W124 Coupe cabin heater permanently hot

[CreosoteChris]

Symptoms

220CE (no air-con) cabin heater is currently blowing full heat, both sides, rotating the heat controls makes no difference.

I had a good trawl around the various threads here dealing with this common issue, and did the following investigation:

Procedure

- Measured the voltage across the duovalve connector terminal (same procedure as this YouTube video) - no voltage, regardless of the heater control settings

- “Tissue test” against the cabin temp sensor vent next to sunroof control switch – no sign of any air current / tissue falls off

- Removed interior light / sunroof control panel, checked vent for blockage – no blockage

- Removed glove-box and checked for operation of cabin temp sensor fan – not turning.

- Removed cabin temp sensor fan and bench-tested – fan spins OK

- Measured voltage across cabin temp sensor fan connector terminals – 0.3V

At this point, I ran out of daylight, and also ideas about how to proceed to a root-cause diagnosis - I’m no expert auto-electrician (or mechanic for that matter)

Can anyone provide any guidance about where to go from here – all advice appreciated


Cheers, Chris
93 220CE, auto, red / black leather, 101k, Manchester

 

[bolide]

You've got further than most people in diagnosing the problem. It pretty much has to be a wiring fault. I'd suspect every join in the relevant cables, and every relevant earth point

Probably a connector has corroded and you've lost the voltage to the fan. You could try taking a post-ignition feed from the back of the stereo and linking it to the fan to make it run continuously. If that works I suspect that your desire to look further may wane significantly...

Nick Froome

 

[grober]

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1995/E320/SYSTEM WIRING DIAGRAMS/5407.pdf

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1995/E320/SYSTEM WIRING DIAGRAMS/5408.pdf

 

[CreosoteChris]

Thanks very much Nick and Graeme – think I owe you both a few drinks

Luckily it’s not the worst time of year to be suffering from an over-zealous heater – and I can still use the dash centre vents to let in cold air and balance out the overall temperature – but I plan to get to the bottom of this eventually, even if it takes a while to accomplish.

Rooting around for info, I was just about to start grappling with the (gulp) 275-page Electrical Troubleshooting Manual found here: http://www.w124-zone.com/downloads/M...am/ETM/ETM.pdf The linked PDFs look a lot more digestible for someone of my capabilities....

…. what with this, and the various other jobs I’ve done on my W124 – I’m actually in danger of becoming competent at messing with cars.


Cheers, Chris
93 220CE, auto, red / black leather, 101k, Manchester

 

[CreosoteChris]

For the second time in a week (see http://www.mbclub.co.uk/forums/electronics/183741-93-w124-coupe-stereo-inoperative-source-unswitched-12v-power.html)……..

……it turned out to be a partially failed fuse – looked OK in the holder, but

- when removed, proved to be in a severely degraded condition
- when exchanged, fixed the issue

As I noted previously, I’m no expert on this subject, and am probably making statements of the bleedin’ obvious for most of the community reading here – but those barrel-type fuses on ageing W124s are prone to partial failure without actually breaking in an obvious way – causing voltage drops which progressively disable the connected equipment dependent on how tolerant it is of the reduced voltage. Not everything fails at the same time - in this case the blower motor was still working via the same fuse.

I’ve already replaced 50% of the fuses in my box, net job is to do the rest of ‘em before I suffer a further similar episode on the next electrical system.


Anyway, I’m learning fast, and have a W124 coupe with no major issues - I’m going out for a test drive!

Cheers, Chris
93 220CE, auto, red / black leather, 101k, Manchester