300SDL temp selector wheel - PeachParts Mercedes-Benz Forum

pilot53 · #1 06-14-2016, 06:28 PM

Im having an issue with the climate control on my 86 sdl, the system seams to be working perfectly aside from the fact that the system doesnt maintain the temp on the wheel. I have tested the system according to the fsm, all of the doors and flaps work as they should in every setting, the system blows hot and cold air when it should, the blower functions as it should, the a/c is engaging, the internal temp sensor has the correct resistance readings and the aspirator motor is working and providing suction as well. The system will respond with changes in blower speed and air temp as I move the wheel, but its as if the system wants to maintain 75 degrees when the wheel is at the 60 degree mark.
Is it likely that something is wrong with the selector wheel itself? Are these serviceable?

Diseasel300 · #2 06-14-2016, 09:27 PM

Anything is serviceable if you have the knowledge, tools, and parts to repair it. It's entirely possible that the wheel could be the issue, however have you checked to make sure heater core isn't hot and that the A/C is actually cooling like it should?

If you're low on freon or the system isn't working like it should (heat stuck on for example), you won't ever get to the target temp, or if you do, it'll take a while.

pilot53 · #3 06-15-2016, 11:42 AM

Actually, my blower fuse blew recently and it seems to have melted the plastic slightly, thinking about replacing the blower first. I have read horror stories about it being hell to replace, but the w124 manual says you only need to remove the wiper assembly and it is accessible from the top below the wiper motor. Is it this easy on an 86sdl?

86-300sdl · #4 06-15-2016, 04:50 PM

In the SDL the "blower" is accessed from under the dash / leg compartment area on the passenger side.

If I ever have to do it again, I'll be pulling the passenger seat first. At 6'1 215 lbs I had the seat fully back & reclined with my feet sticking out the rear door. Lucky I didn't have a stroke...LOL

The sampler fan may also be your temp wheel problem. Located to the right of the glove box compartment at about the same level.

Diseasel300 · #5 06-15-2016, 06:12 PM

Before replacing the blower, does it actually NEED replacing? Is it making any noises? Running too slow? Bogging down at weird times? If not, don't fix what ain't broke.

Does your SDL have the blower fuse in the #16 slot in the fuse box, or is it in a separate holder outside the fuse box? At some point during the production run, the blower motor fuse was changed to a 30A fuse and moved to an external holder next to the fuse box.

If you have the original fuse setup, consider upgrading.

Demothen · #6 06-15-2016, 06:39 PM

(This is all assuming the Hvac system is similar to a late w123)

Essentially every part and system in a car can be diagnosed before replacing anything "blind". That is unless you value your time much higher than your money. Though to be fair, testing circuit boards typically requires specialist knowledge and equipment.

I cant imagine a fan will affect the operation of your ACC thermomostatic operation. Essentially that is a feedback loop:
Is the car warmer than I want it to be set? Yes: run the AC some more. No: turn the AC off or drop the cold air output
Is the car cooler than I want it to be set? Yes: turn the AC off or drop the cold air output. No: run the AC some more.

Its a bit more complicated than that, but you get the idea. Even if the fan was not cooling well, it would probably just run the AC more to try to achieve the same temperature.

Before you throw money at it, I would suggest you check each component of the system.

First, does your system get properly cool when it is set to MIN?

Does your AC system produce cold enough air?

Is your heater competing with your AC?

It sounds like you have tested your cabin air temperature sensor already. I would suggest that you pull the switch unit and inspect it. Check for burnt spots on the case, and if you can open the case and check the circuit boards for damage. May be worth reflowing the solder points.

The selector wheel may be removable. If so, take it off and figure out which pins vary in resistance when you roll the wheel. There may be specs available to indicate expected resistance at specific temperature settings. You may be able to take it apart to clean it, do so if possible using some electrical contact cleaner.

Check your grounds. Some sensors measure a voltage drop to ground, if your grounds are bad this can throw off the readings.

Check your fan before you replace it. If the bearings and brushes are good, chances are its fine.

