The car is a 1979 300SD, 160,000 miles living in New York City. Some of you may have read my posts about how I keep blowing the #3 (25 amp) fuse with my NEW BOSCH blower motor. I replaced the blower motor and all was good for 2 weeks. Then the fuse started to pop. I got a second brand new motor and all was good for 2 weeks and then the fuses started to pop again. I went to my local indy and he suggested to disconect the aux. water pump. I did this and now I have nothing!!! I was getting heat if I pushed the acc while driving. Even though the blower motor wasn't working driving at speed forced hot air into the cabin. I was able to control the temp with the degree selector dial so it wasn't so bad. Now all I get is air the same temperature as the air outside. The degree selector dial is now doing nothing. Is it the servo? Bad relay? Broken solders in the ACC? Please help!!! P E H any ideas? Guys I need your help. This is the last draw. If I can't fix this the car is history...

Safest thing with the aux pump is to put a weak inline fuse in the pump as they are known to ruin the CCU.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

NyDiesel,

Try a 10 amp fuse in series with the auxiliary waterpump (AWP)like BoostnBenz says. If that burnes out, you might have to bite the bullet and replace the AWP.

I thought the AWP was only needed when the engine was idleing to circulate the water thru the heater core but your car seems different.

P E H

I think even at idle the motor could handle circulating the water, I'm under the impression the aux pump was put there so you can leave the car off and still get heat after a short run. However if the pump is there and not working it blocks the path the coolant should take.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Boost,

You could be right about the AWP blocking the flow if it is not working.

I plumbed around the AWP and get heat all the time, even at idling. In fact the car gets the hottest when the car is not moving.

P E H

Hmm, what I mean is without the pump in place or in essence there is no fitting there just one continuous hose. If the AWP fails then it is a obstacle for the water to get through and is very hard for it to do this. Did you have an AWP inline or was this completely without it?

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

[QUOTE I keep blowing the #3 (25 amp) fuse ... QUOTE]

Have you done a good contact cleaning/scraping on that particular fuse holder ? These fuse blocks are notorious for developing poor contact over time. If this fuse is arcing at the contacts, it will be getting very hot and then eventually doing a meltdown. (Arc welding like!) My guess, without being there or re-/looking at your previous posts.

As for the AWP, I've had one apart and am confident that fluid wil flow through this even when off/disconnected. With it off the car, I think you will find that you can pour water right through it. As I recall, a small bypassable plastic impeller is inside.

Some good, logical electrical troubleshooting should find your problem. Don't let a little electrical problem get to ya !

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Diesel-guy

A shorted A/C lo pressure switch (on the drier) will casue a blown #3 and often #9 fuse in the 116...Robert

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership