Heater not working 280SEL '75

1973-1980 : 280S, 280SE, 280SEL, 300SD, 350SE, 350SEL, 450SE, 450SEL, 450SEL 6.9
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tyrrellnick
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Heater not working 280SEL '75

Post by tyrrellnick » Thu 27 Apr, 2017 9:01 am

Good morning all! After a couple of months of stuffing about diagnosing and fixing a very rough-running engine (dreaded injection points in the distributor with my D-Jet), she's purring like a kitten.

Only issue is I live near Canberra and it just got bloody cold, which doesn't go well with driving the missus to work without a functioning heater. The fan works fine, the HVAC controls are all good, but it only blows cold air.

Any tips on where to start my diagnostics? I'm hoping my car has a simpler setup for the heater than it does for the injection system...

Thanks!
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CraigB
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Re: Heater not working 280SEL '75

Post by CraigB » Thu 27 Apr, 2017 11:41 am

I wouldn't call it a simple heater, but the first place i would check is the heater switch under the bonnet near the firewall (from memory) It will have a vacuum line going to it and you can manually operate it, but will spring back again from memory. I can't remember which way it works - vac opens it, in which case that could be your problem and could be a vac leak on a line (maybe bumped off while doing other work?) or the diaphragm. Anyuway, what ever position it is in, I would wire it in the opposite direction and see if heater works. If so then start chasing a vac leak - mityvac pumps or equivalents seem to be really cheap on ebay these days.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

tyrrellnick
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Re: Heater not working 280SEL '75

Post by tyrrellnick » Fri 28 Apr, 2017 8:53 pm

OK... So I picked up a new thermostat from MB Spares today and popped it in tonight (whoever designed the M110 to have that circular unit - is it for vapour return? - right above the thermostat housing deserves to be shot).

Unfortunately it's made no difference to either the temperature gauge (still reading 45 degrees even after 20 minutes of driving) or the heater (still pumping only ambient temperature air).

I'm assuming the two correlate? The only other explanation is there's something wrong with the temperature sensor AND the heater relay.

Any thoughts?

tyrrellnick
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Re: Heater not working 280SEL '75

Post by tyrrellnick » Fri 28 Apr, 2017 10:12 pm

CraigB wrote:I wouldn't call it a simple heater, but the first place i would check is the heater switch under the bonnet near the firewall (from memory) It will have a vacuum line going to it and you can manually operate it, but will spring back again from memory. I can't remember which way it works - vac opens it, in which case that could be your problem and could be a vac leak on a line (maybe bumped off while doing other work?) or the diaphragm. Anyuway, what ever position it is in, I would wire it in the opposite direction and see if heater works. If so then start chasing a vac leak - mityvac pumps or equivalents seem to be really cheap on ebay these days.
Turns out you were on the money! Mind you, the thermostat I took out was pretty shabby so worthwhile doing anyway I suppose.

In any case the heater switch/valve had been zip-tied into the closed position by the previous owner. It did seem to be fairly stiff to move open & closed, so I'm guessing I might need to replace it. I'll do a bit more testing in the morning to make sure the red line has vacuum though before I go spending money on the new part!

CraigB
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Re: Heater not working 280SEL '75

Post by CraigB » Fri 28 Apr, 2017 10:36 pm

I just had a quick look and this sort of tool

http://www.ebay.com.au/itm/Hand-Held-Br ... 1456286108

you can see if the tap is working and holding vacuum and test the line and see if that is and trace the leak from listening or touching, closing off certain sections and trying to narrow down where the leak or leaks are coming from.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

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Chai
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Re: Heater not working 280SEL '75

Post by Chai » Fri 28 Apr, 2017 10:58 pm

Aaah! The old zip-tie workaround trick for a leaking shut-off valve.

There are two possibilities. The most likely is the vacuum diaphragm that opens and shuts the water supply to the hose has a tear and leaks air. Less likely is that the interior heater controls (think your model have the vertical levers) are leaking vacuum. The latter may be as simple as a dislodged vacuum line or as complex as misaligned heater control valve at the end of the interior levers.

The default mode when there is no vacuum supply to the heater shut-off valve is HEAT-ON. Hence, the zip-tie to shut off the water supply in summer.
Chai
1974 450SLC

tyrrellnick
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Re: Heater not working 280SEL '75

Post by tyrrellnick » Sun 30 Apr, 2017 11:10 pm

Chai wrote:Aaah! The old zip-tie workaround trick for a leaking shut-off valve.

There are two possibilities. The most likely is the vacuum diaphragm that opens and shuts the water supply to the hose has a tear and leaks air. Less likely is that the interior heater controls (think your model have the vertical levers) are leaking vacuum. The latter may be as simple as a dislodged vacuum line or as complex as misaligned heater control valve at the end of the interior levers.

The default mode when there is no vacuum supply to the heater shut-off valve is HEAT-ON. Hence, the zip-tie to shut off the water supply in summer.
Hi Chai, thanks for the advice! Do you know if it's possible to replace the diaphragm, or must I replace the entire unit on the firewall? I've seen some plastic units online etc, maybe MB Spares has some...

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Chai
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Re: Heater not working 280SEL '75

Post by Chai » Sun 14 May, 2017 10:20 pm

I never found a supplier for the rubber diaphragm inside it, so I replaced the assembly.
The replacement part looks like this: http://www.ebay.com.au/itm/Mercedes-w11 ... Swl9BWGiSr
Chai
1974 450SLC

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