When I bought the car I neglected to notice they had the fuse rigged with a paper clip. Replaced it with a red 18amp fuse, turn AC on it popped, just about as soon as the nice cold air started coming out of the vents. Replaced it again, it popped again, takes about 20 seconds.

A friend suggested the bearings in the blower motor might be going bad, I took the blower motor down, cleaned it nice, wd40'd it nice...it spins nice, dosent feel like anything is galled in there...put it back together, fresh fuse...POP!

Local tech suggested "reostat behind the fan switch" did he mean blower motor resistor?

Any suggestions on troubleshooting this issue?

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-SpecialDelivery
1985 300CD Silver/Blue H&R Suspension (Sold, still cryin over that)
1982 300SD Silver/Blue '85 OM617 (Sold)
1982 300D - Blue/Blue (Sold)

How are the Blower Brushes?

How does the Blower run when you use jumper wires.

The Bearings are supposed to be impregnated with Oil but the Oil Drys up. Something like 3 and 1 Oil, Sewing Machine Oil or Gun Oil (not LSA) is closer to what is needed. You need to let that Oil soak and re-apply for several days.

I believe the Blower Resistor is that Square Sheet Metal Box under the Hood on the Right Fender Well. Mine was full of Leaves and other junk and the Connections looked corroded.

I looked up the below:
Rheostat = adjustable resistor: a resistor designed to allow variation in resistance without breaking the electrical circuit of which it is a part.

It is also possible for the Motor Windings to be short circuited. That can be checked with an Ohm Meter but I don't remember how to do it. I bet there is someplace on the Internet that would have that.

If the Motor Brushes are in the rear you likely have a Bosch Motor. If you cannot see the Brushes like on Mine you may have a Siemens Motor that has the Brushes up front.

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84 300D, 82 Volvo 244Gl Diesel

Couple things --

First WD40 is bad juju. You need an actual lubricating oil. Clean motor oil or ATF applied with a syringe works great.

Second did you remove the blower motor from the housing to service the other bearing that is inside the can? Also verify the brushes are good, if they are totally worn down that eats the commutator and that destroys the motor.

Your 300D uses a tapped resistor controlled by relays in the pushbutton controller. The relays jump out sections of the resistor, when you are on high speed the resistor is totally cut out.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

hmmmm I will re-remove it and examine, post photos, re-lube. IIRC it seemed to work fine when jumped.

Thanks for the replys fellas!

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-SpecialDelivery
1985 300CD Silver/Blue H&R Suspension (Sold, still cryin over that)
1982 300SD Silver/Blue '85 OM617 (Sold)
1982 300D - Blue/Blue (Sold)

IIRC, my 1985 300D kept blowing that same fuse. It happened as soon as I took it out of Park. It ended up being the wires that cross the back of the transmission. The insulation on the wiring wore through and it would make contact with the trans resulting with a blown fuse. Hope this might help.

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'85 300D 190K miles
running on B100 Biodiesel since 156K, SCRATCH THAT! I can only find Biodiesel at 1 location in Houston! (not very convenient)