Hi all,

I'm having issues with my 1984 300D blower motor working intermittently.

Sometimes it works, sometimes it doesn't, and when it is working, after driving for 10-15 minutes, it cuts out.

When power is hooked up directly to the blower, it will run indefinitely. I also have a few known good motors from other Benzes that I can swap in if needed.

Where is the relay for the blower motor located?

Thanks in advance for any replies,

Bryan

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1984 240D Euro 5 Speed 201,000 miles
1984 300D Euro 4 Speed (runs great but ugly)
1984 300TD 185,000 miles
1985 300TD, 240,000 miles
1972 220D, 240,000 miles w/ a stuck odometer
1984 300D, 214,000 miles
1984 300TD, 255,000 miles rustbucket parts car

There is a blower motor relay but the symptoms you describe are usually a fault with the pushbutton control panel (aka pushbutton unit, or PBU).

The typical fault is that the blower won't start up when the PBU is in the "ECON" position (second from right button depressed), but will start when the AC button (second from left) is pushed, then the ECON button a few seconds after. Or sometimes it requires DEF to be engaged, then another button.

Once it wears out more, the fan cuts out after a few minutes on ECON, and/or when you come to a stop.

The relay should be behind the glove compartment, attached to the bulkhead. You need to pull out the glove compartment insert (bin) to access it. (At least that is where it is on my '80, I never checked my '85 to see if it's in the same place.)

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Thank you for the reply.

The symptoms are odd. When the blower doesn't want to work, it doesn't matter which position the climate control is in, and the same goes for when it does want to work. Whenever it stops working, I will shut it off, and if I wait a while, I can turn it back on, but it will only last for 5-20 seconds until it shuts off again.

Bryan

__________________
1984 240D Euro 5 Speed 201,000 miles
1984 300D Euro 4 Speed (runs great but ugly)
1984 300TD 185,000 miles
1985 300TD, 240,000 miles
1972 220D, 240,000 miles w/ a stuck odometer
1984 300D, 214,000 miles
1984 300TD, 255,000 miles rustbucket parts car

I still vote for the climate control pushbutton unit. They can produce weird symptoms that are ambient-temp related. For example, my old one would only work on DEF on very cold mornings. Once the cabin warmed, it would work on other settings.

Also there is an issue with that climate control system's wiring whereby if the auxiliary electric coolant pump (under the hood) seizes, it can send a voltage spike up to the pushbutton panel that fries some or all of the circuit board inside, or in extreme cases partly melt the unit. If you search you will find info on this (and on how to install a 2A fuse in line to prevent it). Symptom of aux pump seizing is lack of heat at idle when the car is warmed up.

Other than that, you could do worse than to look for the relay and make sure the connector is firmly plugged in. I had weird issues on my 1980 wagon that went away after I found the connector had worked a bit loose.

Personally I haven't replaced any relays on any Mercedes I have owned, other than the one for the turn signals/flasher. So I don't usually go hunting them as the cause of anything.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Yeah, the PBU has been know to exhibit the symptoms you mention.

The FSM won't open, "component location".

Can't find it in the epc.

I have replaced a relay in the SD.

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83 SD

84 CD