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1980 380SL
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65 Posts
Discussion Starter #1
My heater fan runs continuously when the car is running, it doesn't make any difference where the control is set the fan runs at full speed the whole time.
Any suggestions as to where the fault may lie as it's driving me nuts!
Regards
Graham
 

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1979 450 SLC 5.0
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486 Posts
Having just spent over a month, and $300 to diagnose and fix my fan that would not run at all, I think I can safely weigh in on this situation.

The Fan Switch gets power from Fuse #4 on pin 5. Then the four fan speeds are output to the blower resistor pack on pins 1-4.

The blower resistor pack receives a ground on pin 3, and the only time it runs at full speed is when there is power on pin 4 (I think it is pin 4, off the top of my head). This input pin by-passes all of the four resistor coils in the pack, and gives the full 12V to the blower motor.

The remainder of the time the 12V coming in on one of the remaining 5 pins (yes 5, even though there are only four fan speeds) drops the voltage from 12 to ~ to reduce the fan speed depending on how many coils it is routed to.

There is one more source of power to turn the fan on, but it is miniscule even compared to fan speed 1. That is the "Anti Freeze Up" automatic fan on when the A/C is switched on. This will run the fan at about 1/2 of fan speed position 1, to keep the A/C Evaporator from freezing up if us dumb humans turn on the A/C without turning on the fan.

The power source for this auto A/C comes from the Freon Temperature Switch mounted on the Receiver Dryer.

Easy Trouble Shooting. Pull the fuse for your Blower motor, if it is still running it is the power source for auto fan from the A/C unit. (But that would not explain yours running at MAX the whole time.)

If it quits with the fuse out, then you have a shorted fan switch, and it's never turning off the power to the fan.

What would explain yours running full blast with the fuse removed is the Bi-Metalic Spring in the Resistor Pack. It closes to run the fan at full speed if the resistor pack overheats. (But there would still have to be a source of power from SOMEWHERE, and my guess would be the auto A/C input, with the Bi-Metallic Spring shorted out could still do it at that point.)

JetPaul
 

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1980 380SL
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65 Posts
Discussion Starter #4
Thanks for the info guys when I get home from work in 6 weeks I'll give it a severe looking at, I need to pull it out anyway as the bearings are in need of a lube.
Many Thanks
Graham
 

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Registered
1980 380SL
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65 Posts
Discussion Starter #6
Although I've been home about a week now the weather has been terrible so haven't ventured out much, managed to clear a space in the garage yesterday and got the beast indoors to get the top dry so I can refit the hardtop, once that's on I'll attack the fan. When I went out the other day it was off when I first set off, switched it on for demist and when I got to town it wouldn't stop so does seem to be temperature related and the bimetal switch is the prime suspect. When I get the motor out is the switch attached to it or the resistor pack?
Thanks for your support
Graham
 

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Registered
1979 450 SLC 5.0
Joined
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486 Posts
No need to pull the blower at all.

Take the top cover off and the Bi-Metal spring switch is in the resistor pack.

Look at all the coils for evidence of overheat or other damage. If they all look good put a bit more tension (by bending) on the bi-metal spring to hold it open better.

I bent the small metal tabs on the resistor pack "cage" and reinstalled it on mine without any problem.

JetPaul

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