'91 500 SL (R129), 55K miles... ACC blower motor no longer operates at high speed...works fine in first three speed settings, with no abnormal noises; just no high speed.

I understand the blower motor regulators are problematic for the W140, but what about the R129? Correct regulator for this vehicle is #129 821 33 51, which lists for over $525.

How does one differentiate between a failed regulator vs. a faulty blower motor? Hate to buy a part I don't need.

BTW, where is the regulator located on this vehicle?

Safe to assume that since the fan works in three out of four speed settings the fan is not faulty?

Sure sounds like your blower motor regulator switch is going south. However, before you change it, try to toggle the key in the ignition switch with the fan setting on high. It may just be the ignition switch that needs replacement.

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Reno

'93 300E 4Matic
'98 Nissan Altima
'00 Yamaha Road Star Silverado

Jiggling the key doesn't help....there is just no high fan speed, whether in manual or automatic mode.

Unlike the W140, a replacement blower motor for the R129 does not come equipped with a regulator. But is it prudent to replace the motor when a new regulator is installed?

I assume blower motor function could easily be determined by supplying 12 v direct and checking function and amperage draw; high speed operation in the absence of the regulator would rule out a blower motor problem, provided the amperage is within specs.

Could a pushbutton control unit malfunction also result in loss of high fan speed, while leaving the three lower speeds functional?