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Old 06-02-2003, 01:31 AM
Posts: n/a
Too high control voltage on ACC?

My blower had an occasional squeal when I got the car, then went intermittent, then completely stopped working. Before it stopped working I had variable speed, indicating the regulator was functioning.
I pulled the fan out yesterday to lube the bearings and replace the brushes. Then I got sidetracked trying to grease the windshield wiper gearbox. (separate post)

The brushes were ok, but since I had ordered a set, I installed them (not like you can pick up a pair at 7-11). I lubed the bearings, and reinstalled the fan. The fan seemed stiff, even after lubing. It will not freely spin and stops immediately after removing my finger. I do not know if this is normal.
Started the car and the fan was blowing. It ran for at least 5 minutes, then I started pushing different buttons on the ACC. The fan stopped. No combination of buttons will restart it.
I pulled out the meter, and checked voltage on the firewall. Had battery voltage at the red wire. On the yellow, it is 1+ volt on low speed, 3+ on auto, but measures 10+ volts on high. Most of the posts here indicate 7+v is the upper limit.

Is this indicative of a faulty ACC? Or does it sound like the regulator is toast? Lastly, should the fan freely spin?
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Old 06-02-2003, 04:14 AM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,114
The fan should spin pretty freely but I don't think it goes for a long time after you stop applying spinning force. I don't think you can spin the motor shaft without cage fast enough to get it to spin after you lift your fingers.

Did you check for axial play in the bearings? My SDL fan makes noise at anything but max. It usually stops if it gets to min. I tried the SE fan in the SDL and it works as it should. The only difference I can tell is that the SDL motor has more axial play. I didn't try the SDL blower in the SE.

As for the voltage, what does the fan see? It's a strange (to me) set-up where one wire sees constant +12V and the other wire sees anything from close to 0V (max) to something close to +12V (low). FWIW, I think I saw a max of +7.4V on max at the firewall when I was trying to figure out what was wrong with the SDL.

91 300SE
87 300SDL
83 300SD
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Old 06-02-2003, 04:58 AM
Posts: n/a

The bearings do not seem to have any play at all. I really expected the fan to turn more easily after oiling, but this did not happen. And since this is balanced as an assembly, I did not remove the cages. It stops dead as soon as I let go with the cages on.

For the voltage, it appears the regulator uses the control voltage to determine how much of the 12v (red wire) to send to the motor, and grounds the remainder (converts to heat, ergo the pentium-class heatsink). At least this is my current understanding. (no pun intended)
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Old 06-02-2003, 08:18 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The fan should spin free.

The normal bearing problem is that the shaft seizes to the bushing. at that point the bushing spins in the housing. That is not a bearing and lubrication won't help for long. You must be sure that the shaft spins in the bushing not the bushing in the housing.

I can't tell you whether the controller goes that high, it doesn't on the later systems. but it might on yours.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 06-02-2003, 01:50 PM
Posts: n/a

Sounds like a new fan is in order.
I have seen at least one post indicating that a stalling fan can burn out the regulator.
Is there any way to test for damage to the regulator or is this just a go/no-go proposition?
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