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Marty
Marty, ASE Certified Technician
Category: Car
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Experience:  ASE & Bosch Certified Master Tech. Over 30 years experience.
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1988 Mercedes Benz 300E: Heater fan..tested..battery voltage

Resolved Question:

1988 Mercedes Benz 300E question: Heater fan not working. I have a fan from my '86 ( same make and model). Can fan be tested with battery voltage before tearing area all apart to replace and how ? Tt has 2 connectors (stakon type) to brushes. Seemingly no ground (maybe case is ground).
Submitted: 3 years ago.
Category: Car
Expert:  Marty replied 3 years ago.

eurotec :

Hi, you cannot actually test the fan motor itself without getting right down to it because the motor circuit goes through the blower regulator first.

eurotec :

You can put power and ground to your used fan motor to test it however.

eurotec :

I can tell you how to test for power to the regulator and from the pushbutton switch if you need help with that.

eurotec :

Before you go to far you need to make sure that the blower motor fuse is good. The fuse is not inside the main fuse box. It is located just outside the box in a separate fuse holder.

eurotec :

Open the lid of the small fuse holder and see if the strip fuse is cracked in the center.

eurotec :

Full Size Image

eurotec :

I've attached a wiring diagram of the blower circuit below.

eurotec :

Full Size Image

Customer :

I have checked that R15 fuse and it is good --12v both sides-- with ignition on. I have also tested that--what I would call--the regulator connection (behind master brake cylinder unit)--a three wire connector red,yellow and black. With ignition on I have 12 volts on red. 8-10 volts on yellow and 0 volts on black (ground?). How (where) are these leads conneced to blower motor. Like I said there are two connections on the motor (replacement from '86) push on single tip one male one female. They do seem to be connected to the brushes of motor.

Customer :

I also hve a full electrical diagram of the '88 (and also '86 --pretty much the same) and a partial repair manual

eurotec :

Ok, then you have done all of the correct tests to this point. You either have a bad blower motor or blower regulator.

eurotec :

You need to take it apart and individually check the blower when you get it apart.

eurotec :

The regulator is mounted underneath the blower motor.

Customer :

How do I check the replacement motor before I take it all apart ( I have a procedure for that) and find my replacement motor 9 from my old '86 (which WAS working) is also bad.

eurotec :

As far as your used motor you can bench test it by applying power and ground to the terminals.

Customer :

I have leads and extra alligator clips for testing

eurotec :

Full Size Image

eurotec :

Just connect the red lead to the positive battery cable and the blue lead to the negative cable.

Customer :

any paticular polarity for either clip

eurotec :

Make sure a helper is holding the motor.

eurotec :

It doesn't matter

eurotec :

but it does matter when you install it.

Customer :

I have NO wires on the old clips are fastened right to the body of blower no colors male and female though

eurotec :

ok, just connect one wire to each terminal

eurotec :

It will either run forwards or backwards

Customer :

ok I'll try it and get back to you.

eurotec :

ok

Customer :

Well the blower fan seems to work fine YEAH!!!! Thanks for the info on that and it was well worth the price just for that but am wondering about this regulator. What does it look like and do you know if it clips in or screwed in? Why I'm asking is because both procedures I've seen for removing the fan NEVER mention the/a regulator unit near the fan/blower motor and they were quite explicit on the discription of the replacement process(any pictures of it).

eurotec :

That's because the blower regulator is underneath the motor bracket and doesn't get disturbed in the process.

Customer :

Would you have a part # XXXXX would it be on it. The fan had been acting pretty funcky in the last year or so coming on at times (or not---squealing and such) so I was thinking it was the fan but it may be the regulator if it cotrols the speed of the fan in some fashion. No big deal if your not sure. just thought I cut down on research time. before I start this deal. Pretty cold here in Maine right now if the speed up there is SOME heat but not a lot.

eurotec :

124 821 21 51, Blower Regulator with dust filter

eurotec :

124 820 27 10, Blower Regulator without dust filter

Customer :

OK. Thats all I need for now thanks alot

Marty, ASE Certified Technician
Category: Car
Satisfied Customers: 49962
Experience: ASE & Bosch Certified Master Tech. Over 30 years experience.
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