My 91 300 E 4matic blower quit. It had been making some squeal noise in the past weeks. I replaced the blower. Still not work. The fuses are OK, both the 8amp and 25 amp inline. Tested the aux coolant pump and had .81 amps and then went to 1.0 amps when warmed up. My book says replace the aux over .8 amps. I shot some WD40 in the aux motor bearings and amps dropped to .5. Still no blower on any setting. I have 14+ volts to the blower(red wire) and -.22 volts (blue wire). If I take the blue terminal to ground the blower runs. Should I replace the aux pump or is there something that I am missing and should check out first. Thanks for any and all shared knowledge.

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Handyjay

I just checked the 3 wire plug behind the brake booster. I have 12+ volts on the red wire, 8 volts on the yellow and a good ground on the black wire. These go to the blower regulator. My research says that with these readings the climate control is OK. Next I ground the blue terminal on the blower and the motor runs at high speed. Blower motor OK. I am down to the regulator or the aux coolant pump. The pump was pulling over .8amp and then I lubed it with WD40 and it draws .5 amp. My book says it must be the regulator. They don't give these away so I am looking for a procedure to confirm the regulator is bad. Any ideas?

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Handyjay

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You have already confirmed the reg as bad .
You state blower works fine if you ground the blue wire .. that by-passes the reg out of the circuit and verifies both blower power feed and good blower motor.
..and you state that you have trigger voltage [ yellow wire ] to the regulator.. that triggers the regulator, which is not happening b/c the reg is not conducting to ground even though it has trigger voltage.
.Diagnosis....bad reg...................................

Cheap out/temp fix ....... cabin ground switch on blue wire ... high fan only.

Arthur, thanks for the feedback. Is there any fixing on a regulator like a diode or internal fuse? Would a bad blower motor cause the regulator to fail? Should I be concerned about the aux coolant pump motor? Thanks again.

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Handyjay

The reg is a voltage regulator circuit consisting mainly of a high power switching transistor that is regulated by a variable low voltage trigger , [ yellw wire from CP]. This transistor is encased in epoxy and is non sevicable.

The blower motor can be the cause of reg failure if it is exceeding spec draw [ load amps] I always take a blower motor amp/current reading before changing the reg to verify blower brushes/bearing condition, You are looking for 25 A max @ full speed.
Many reg. failures are due to improper air circulation across the reg. heat sink , usually caused by blocked air filters [ on systems using such]
Pump --No

I have a regulator that is working from my '91 300d. $100. I thought it was defective, so I replaced it with a new one, but it was my motor that was bad.

jeff

That's why the amp draw test is cool........

What's an amp test draw and how do you do that?

jeff

You put a meter [ ammeter] in series with the motor power feed and it wil tell you how much current [Amps] the motor [or any other eletrical device you want to load test] draws. If the test show too high a draw , there is something wrong with the item being tested.
In the case of blower motor , that usually means bad bearings or brushes in the motor. This too high a draw will rapidly burn out the regulator/wires and fuses, as the draw exceeds the design specifications of the system.

All is well. Blower works perfect. Replaced the blower motor and regulator. Cost including shipping about $400. Wonder what a dealer would have charged for the same job. Thanks for all the help.

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Handyjay