back together and did more diagnostics on the resistor. I now have power to each terminal of the resistor but I suspect the resistor is shorted out. If you look at the schematic I posted earlier, you will see a light blue line which is supposed to go to the resistor. I found that yesterday in the harness, it had been cut off.

Previously, when I hooked a ground wire to the ground terminal of the resistor, it immediately blew a fuse. That would explain why the original owner's "mechanic" disconnected it, but would it have been so difficult to replace the resistor itself and maintain full functionality of the system? Apparently to his "mechanic" it would have been.

Regardless, I did retest the unit with the system on and wires hooked to the resistor. All poles read the same 12.5 volts, so it definitely is not working. Now on the hunt for another one.
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Old 11-05-2020, 11:58 AM   #8
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The resistors will NOT drop voltage, they will drop current and that is what limits fan speed. https://eepower.com/resistor-guide/r...wer-resistor/#
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Old 11-05-2020, 12:08 PM   #9
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The resistors will NOT drop voltage, they will drop current and that is what limits fan speed. https://eepower.com/resistor-guide/r...wer-resistor/#
Resistance will also have an effect on voltage.

https://gm-techlink.com/?p=10476
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Old 11-05-2020, 02:32 PM   #10
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RK, as to your compressor clutch. The one feed on my block feeds the high speed fan relay for the heater and that is working now. The other runs via a relay to the condenser fan. Have you found that relay?
No My Freightliner wiring schematic does not show a relay i does show the fuse block where the high speed fan fuse is located and the fuse next to it will power up the condenser fan for a while then will blow. Thad fuse is shown to be 30 amps and I have gone up to a 40 amp and the results was just a little longer before it blew. Bottom line when that fuse is not blown the intermediate evaporator blower fan speeds will work. I know the fan is in its last legs as I am sure the bearings/bushings are bad as there is resistance when you try to turn the fan blades by hand. Again my diagrams do not show a relay or a multi-breaker.
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Old 11-05-2020, 02:40 PM   #11
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Resistance will also have an effect on voltage.

https://gm-techlink.com/?p=10476
Yes, but it's not the voltage drop that controls the fan speed, it's the current drop.
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Old 11-05-2020, 02:54 PM   #12
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Was always taught in dc app voltage was speed and current was torque. That why on our 2000 hp dc traction motors the voltage was regulated from 0 to 800 VDC. We had current limits we could set according to pipe size bur voltage was set by the throttle rheostat and we called that RPM.
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Old 11-05-2020, 02:56 PM   #13
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Good job of troubleshooting, don’t you love it when it took more work to bury something then to just mount it in the open
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Old 11-05-2020, 11:20 PM   #14
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Good job of troubleshooting, don’t you love it when it took more work to bury something then to just mount it in the open
Thank you! Perhaps tomorrow I'll have a chance to get the new resistor mounted and test it out. If it fails, I will be at a loss, everything else has been eliminated except the fan itself.

RK, you have a slightly newer chassis, so probably not exactly the same wiring. Would bet you are right, the fan is shot. I don't recall that being a difficult thing to get to, directly under the front of the coach. Mine runs quite bit when I'm in a traffic jam during the higher summer temps.
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