There's probably other checks you can run, but that's what I can think of off-hand.

pilot53 · #7 06-15-2016, 07:47 PM

The reason I wanted to replace the blower was because even with the new fuse in, it has become discolored from heat, so it has not blown again but it looks like its getting close. But now I think i will live with it unless it actually goes for sure, dont want to do that whole dash removal nightmare. The car actually cools fine if I bring the wheel down low enough, but the wheels 60 degree mark seems to correspond to 72 degrees cabin temp. If I set 70 degrees cabin temp on the wheel it feels like about 80 in the cabin. I've tested the functions of all of the switch positions and they do what they should according to the fsm, and the flaps and doors are all working as well. The cabin temp sensor is spot on resistance wise, and the aspirator motor for the sensor is brand new and drawing air correctly. I pulled the climate control panel yesterday, all looked good except the part number for the unit was for an 88-91, not 86-87 sdl. I know the p.o. replaced it, maybe they are different. I measured the resistance at the selector wheel, but cant find any specs for it.

Diseasel300 · #8 06-15-2016, 07:56 PM

You still haven't said if you have the fuse in #16 in the fusebox, or the strap style in the external carrier. The upgraded strap style fuse is 30A to handle the draw of the blower motor.

Heat in the fuse (assuming you have the cartridge style in #16) will be coming from a hot connection between the fuse and the fuseholder. If you still have this style of fuse, try cleaning the seating points (disconnect the battery first!) to get a better connection.

The fans are easy to get to in the W126, if you have the tools and know how to get the kick panel out, you can get the fan out in less than 10 minutes.

That said, they're expensive, and I'd almost be willing to bet money it isn't your problem. If you have a hot connection in the fusebox, or still have the original fuse setup, replacing the fan motor won't cure your problems.

pilot53 · #9 06-15-2016, 09:37 PM

Good to know, I still have the original fuse. I'll try cleaning the contacts.

Diseasel300 · #10 06-15-2016, 09:49 PM

The attached PDF shows the FSM procedure for moving the blower fuse for cars built before 06/86. Apparently fuse failure was a common issue, hence the design revision to a higher capacity fuse in the external holder.

My car was built 06/86, so already has the fuse moved to the external holder. AFAIK, the blower motor is original and still works fine with no noises. Fuse is original too, and surprisingly not corroded and falling apart like some of the other ones in the fusebox (since replaced with brass fuses).

Diseasel300 · #11 06-15-2016, 09:51 PM

I should also add, I doubt you'll find the OEM parts to move the fuse, but you can always use a standard ATC or cartridge style fuse holder with a Western 30A fuse.

Demothen · #12 06-16-2016, 11:19 AM

FYI - Old VW's (air-cooled era) use an inline Bosch style fuse holder to power the backup lights. (AKA the same style that our MB's use) - You might look for those from a VW specialist, if you want to only need to carry one style.

pilot53 · #13 07-07-2016, 07:35 PM

To my surprise the local mb dealer was able to get all of the parts for the fuse relocation within two days. After relocating the fuse the a/c is working better and more consistently but I have noticed that when its very hot and it starts to rain, thats when the ac acts up. Just today for example the a/c was working fine even though it was 90 out, with the temp selector set at 22c, the system maintained the temperature perfectly, but then I drove into a small rainstorm and as soon as the rain started, the air stopped blowing out from the center vents, got hot, and the blower slowed down. So I moved the selctor wheel to "min" and initially the air out of the vents got slightly cool again, but when the rain stopped the a/c started to blow out nice and hard again from all of the front vents. I moved the selector back to 22 deg and it continued working normally. Now I can recall similar behavior in the past when driving through the rain, one day the a/c stopped and the windows fogged up because it got so humid. It seems the system has an issue with moisture, any idea what may be wrong?

jay_bob · #14 07-07-2016, 10:48 PM

Intake air sensor getting wet?

Or: compressor clutch slipping when wet?

pilot53 · #15 07-08-2016, 11:15 PM

Quote:

Originally Posted by jay_bob

Intake air sensor getting wet?

Or: compressor clutch slipping when wet?

Is this a normal thing to have happen to these cars? I dont think the compressor clutch is wet because it seems to happen right away as soon as its gets rainy and humid, and the actual temperature of the air gets hot, like the heat is coming on. Is it possible that the outside air temp sensor may be getting wet and causing the system to think its colder than it is, or just be defective? Can anyone tell me how to locate this sensor on the sdl?