I have a 79 300td and the only two buttons that work on the climate control unit is the defroster button and the off button. It turns on the fan full blast and I get heat. Nothing else on the push button unit works.

Now lately sometimes when I push the defroster button, the fan wont go on, or when the fan is on I cant turn it off!

I notice when I put my finger in the defroster vent when the fan isnt running I'm feeling heat in there, so I'm wondering two things: is it just the push button unit and most importantly:

For now can I just rig a switch right at the fan, Im saying just run a + and - wire with a on/off switch right to the fan, to operate it manually for now. Its winter, and I need the heat! Any suggestions, advice or anyone out there that did this?

Thanks!
Mike

when the pushbutton unit fails all heat to the defrost units is the fail safe mode. I guess the engineers felt this to be a safety feature.
I would guess that your pushbutton unit is bad. YOur fan if original may be operating this way because the brushes are worn . You can disconnect the fan easily and test.
Good luck

If it is the fan then why does it not shut off sometimes? The only way is to unplug it. But today, it doesnt go on, which gives me the idea to run power right to the fan and a switch, but I have not read in the forum of anyone doing this.
mike

If your push button unit is indeed the cause, than your idea should work. You may also need to DISCONNECT power to the mono-valve, so that it opens, and allows hot coolant to flow through to the heater core.


Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I took mine out of my 300D and cleaned the brushes. it works great. the cooling buttons don't do any thing but then I don't like ac..
Fixed Blower Motor For $6.03 try this thread. It helped me hope it helps...

A '79 would not have a monovalve, it has the infamous servo. It may have gone south and it would cause the problems you describe. Another is that you have a vacuum problem and the flaps are not being pulled into place in positions other than DEF. The fan won't come on until the flaps are in place.

All this and more is explained in George Murphy's $5 troubleshooting guide. Give him a call at 865.482.9175.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks for all your replies and help!

I discovered that the blower fan fuse had blown, and apon replacing it, found it is still stuck on!

Does anyone know where the "dual contact relay" is? from studying the electrical diagram in the haynes manual, this relay is what interupts the power from the fuse (#10) to the fan... I think. If so, I wouldnt mind testing if power is reaching it, or if I'm putting a manual switch, maybe I could spice in there.

Its been below freezing and snows on the way. Engine still cold starting fine!

Mike

does yours have a fan relay and temp relay, black and silver boxes located on or near the fan housing. points may be stuck