Sounds like a bad regulator to me. They're basicly a heavy duty FET. They can fail so that they will show voltage at the load but not supply any current.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

This is a very expensive part. How can i be sure that it is bad?

http://catalog.peachparts.com/ShopByVehicle.epc?q=1989-Mb-4--0sel-Climate--Control&yearid=1989%40%401989&makeid=MB%40%40MB%40%40X&modelid=420%2DSEL%2D001%40%40420SEL&catid=R%40%40Climate+Control&subcatid=R2022@@Blower+Regulator&mode=PA

See the red and blu wires that goes to the blower??
Jumper wire from the blu to ground ..if that gives you high fan, reg is suspect.
But you see that 3 wire plug?..that should have a yellow wire that is variable V from ther Control Panel..that is what triggers the regulator and if no V there , the problem is not the reg.
You should have 12V ay the red also.

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A Dalton

I will test it tomorrow.

420 benz, not sure if your 420SEL has a 30-amp strip fuse on the driver's side shock tower. It's enclosed in a small black plastic case and has a philips head screw on either end. Typically the strip fuse develops a hairline crack which results in the blower motor not working. If you loosen the two screws it will be obvious if the strip fuse is cracked. If so, replace it.

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Fred Hoelzle

Arthur informed me about that fuse.I did remove it and it is ok.Too Bad that would be too easy. Thanks

Tested Blue wire to ground"nothing"
Tested three wire plug.Yellow"nothing"-White 7.6v
Green/Black 12v Note:With the BM cover off and the key on with both red and blue wires attached to the BM The fan turned very slow.That was this afternoon. I just tried it again and nothing.