93 300E Blower Motor Surges on Auto & High
 

I am trying to figure out what is causing the Blower motor to surge when the AC is set to "auto" or "high". It works fine on low. When put on high setting, the fan runs fine for about 10 seconds, then surges between high and low speeds, then stops. Fan runs smooth and quiet, it just won't maintain higher speeds??? Any solutions?

Thanks

To me, it sounds like a poor connection to the motor. That is what a D.C. motor will do when it is not getting its full voltage, or is starved for current, because that's what Ohm's law would have it do in that situation. I would check both the power and ground sides of the circuit. Maybe start by putting a voltmeter across the motor and see what the voltage does as the motor surges. If it stays constant, it's probably a motor problem, while if it goes up and down with the motor surge, it's a power problem.

OR - maybe a weak battery causes the slow running (esp. if there is other current drag on the system, such as lights), and the surging is caused by the voltage regulator kicking the alternator into "charge" mode?

most prob the blower motor regulator going out[the regulator supplies the ground] take down the panel under glovebox to gain access and run a ground wire direct to motor to see if it will run full speed and remain so.

You can not get to the motor from glovebox or panel on that chassis.
It is under the screens , under the hood.
Ground the Blu wire to check for high fan.

RE: Blower Fan Surges on 300E
 

Many Thanks for the Input. The SAGA gets deeper.

As I was checking the wiring from the blower motor, it appears someone wired the power direct to wiring under the fuse block. They essentially just stripped the wire insulator and soldered the blower power wire in. I essentially stopped right here and told my son to wait until we could figure out the proper power connection before driving it again.

Where should the power wire connect? It appears to be a Red 10ga power wire coming out of the blower box, run along the firewall, and connected to the underhood fuse block wiring. I called the previous owner and found out they took the car in to a local gas station (in their hometown) for this repair - They were not Mercedes mechanics evidently...

Any Guidance would be Greatly Appreciated

Thanks - Brett

Go to www.autolib.diakom.ru/CAR/MercedesBenz/. In the first dropdown box, select year for your car. The screen will change to a second page where you select model in the second dropdown box. The screen will change again where you select SYSTEM WIRING DIAGRAMS in the third dropdown box. The page will display all the wiring diagrams for your car. Select Air Conditioning Circuits, page 1. This should show the circuit for your blower motor.

RE: Blower Fan Surges on 300E
 

Thank You So Much for the Great Link!!!!

Brett

Glad to help!!!