HVAC - What burned up? - PeachParts Mercedes-Benz Forum

mkrazy1 · #1 05-06-2008, 10:01 AM

I have an 87 300D with just over a 100K - Love the car! I have been researching issues on the board trying to figure out some on-going hvac issues, including figuring out why the air only comes out the dash vents and why the a/c would just stop working until I restarted the car. The last issue was fixed by just cleaning the A/C clutch. I figured I would start troubleshooting the vent issue soon. Anyway, a new issue came up yesterday. I had the A/C on and the fan set on "auto". The fan all of a sudden just shut off from being on high speed. After a minute it came back on high and then ran for a minute or two and shut back off. I pushed the button to just high and it didn't come on, so I hit it off and then back on and still nothing. Then it came back on high again. After a minute I head a "pop" from under the dash and plume of smoke came out of the vents and the fan quit. I figured there was probably a resistor on the fan the burned out, but this morning the fan worked like it should, meaning it varied in speed. Any ideas on what burned up. It was definitely an acrid smelling odor that even lingered in the car this morning.

vstech · #2 05-06-2008, 10:52 AM

any sign of rodents in the car?...

mkrazy1 · #3 05-06-2008, 12:03 PM

None that I have seen, but before I bought it I have a feeling it sat for years. I've been driving it for about 6 months now and haven't seen any nesting material come out the vents or anything.

SD Blue · #4 05-06-2008, 12:06 PM

Someone recently had similar issues, on the same year/model, and it turned up as a bad ignition switch.

jcyuhn · #5 05-06-2008, 12:18 PM

This car does not have a resistor set to control fan speed. It uses a power transistor to rapidly switch 12 volts on and off to the fan to achieve the desired fan speed. They are known to fail on occasion. The controller is located near the fan - it's actually cooled by the airflow. If it smoked, you would definately smell it in the cabin. It's unusual for electronic components to let the smoke out and still work afterwards, though.

Try searching on something like "124 fan speed control" to read up on the subject.

- JimY

mkrazy1 · #6 05-06-2008, 01:26 PM

That's why I'm baffled. Since the smoke came out of the vents and it was definitely electrical, I know it had to be something buried within the duct system, but the fan still actually works?

babymog · #7 05-06-2008, 10:29 PM

Odd failure mode, when it doesn't fail.

The first symptoms are reminscent of my failing CCu on the '87, but not the pop and smoke part.

If the motor fried part of the windings, but somehow cooled and worked, it would be of lottery-winning odds, ... the speed controller is even less likely to heal after a failure.

I don't have a clue, hope it was a mechanical impediment to the blower and see if it continues to work correctly now.

The dash-only air is probably the vacuum-pods that are accessible (barely) behind the radio.

mkrazy1 · #8 05-07-2008, 12:00 PM

Well, it's still working as far as blower continueing to blow air at variable speeds, and cooling and heating. Hopefully I'll get a chance to start pulling it apart to look at what actually blew here in a week or so.

mkrazy1 · #9 05-09-2008, 11:37 AM

I decided to do a little bit with this last night... Figured I would look to see why the air never really changes from blowing out from all vents all the time... I figured I would do the checking on the different pods to see which are bad so I could order them prior to taking the dash apart. I pulled the glove box and checked the various vacuum lines with a mity-vac. Now I am wondering which lines go to which pod?

Here is what I came up with:

Line coming in from the top - with engine running has 10" vacuum - Is that correct? I would think it should be higher.

With engine off, I used the mity vac to see if each would hold a vacuum.

1 - zero
2 - holds 5" only
3 - slowly leaks down
4 - holds 25"
5 - holds 25"
6 - only holds 5"
7 - holds 25"

With the engine running, if I push the various buttons I can hear the vacuum manifold click but nothing ever really changes as far as air coming out the floor only or defrost, and air always comes out the dash vents (all).

a2t · #10 04-06-2009, 07:09 PM

same problem w/smoke in cabin and blower acting crazy just happened on my 87 300D.

Was working fine when I shut it off after lunch, then got back in tonight to drive home from work.

Blower on ALL the time (with key on), seems like 3/4 speed (regardless of push button setting lo, hi, auto). Smells like something burning, smoke coming from vents. Temp wheel works, AC still working.

Tried shutting off and back on, same problem. Blower only stops when cranking or key off.

Any ideas? Im thinking the power transistor on the blower motor? Any easy way to get at this to diagnose?

gsxr · #11 04-06-2009, 08:01 PM

Sure sounds like the regulator is fried. It's also called a "porcupine", due to the appearance of the finned heat sink. Since it sits directly under the blower, if it fails, the smoke would blow into the cabin. See photo below. That's my guess, anyway... if the blower motor itself was bad, it wouldn't be running!

While the motor is out, inspect the brushes... if they are almost worn out, plan on replacing the blower motor as well in the near future. Put a few drops of ATF on the shaft bushings to help lubricate things a bit, this can quiet a squeaky motor.

a2t · #12 04-06-2009, 08:56 PM

thanks dave. Do you have a removal procedure for blower on this car? does blower have to come out to replace it?

SD Blue · #13 04-06-2009, 09:23 PM

Also, ensure it isn't the pushbutton controller. There is a situation where the aux. water pump causes the controller to overheat when the pump is starting to fail and begins to draw more current. One poor guy lost his car to fire because of this. To prevent this, there have been several mentions of installing an in-line fuse to the aux. water pump.

a2t · #14 04-06-2009, 09:29 PM

yeah ive read about that. its a new aux pump in there.

the fact that fan is running fast regardless of pushbutton speed setting leads me to belive the blower control transistor is shorting, plus the smoke coming thru vents. F$CJK. need to figure out how to get the wiper motor out...

guess theres no easy way to get to the transistor?

a2t · #15 04-06-2009, 11:38 PM

nevermind i got the whole thing out in about an hour, not as bad a job as i thought.

the transistor is HUGE, im surprised its only $235. it smells like the burnt wiring im smelling in the car, guess its toast.

blower motor doesnt spin all that freely - should it? the brushes are about gone, guess i will replace it too.

is it possible the blower motor caused this?

i can get an ACM brand blower w.cage for $178 or Behr brand w.cage for $237 from worldpac. Anyone ever use ACM ?? Wouldnt mind saving $.

wonder if any of this stuff is interchangeable with similar model SDL Im about to buy...

oh yeah, ill post pics of the evaporator or condensor whatever its called sitting behind all this. im amazed i got any cold air, the thing is totally clogged with *****. cant believe ive been breathing that for 3 yrs!