87 190D odd blower motor/climate control issue

[OmegaBelly]

Hello everyone, longtime lurker here and I hope this is in the right topic place. My recently acquired Benz has an issue I’ve never dealt with nor could find any information on. The blower motor only works with the key in the run position, but shuts down once the car starts. I replaced the speed switch, the current climate controller is a rebuilt unit from the previous owner. I plan on pulling everything out to chase a potential break in the wiring this weekend. Also my blower motor screams like a banshee, plans on replacing that is in the works. Anyone deal with this before, or know of this issue? I’ve owned a couple 201’s and usually get the ticking or sporadic on/off. The cold is coming and I need a defroster!

TL;DR blower works on start position, but shuts off when engine runs. Any advice is appreciated! ~275k miles

Additional tidbits: 2.5 NA, automatic, replaced lifters, chain tensioner, master cylinder, fuel lines and filter, oil change and filter and interior swap from my wrecked 84 2.2 (may it rest in pieces)

[Diseasel300]

A blower that works in start, but not run is likely an issue with the ignition switch itself. Squealing motor is one that needs replaced or lubricated. If it's been squealing for a while, you're better off replacing it. If it hasn't been making noise for long and the shaft isn't tight or excessively sloppy, you may be able to buy some time by oiling the bearings with oil made for electric motors (Zoom Spout or 3-in-1 in a blue bottle).

[OmegaBelly]

That was an area I was going to check tomorrow along with the climate control unit. I assume the motor has been squeaking for a while. I’ll replace the blower once I get this bug out of the way. I did find a rodents nest behind the radio when I pulled the original interior out, but found no chewed wires. I’ll pull the ignition switch out and spray it with QD contact cleaner and see if there’s a dead terminal. I’ve yet to drive the darn thing more than a couple miles, but she seems like a promising car to fill my diesel void

[Mxfrank]

Before you go further, try unplugging the aux water pump. A bad pump shuts down the CC system.

[Diseasel300]

Originally Posted by Mxfrank

Before you go further, try unplugging the aux water pump. A bad pump shuts down the CC system.

Has anyone ever proven that? I know there's nothing in my '86 to shut it down, unless it burns open the Darlington transistor pack. My pump was seized and the ACC still worked just fine.

[Mxfrank]

It's a fact. When the aux pump begins to develop leaky seals, the motor shorts and draws a ton of current. On the '87 there's an automatic circuit breaker. If the pump is unplugged, everything else will start to work. Earlier cars had fuses inconveniently located on the backside of the head unit, This is from the FSM:

"The circuit board in the tempmatic pushbutton control
unit is protected against shorts from any of its externally
connected circuits. The respective output
signal, i.e. blower and A/C compressor, is switched
off, and the blend air flap remains in its present position.
The respective output signals are switched on
again after the short-circuit has been eliminated.
Due to this modification, the two ampere fuse in the
tempmatic pushbutton control unit is no longer required."

[Diseasel300]

The FSM blurb suggests that the output affected by a short or overload is protected, not actually shutting the entire CC unit down. Having gone through the ACC in the '86 (which should be the same in the '87) there's no fuse and certainly no "circuit breaker". I suspect the protection system senses the current on the base of the transistor pack and shuts it off.

In the case of the OP, I'd still be looking at the ignition switch. If the ACC were shutting down, it should do it in "crank" just like it would in "run". Since the blower runs in "crank" but not in "run", I'd suspect worn or burnt contacts in the switch. There is a dedicated high current contact in there to control the blower.

[OmegaBelly]

Ok folks, it was the ignition switch paired with a crusty mechanism. I replaced the switch and cleaned/lubed the mechanism. Should’ve figured since the cylinder felt gummy. I pulled everything behind the dash to make sure there were no chewed wires from rodents and incompetent previous owners. Thankfully nothing was messed up, just a case of wear and tear. Now the starter is taking a dump, so another solution for another day! What I’ve gathered here is whenever you pull the gauges, take some time to pull the steering lock assembly along with the switch and give it a good cleaning. Thanks everyone for the advice and how quickly it all came in! Catch you all soon