No blower operation below ~20 degrees - PeachParts Mercedes-Benz Forum

MattBelliveau · #1 01-14-2009, 10:19 AM

At the last of my wits here. I've resoldered the CCU, installed new brushes in the blower motor. It works great when its a luxery. When it drops below 20 degrees or so, I just can't get it to kick on, even when its in defrost mode (which I've been told it should run at full blast no matter what). Am I looking at a new blower unit? I will be testing it tonight to make sure its getting proper power.

kerry · #2 01-14-2009, 10:50 AM

I feel your pain. I'm having similar blower problems on my 79SD. It works periodically but most of the time it doesn't. Can't figure out why.
Have you checked the cold engine cut out circuit? It's on my list. I know that defrost mode should bypass it but I'm going to try grounding the sensor once I get some time to work on it.

bustedbenz · #3 01-14-2009, 11:01 AM

Quote:

Originally Posted by MattBelliveau

At the last of my wits here. I've resoldered the CCU, installed new brushes in the blower motor. It works great when its a luxery. When it drops below 20 degrees or so, I just can't get it to kick on, even when its in defrost mode (which I've been told it should run at full blast no matter what). Am I looking at a new blower unit? I will be testing it tonight to make sure its getting proper power.

Mine's similar. When it works, it really works. And then some days, particularly when it's cold and you NEED the defroster, like you describe, mine just sort of loses its mind. You try to turn it on and hear a relay clicking two or three times a second back in there. Incessantly. You try again a little later and can't get anything. You give up, shut the ignition off to reset everything, restart it, and it works fine and/or goes back to clicking. Repeat enough times and it eventually starts responding again.

The REALLY fun one mine does is take forever to beg it to deliver heat, and then it won't STOP delivering heat. You push Off and it all just keeps right on going. You try to turn the fan down to low and it stays on high. You try to set the temp to max cold and it just ticks right on along on overdrive heat.

Sometimes I can cure it all by shutting everything off including the ignition switch and restarting. Other times I give up and open the roof to let the heat out.

rrgrassi · #4 01-14-2009, 11:18 AM

There is a temp switch, IIRC on the tstat housing next to the vacuum switch, that closes to ground when the coolant temp reaches near 40c to allow the blower fan to operate.

MattBelliveau · #5 01-14-2009, 11:50 AM

Quote:

Originally Posted by rrgrassi

There is a temp switch, IIRC on the tstat housing next to the vacuum switch, that closes to ground when the coolant temp reaches near 40c to allow the blower fan to operate.

I might add that I DO have that temp switch grounded...

rrgrassi · #6 01-14-2009, 11:53 AM

Have you checked power and grounding at the connector on the blower? Also check the resistor pack/cage.

MattBelliveau · #7 01-14-2009, 11:56 AM

Quote:

Originally Posted by rrgrassi

Have you checked power and grounding at the connector on the blower? Also check the resistor pack/cage.

I plan on doing this tonight. What/where is this resistor pack cage? Is that the little spring box thing on passenger side fenderwell? Thanks.

rrgrassi · #8 01-14-2009, 12:02 PM

Yes it's the spring box on the passenger side. It does get quite hot when the blower is running.

MattBelliveau · #9 01-14-2009, 12:33 PM

Quote:

Originally Posted by rrgrassi

Yes it's the spring box on the passenger side. It does get quite hot when the blower is running.

What should I look for? Cracks in the springs or something? I guess the question is how do I test this? What would be the fix if this was the problem (anything other than replacement)? I always wondered what that little thing was...

kerry · #10 01-14-2009, 01:45 PM

I linked an on-line version of the evil servo system manual a few weeks ago. In looking at the manual, it states that the switch which detects coolant temperature and permits blower to blow on non-defrost settings only above 125 degrees is on the bottom of the servo. When I ground the sensor in the thermostat housing on my 79SD it turns on the auxilliary radiator fan.
There is also a main vacuum switch which only allows the fan to blow if a minimum level of vacuum is reached. This switch is the left one of the two behind the glove box on my 79SD. You could try jumping that switch.

MattBelliveau · #11 01-14-2009, 02:01 PM

I'm really thinking of just jumping all the CCU BS and just running a new circuit controlled by a toggle switch. If I were to do this, does anyone know the amperage of the motor? Just for the record, my monovalve is rigged to keep hot air in the car. I don't reallty understand how vaccuum would have an effect on this, as it seems to be the outside temp that is the culprit (even when the car fully warms up, it won't come on).

Freezing my nads off,

Matt.

MattBelliveau · #12 01-14-2009, 02:02 PM

Quote:

Originally Posted by kerry

There is also a main vacuum switch which only allows the fan to blow if a minimum level of vacuum is reached. This switch is the left one of the two behind the glove box on my 79SD. You could try jumping that switch.

Is this switch an electrical one, or a mechanical vac one?

kerry · #13 01-14-2009, 03:15 PM

Quote:

Originally Posted by MattBelliveau

Is this switch an electrical one, or a mechanical vac one?

It's both. Vacuum controls an electrical connection. this is on models with the evil servo.

rrgrassi · #14 01-14-2009, 03:19 PM

The '83 does not the servo, so servo info will not help out in this case.

Matt, I have not tested the resistor pack yet, so I do not know the output values. Input should be the normal voltage 12-14 volts.

kerry · #15 01-16-2009, 02:19 PM

I'm now beginning to think that my symptoms are mimicking yours. It's warm today (45 degrees) and blower is working fine. It wasn't working a few days ago when it was cold.