Our 1986 420 SEL suddenly quit blowing air. The fuse is fine and I pulled the blower motor and it works fine on the bench. Is there a separate relay or does this mean our fan controls are broken? thanks, Mark

Three possibilities:

Fan speed control bad (expensive)

Pushbutton unit bad

Blown ACC fuse (probably the one you checked) or blown blower fuse. The blower fuse may or may not be in the fuse box (not sure on the W126 chassis) -- if in the box, it's a big red one. Replace -- they crack and can look fine but not transmit current. Else check the ACC fuse, there is one for the control unit too, and if bad the fan won't run. If the fuse is NOT in the fusebox, it will be in a small coffin shaped box, and consist of an aluminum strap fuse. These often fail by cracking across, look fine until you remove one screw when it falls apart.

Check the speed control by checking for a switched ground (fan is hot all the time on the supply side). If absent, check for 0-6V signal voltage on the yellow wire to the switching unit (big thing with fingers on it in the blower box). If present, switching unit is bad, if not, PBCU is bad.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter,
Thanks, I'll get to work and see what I can find. Thanks, mark

Peter,
Thanks for your help. I replaced the blower fuse and ACC fuse and checked them with an ohmeter to make sure they were fine. I apologize for being dense, but explain how you wanted me to check for a switched ground. Also I unmounted the switching unit from the blower cover, and I see the yellow wire, but there's no contact point to test - it's encased. Should I unbolt the black wire block from the switching unit to see the back side? Thanks so much for your help. Mark