Fan Voltage on AC unit - PeachParts Mercedes-Benz Forum

Ken Griffith · #1 07-10-2003, 12:35 PM

My fan speed regulator went out months ago and I had it replaced. Everthing works OK now except the fan doesn't seem to blow hard enough. When I check the voltage through the front panel it won't go higher than 4.8. This is on my 96 S-500. On my 98 430E it goes to 6.0. Looks to me to be the same units. What should the s-500 go to? IF I'm not there is there anything to do? I've manually put the AC to "LOW" and maxed out the fan speed. 4.8 is it.

KG

JimF · #2 07-10-2003, 08:53 PM

sounds like the blower regulator is gone again.

Not sure of the numbers but it blower should go from hi to low speed as you spin the dial (Min - Auto - Max).

Maybe good news it's still under warranty. Also make sure that the 'white' grease make good contact with the blower's housing to dissipate the heat in the regulator.

KenP · #3 07-12-2003, 07:58 AM

Ken,
Went out and checked my 1996 S500 Coupe...
You're reading out the blower voltage on your climate control pushbutton unit, right?
Service manual says maximum of 6 volts.
My S500 blower works great; lots of air through the vents. When I manually max out the blower fan speed, my reading is the same as yours... 4.8 volts.
Next step down is 3.2... and then down from there.
My A/C and blower work fine. I think you're OK electrically.
There are a number of pushbutton units (starting in '96) that LOOK the same, but they are not... so, you can't go by that.
If your vent flow rate is low, its probably something else. How long since you changed your main filter? or, maybe you need to replace the blower motor itself. I replaced mine recently (it started to squeek/squeel) and the flow was much improved.
Hope this helps, KenP

stevebfl · #4 07-12-2003, 01:27 PM

The real way to test your problem is to evaluate the voltage drop from the blower neg side to ground. When in high blow the drop should be less than 1v. In theory it should be zero but you are dealing with a transistor amplifier and it does not become a perfect conductor.

The regulator regulates by closing the ground side of the circuit. If you watch this voltage drop it will be over 6v at low speed and drop as the speed increases.

KenP · #5 07-13-2003, 07:17 AM

Steve, can you explain a bit more? I certainly understand regulating the speed of the motor by varying the ground side/path of the motor, but something doesn't add up.
If the voltage drop to ground is 1 volt or less at high speed, what is the "blower motor voltage" shown in the diagnostic mode of the climate control head? Maybe its not really blower motor voltage?
If hot side is 12-14 volts, how can "blower motor voltage" only be 4.8 volts with a 1 volt drop?
Thanks much, KenP

stevebfl · #6 07-13-2003, 08:27 AM

Well I planned on writing you a private answer since today I am not feeling very generous, but your profile states you won't accept email from the group. Since such access is the reason I don't feel generous this morning I leave it unsaid.

stevebfl · #7 07-13-2003, 10:34 AM

At least you are protected from random violence, maybe I should consider being anonomous. But, I am over it and I am sorry, so here is your answer.

The voltage given in the display is the control voltage. if you made the circuit into an amplifier transistor device this voltage would be the base biasing voltage. The transistor then would alow current to flow in proportion to the bias strength. Transistors are used as amplifiers or switches. In this case they are being used as an amplifier.

The voltage I was mentioning; the voltage drop between the blower ground and real ground is a measurement of the level of amplification. If the transistor was at high blow there would be theoretically no voltage drop as the ground of the motor would be pulled all the way to ground. In low blow the blower ground is only pulled to maybe 6v. The regulator/transistor looking like a resistor to the circuit.

The test is easy as the blower circuitry is easily accessible under the large plastic cover at the rear of the engine compartment. There you can verify that the bias voltage displayed on the pushbutton controller is the actual voltage seen by the regulator AND using the voltage drop method one can see how the regulator responded to command.

KenP · #8 07-13-2003, 07:50 PM

Steve,
Thanks. Not sure I entirely understand, but I'm going to pull the schematic and see if I can noodle it out with your added help. I'm a professional engineer by training... the way these cars are laid out electrically and mechanically interest me... which is the primary reason I bought one.
Don't understand why you ran into a mail problem with my profile. Others have sent me mail. Something must have changed... I'll check. Anyone can reach me direct at gearhead285@hotmail.com.
No intention of trying to stay unknown to the group or anyone else for that matter.
Regards, KenP