W124 climate control... fan surges
 

1991 300E 4M ... the heater works great when the fan setting is on High, but if the fan is set on Auto or Low the fan very quickly and annoyingly alternates between a whisper and a roar. The period ranges from maybe 30 seconds to a couple of minutes.

Any ideas before I start throwing parts at it? I would ordinarily suspect the push-button box, but it seems odd that the Low setting is acting funny as well as the Auto setting. Makes me wonder if it could be the blower motor?

Matthew
87 560SEL 130K
91 300E 4Matic 100K

It could be a host of things like an intermittant contact at the in car temp sensor or a faulty blower regulator.

Its most likely the solder joints on the climate control board. They develop microcracks over time and you have intermitant and eratic operation. I have seen lots of cracks where the temperature control pot is soldered to the main board. With care and skill one can open the controller and re-solder the bad connections.

I'd bet on the regulator starting to fail. I had this happen to me. The only problem with all of this is the regulator is buried under the fan in the cowling box. Pain in the "A" to take apart and get to, especially in an older car (plastic parts can be really brittle). Also, just to be sure, check the connector where it comes thru the firewall(behind and to the right of the brake vacuum boost can). If this is corroded, it can give intermittent control voltages to the regulator and it can cause these symptoms. Make sure it's clean. Check the voltages here as well. The red wire connector should be system voltage, the yellow (with the fan on high) will be some where near 7.0-7.5 volts. The regulator is the ground source for the fan. Good luck. I wouldn't bet on the ACC failing just yet. BTW, make sure your voltage regulator is regulating the system voltage properly. If it's too high, this will also cause the fan regulator to act up.