HVAC Going... going... gone (no blower) 1985 300TD - PeachParts Mercedes-Benz Forum

WarTowels · #1 01-19-2015, 08:01 PM

I could use some guidance with my 85' TD.

When I purchased the vehicle not so long ago, I had two options on the HVAC:

1. Full blast heat to defrost area and main circle vents
2. Full blast with slightly less heat to defrost area and main circle vents

I was totally cool with this mostly nonoperational system as it's winter here and I could actually stay warm. However, yesterday the blower didn't come on. Ut-oh.

Today, I tried the hvac with just the ignition on and the blower motor came on but seemed weaker. Then later today when I ran an errand back to nothing.

I've checked the main fuse box and everything looked OK. Ive tried finding a separate out of the main fuse box fuse but haven't seen anything. That's about as far as my current knowledge extends.

I can say when the blower was working it was very powerful at it's max setting and was working fine one day and then nothing at all the next.

toomany MBZ · #2 01-20-2015, 08:01 AM

Inspect the push button unit for cold solders.

You'll need to remove it from the car, open it up to inspect.

A common failure on these ancient units.

JamesDean · #3 01-20-2015, 08:49 AM

Most likely cold solder joints on the push button unit. Very common.

If you are interested, I service the HVAC components on these cars and would be happy to help.

Here are some photos of a recent one that I did:
W123 300D (1983) - Climate Control PBU - BC - Imgur

Junkman · #4 01-20-2015, 09:40 AM

I had a blows fail and replaced with used from the parts car. It seems that the plug was easily accessabl. I would be easy to test at he plug to confirm voltage.

Rogviler · #5 01-20-2015, 09:45 AM

On mine it was the blower. I pulled it out to see if it could be salvaged and not only were the brushes gone but they had worn completely through the copper on the shaft. A good used one fixed the problem.

-Rog

WarTowels · #6 01-20-2015, 09:51 AM

If I disconnect the plug on the motor, should be able to test for voltage yeah?

If it does have voltage, would that rule out the push buttons?

toomany MBZ · #7 01-20-2015, 11:21 AM

Depends, with a cold solder condition, sometimes it'll work, sometimes not.

The push button unit tells the blower, flaps, etc. what to do, so if you have an intermittent connection, it's guessing.

funola · #8 01-20-2015, 11:30 AM

Quote:

Originally Posted by WarTowels

If I disconnect the plug on the motor, should be able to test for voltage yeah?

If it does have voltage, would that rule out the push buttons?

Yes. You can also apply power to the blower to test it. It draws 16 to 18 amps so use appropriate gauge wires. If you need a blower I have 2 good used ones.

Mxfrank · #9 01-20-2015, 12:18 PM

Try disconnecting your auxiliary water pump. Just unplug the power. If the fan comes back, the pump is bad. I know it doesn't make sense, but this is what they do.

WarTowels · #10 01-20-2015, 02:33 PM

Well, took the multimeter out and tested the plug for voltage.

The results mirror my description about the HIGH settings working and the LOW settings not working. Also none of the "floor" only buttons produced voltage.

So with fan setting set to HIGH and top vents selected the plug sees 12+ volts. Various combinations of HIGH and defrost or other combination settings produce 12+ volts.

Fan did NOT turn on.

I proceeded to give the motor a good whack with my hand and oh hey it turned on. But it seems very WEAK.

So now I'm thinking the actual blower motor may have had a sudden heart attack since it was working so lovely a few days ago.

Should I just replace it?

Assume it is the issue, why might the low settings produce zero voltage? Also, why would the floor settings produce zero voltage as well? Seems too specific to be a soldering issue, although perhaps it's possible the low fan speed... and all floor settings are not working?

WarTowels · #11 01-20-2015, 02:48 PM

Thanks for the replies thus far btw!

Junkman · #12 01-20-2015, 03:46 PM

I would replace it or at least pull it and see whether it will run wired direct to the battery. I have wires with alligator clips for such things. I should wire in a fuses but haven't.

The good blower ran easily. The bad stopped and wires got hot. I was using a 12v boat battery with the blower removed and set on 2x4s on the shop floor.

Your aux water pump is likely bad and cat ruin your push control. You can disconnect. My SD heat is sufficient with the pump disconnected and TN winters.

toomany MBZ · #13 01-20-2015, 03:57 PM

^ Agreed, perhaps the brushes have lived a full life and need retiring.

If they get too short, you'll have fuse issues.

WarTowels · #14 01-21-2015, 09:38 AM

I applied power to the blower (maybe reversed...) and it sparked and started to spin and that was that. D.E.D.

So plan is to replace the unit.

What's this about a secondary water pump affecting the hvac buttons?

JamesDean · #15 01-21-2015, 10:00 AM

Quote:

Originally Posted by WarTowels

I applied power to the blower (maybe reversed...) and it sparked and started to spin and that was that. D.E.D.

So plan is to replace the unit.

What's this about a secondary water pump affecting the hvac buttons?

The secondary water pump is a small electrical pump that helps in certain situations. With age it can seize up or suffer performance degradations that cause it to cause more current than normal. This puts an additional load on the push button unit as it is the supplier. This loading can sometimes draw down other circuits in the PBU causing oddities like described above.

In a small number of cases the aux pump can put such a load on the PBU that it generates significant amounts of heat and damaged the pbu/hosing. Some cases it can even cause fires.

The general recommendation is to install an inline fuse to protect the PBU in the event of an aux pump failure. I believe the fuse should b about 1 amp. I think that's what the consensus was. I tested one of my aux pump and it was sitting around 100-200 mA of current draw.