I hate to bring up heating problems again but it has reared its ugly head again but in a different manner.
1981 300d.
At any speed with EC selected, heat slowly cycles temp up and down at about a 7-8 minute cycle. Seems to bring temp to what is selected then slowly starts blowing cold air then slowly starts blowing warm air up to temp again/etc.etc.
At any time I can select MAX temp and temp rapidly goes to hot as it should.
Again at any speed.

Engine temp stable=85ish
hose behind glove box good

AARGH
Pat

Rig up a test light as in this thread http://www.peachparts.com/shopforum/diesel-discussion/363387-monovalve-acc-info-troubleshooting-thread.html

Shoot a video and post it. The test will tell you if it's a electronic temperature control problem.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I don't know if you've posted issues related to heat before, so I don't know what if anything you've done to the system previously.

1981 should have the 2nd gen climate control system with horizontal push buttons.

Good idea for testing from Funola.

That said, I'd also start by making sure your foam hose is intact between the dash sensor and the blower housing. If the sensor isn't getting proper air flow it could be providing bad readings. That's an easy thing to remove from the equation.

You might also test the see that the sensor is providing proper resistance at various ambient temps.

The next most likely culprit is a flaky push button unit. Cold or cracked solder joints in the unit can lead to a variety of strange behavior. If you're comfortable with circuit board repair you could pull it out, open it up, and resolder all the joints. This has helped many folks. If you've never had any work done on the push button controls and aren't comfortable with doing it yourself you might reach out to JamesDean (forum member here) about having him inspect and repair it.

Third, there is your temp regulator. It's the brains of the system. If it's getting flaky with age it may be reading inputs from the sensors in the cabin incorrectly, or sending improper commands to the monovalve. This unit can also be repaired, but is more challenging. JamesDean may also be able to assist. Or you can look in to a Programa rebuilt unit.

If you're up for doing this stuff yourself, here is a helpful guide on testing the system.

Attached Files

Middle-aged Mercedes Electrical Repairs.pdf (430.5 KB, 103 views)

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