'72 250 - can't get heater fan to work - PeachParts Mercedes-Benz Forum

rgnprof · #1 11-30-2023, 10:32 AM

I have a W114 250 with AC and I'm having trouble getting the switch under the driver's side dash to engage.

The vacuum is working but in the heater mode - with the AC knob turned all the way CCW - the little silver lever is not engaging the switch tabs and switching the power over to the heater fan. I know the fan is working because if I push up on the silver lever to engage the switch the fan operates normally.

Any ideas on how to adjust/fix this?

I am attaching a pic to help: the black circle is around the silver lever and the black tabs that push in to the switch to activate it. I am not getting the lever to come up enough to depress the tabs and activate the switch.

gmog220d · #2 11-30-2023, 11:06 AM

Assuming that the vacuum actuated rod is supposed to push the switch lever down for heat, it appears that the tab on the rod is on the wrong side of the switch lever. If down is the correct direction for the rod to engage the heater fan, then you could remove the switch and replace it with the tab over the switch lever instead of under.

rgnprof · #3 11-30-2023, 11:13 AM

I was wondering about that, but couldn't figure out how it could have gotten messed up in the first place. It's been working correctly for 20 years! And then just quit several months ago...

gmog220d · #4 11-30-2023, 11:26 AM

I looked at the '74, and saw that the tab is over the switch lever instead of under.

gmog220d · #5 11-30-2023, 11:49 AM

If you reset the switch lever and it goes out of whack again, then you'll need to observe the rod and switch in action to see what's going wrong. Could be an adjustment issue. Could also be that the vacuum actuator's rod is free for some reason to move far enough to the side that the tab jumps over the switch lever.

rgnprof · #6 11-30-2023, 11:50 AM

Ok - I moved the tab over the switch and that isn't right. When I did and turned the AC knob completely CCW it does activate the fan and it also turns on the little green AC light indicating the AC is on - NOT the heater (which is backwards - normally the AC is activated when you turn the AC knob CW and then the green light is supposed to come on).

This did help me realize that I was mistaken in which fan was coming on - the heater fan is NOT coming on - it's actually the AC blower that is working.

I did take the cowl off at the base of the windshield and could reach the heater fan - it spins freely so at least it's not seized.

gmog220d · #7 11-30-2023, 11:52 AM

Well then, it's a bit of a mystery.

Please keep us posted as to what the problem really is and how you go about fixing it.

rgnprof · #8 11-30-2023, 11:52 AM

I have had the center section and the AC knob disconnected - assuming it's possible that I installed something 'backwards'...???

gmog220d · #9 11-30-2023, 12:16 PM

Quote:

Originally Posted by rgnprof

I have had the center section and the AC knob disconnected - assuming it's possible that I installed something 'backwards'...???

Maybe the vacuum lines to the AC switch weren't put back in the right positions? There are three vac lines connected to the AC switch. I assume that one is vacuum source (the center one?), and the other two run to actuators under the dash. Might be worth checking those connections again.

rgnprof · #10 11-30-2023, 05:40 PM

Thanks Greg - appreciate the help. I'm still trying to figure things out...

Had a vacuum leak in the other vacuum pod under (under the passenger side dash) - got it fixed but still having same trouble.

meltedpanda · #11 12-01-2023, 11:19 AM

have you checked the 12 v feed to the heater fan?

rgnprof · #12 12-01-2023, 11:32 AM

Quote:

have you checked the 12 v feed to the heater fan?

Funny! I was just trying to figure out how to do that?? I can reach under the cowl at the base of the windshield and spin the fan, but I'm not sure how to get in there to test for voltage...

Sugar Bear · #13 12-01-2023, 11:58 AM

https://patrykrebisz.com/w114/heater_fan.html

Start by confirming power to and through fuse #3
Remove the fuse box and look under it for bad connections for maintenance purposes.

rgnprof · #14 12-01-2023, 12:17 PM

Thanks for the link! I have checked all the fuses for power and don't see a problem there (on my car fuse #3 is for the Automatic Transmission, Fuse #6 is for the heater - regardless both have 12+ volts).

Followed the procedure in the link - ran cable from battery + to the lower left pin and another cable from the battery - to the other 3 pins - NO FAN! Did hear a click when I connected the positive wire to the lower left pin, but no fan.

meltedpanda · #15 12-01-2023, 04:40 PM

lets hope its not the fan , I have been told the car is built around them lol. On my 72 220 I had an issue with the fan and it took me forever to get mine working, but it was "stuck" I managed to get some ATF into the mechanism and it started to work. To get to the fan itself, I have read where many just cut an access through the fire wall ..jeez