What actuates the defrost vents/pod? - PeachParts Mercedes-Benz Forum

ps2cho · #1 05-08-2017, 12:49 AM

So I want to ask if there are any tests I can run to figure out what is causing my defrost vents to stay open at full A/C (my 88 300TE).

I have pulled the glovebox and verified the pod is OK. Holds vacuum fine (replaced it a few years back).
I thought about buying a new changeover valve, but then I wondered if it could be the climate control unit, so I'd rather not waste money without trying to test something else first.

Anybody have any ideas on something I can try to figure out what part is not telling the defrost vents to close?

sixto · #2 05-08-2017, 01:38 AM

Can you get a probe on the defrost solenoid wires to see if it's getting a signal?

Sixto
83 300SD for sale
98 E320 wagon

Diseasel300 · #3 05-08-2017, 09:20 AM

^^^ x2

Now the $64M question - does the changeover valve energize (receive voltage) to send vacuum to the pod, or does it de-energize (no voltage) to send vacuum to the pod? It is important to know what the proper operation mode is.

Assuming the pod is supposed to receive voltage (energize) to send vacuum to the pod - If the changeover valve is energized, but the pod is not receiving vacuum, you probably have a problem with the changeover valve.

It could also be worth checking your vacuum source at the changeover valve itself. It's possible it could be weak, or you may have another pod elsewhere in the system with enough of a leak to reduce the vacuum strength to a level that the defrost pod no longer operates.

Rick76 · #4 05-08-2017, 10:52 AM

I can almost guarantee that the changeover valve has bad solder connections. I've picked up a couple at the wrecker and they as well as the 2 in my W124's had cracks where the solenoids solder to the circuit board.
Later valves can easily be disassembled by just undoing screws but the earlier ones can be a challenge to get apart and back together. Not sure what year that changed but 93, 94 and 95 definitely have the screw together version. If you have a wrecker near you it would be worthwhile picking up the newer one, re-soldering it (add fresh solder) and installing it.

As Diseasel300 suggested, check you vacuum source.

tinypanzer · #5 06-05-2017, 06:45 PM

Quote:

Originally Posted by Rick76

I can almost guarantee that the changeover valve has bad solder connections. I've picked up a couple at the wrecker and they as well as the 2 in my W124's had cracks where the solenoids solder to the circuit board.
Later valves can easily be disassembled by just undoing screws but the earlier ones can be a challenge to get apart and back together. Not sure what year that changed but 93, 94 and 95 definitely have the screw together version. If you have a wrecker near you it would be worthwhile picking up the newer one, re-soldering it (add fresh solder) and installing it.

As Diseasel300 suggested, check you vacuum source.

I second the bad solder joint notion. I had to re-solder several on my 300SE. You've already ruled out the bad pod. It's going to be the solenoid bank or the climate module, probably a cracked solder joint in either case. Mine wouldn't open so I cut away the plastic cover behind where the plug header inserts with a Dremel so I could get access to the pins. Re-soldered them, and all is well.

ps2cho · #6 05-08-2017, 03:00 PM

All the rest of the pods close and function...it will occasionally work and I can tell because I notice the sudden increase in airflow out the vents.

Sounds like I'll just get the new changeover valve!

Hit Man X · #7 05-08-2017, 10:33 PM

I had a similar issue on the 300SEL for as long as I owned it... the defrost would not fully shut, just partially. I would just zip tie it shut in the summer and remove the zip tie in winter. I became tired of this nonsense and had the PBU rebuilt. Same issue.

Pulled the solenoid block and that section plus one other would not energize (one ground, eight total connectors). Firm whack on ground corrected this, for a while. Then back to the same old ****. Bought a new one, $97 later, system works as designed.

Wished I had tried that a decade ago.

Austin85 · #8 06-02-2017, 12:34 AM

Quote:

Originally Posted by Hit Man X

I had a similar issue on the 300SEL for as long as I owned it... the defrost would not fully shut, just partially. I would just zip tie it shut in the summer and remove the zip tie in winter. I became tired of this nonsense and had the PBU rebuilt. Same issue.

Pulled the solenoid block and that section plus one other would not energize (one ground, eight total connectors). Firm whack on ground corrected this, for a while. Then back to the same old ****. Bought a new one, $97 later, system works as designed.

Wished I had tried that a decade ago.

This thread showed up on my search. How did you go about zip tying the defrost vents shut? I have an 81 280SEL now and AC goes out front dash weak and defrost vents about the same. I wouldn't mind trying to zip tie them shut.
Thanks, Austin

tinypanzer · #9 06-05-2017, 06:49 PM

Quote:

Originally Posted by Hit Man X

I had a similar issue on the 300SEL for as long as I owned it... the defrost would not fully shut, just partially. I would just zip tie it shut in the summer and remove the zip tie in winter. I became tired of this nonsense and had the PBU rebuilt. Same issue.

Pulled the solenoid block and that section plus one other would not energize (one ground, eight total connectors). Firm whack on ground corrected this, for a while. Then back to the same old ****. Bought a new one, $97 later, system works as designed.

Wished I had tried that a decade ago.

If you can solder, try re-soldering the backside of the solenoid bank where the plug header inserts. I had to cut away some plastic with a dremel to get to it, but after re-soldering those pins everything started working again. Same symptoms as you're having. The dead giveaway is the intermittency. If the pod is shot, there is no 'sometimes it works'.

mercron · #10 05-09-2017, 01:39 AM

Have you tried this test?

http://www.w124performance.com/service/w124CD1/Program/Climate/83-601.pdf#page=1

The defrost vents only close when temp is set at Min and the selection button is empty Up solid Down arrows. If a temperature is selected, there will be leakage from the defrost vent.

Sent from my iPhone using Tapatalk

Hit Man X · #11 06-02-2017, 01:41 AM

Well... on ACC cars, pull glovebox liner. The defrost pod is to the left. Recycle and fresh air in front of you...the three can be changed in about 30 minutes on non passenger air bag cars. To zip tie the flap shut, just disconnect the pod's rod from the flapper arm and swing it toward you.

If you do not have recycle pods working, AC will not work well as you will be constantly pulling in outside air to cool and remove moisture. I guess you could disconnect them from the door and wedge something in there to prop the door to 100% recycle for now.

Make sure your fan clutch is good same with that aux fan. From memory early cars had a small aux fan, you probably want to upgrade to the later W126 unit. 1984+ area I think the aux fan was increased vastly in size. Every M110 I have seen uses that metal fan like the M116/7 W126s, that pulls quite a bit of air.

Be sure there is no debris by the master cylinder, you do not want a fire there due to the aux fan resistor.

tinypanzer · #12 06-05-2017, 10:54 PM

Here's a picture of the solenoid bank I fixed. Upon seeing it I realized I used a hand jigsaw rather than a dremel. Note the angle I cut it at, exposing only the back of the circuit board and not breaching the pneumatic seal. Works great, just pulled it out of a my 300SE which is off to the junkyard in a couple days. Note - this is a w126 GenII solenoid bank, not sure how similar to yours it is.