91 300 SEL - 145,000 mi., documented regular service.

I've spent the better part of this Sunday searching through your archives, but more confused now than ever. Before I buy a 'for sale' sign', maybe some advice?? (I've tried jiggling the key)

Symptoms:

6 months ago - when purchased - didn't seem to get too much heat under dash, but powerful in defrost and upper vents .

3 months ago - blower fan set on Auto (at operating temp)- moving from high to low erratically and quickly (only happened once a week.) Either heat or A/C worked fine. Fan very powerful.

2 weeks ago - AUTO setting seemed not to be working at all - but high worked fine, low - hard to tell. NO squeaking, clicking, etc.

3 days ago - blower fan won't run on any fan speed, any temp, A/C on or off.
2 days ago - Blower fan working on high again for 30 seconds, then shuts off.

Today - no fan at all, regardless of settings.

A/C seems to be engaging, as engine pitch lowers when the A/C buttons are pushed. Pitch goes up on economy setting.


All roads on this board seem to lead to first replacing the Monovalve - NOT.
Any ideas???

The monovalve could help with the flaky heat. Mine would come and go. The monovalve diaphragm seemed to help. Someone posted a pictorial how-to(diesel 126) awhile back. Same deal.

Monovalve how-to at this URL:

http://mbdieseldiy.tripod.com/monovalve.htm

For the blower, pop the hood - drivers side near firewall - fuse box. Open it up. Look at #5 fuse. If this blows, you loose the blower. Even if the fuse looks good, replace it or at the very least, reseat it, but beware - these ceramic fuses are WEAK. Reseating could break the fuse. I suppose the brushes in the blower motor could be worn. Try the #5 fuse first. Quick & simple if it solves your problem; however, I kinda doubt it will. Sounds like you've been loosing this on & off. Fuses usually are not an on/off affair. Thought I'd mention #5 anyway. Mine has blown twice in the 6 yrs. I've owned the car.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I believe that there is an electronic fan speed control circuit module which is mounted on a heat sink which in turn is mounted on the actual blower motor. As stated in the previous post, your intermittent trouble is not likely to be a fuse. I'd suspect the motor brushes, the speed control circuit module, corroded wires / connections and finally (gulp!) the HVAC computer itself. I can't think of any easy way to trouble shoot this. You're going to have to get a look at that motor. If it is brushes ( they'll be worn down to the springs ), I'd take it to a electric motor rebuild shop. That'll be a lot less expensive than replacing the entire blower motor with an OEM new one. Hopefully, someone else in this group can advise you on how to get to the motor. Good luck.

__________________
Resistance is Futile.

Thanks guys for your insight. However, knowing that six months ago the original problem began as not getting any heat underneath, but getting lots of heat out of the defrost vents has me stumped. I suppose this could be multiple problems, but in your opinion, is it possible that the control head unit (whatever the push button device is behind the dash) could also be faulty?

any way to check the functioning of the blower motor with external power?

Have you seen a post that shows how to extract this unit? I haven't found one yet.

thanks again.