Pike

Advice Wanted:
I have a 1978 300D. In the past month, the car has not started approx once per week. It jumps off just fine. Today I had a battery test performed on it. The battery turned out to be low and was suggested to be replaced. Not a problem. However, the reason the battery is being drained is the fact that there is a short in my Auxiliary Fan. A light test was performed on all my fuses and the short lied within the Auxiliary Fan fuse.
Do you think it is the Auxiliary Fan Assembly or perhaps the Switch?
Also, what is the approx cost of this maintenance? Any help is appreciated.
Please post or email me personally at jsparks01@hotmail.com

Thank You
Pike

mbtech208

It's easy enough to try a new relay first, if that doesn't solve the problem, it's probably the fan itself. I don't have much experience with older cars, so I've not seen it first-hand. Just seems logical to me,

Pike

Thanks for the post. All information is greatly appreciated.

Pike

sph17

Runs about $200 online. Do a Google search for mercedes Benz parts. One site

http://www.mercedesshop.com/catalog/index.php3

I have never used them or know how they are, but it gives you an idea.

My Aux fan Didn't worked for a number of years on my W123. I finaly wirered it with a new relay with power coming straight from the battery, not the fuse box. I wired iy so that whenever my compressor clutch was engergised, the relay would be energized and run my Aux Fan.

In effect, I bypassed the fuse, and original relay that you may be having problems with. I have also had to replace my aux fan in the past and it is not a big job.

Pike

Thanks for the advice. Is it best to replace ASAP or just unplug the battery when not being used and avoid using the AC?
Thanks
Pike

sph17

First I am not an MB mechanic. I have had three W123s and have over 700,000 accumulated milage experience, so take what I say with a grain of salt.

The Aux fan comes on when the A/C is on and some sensor senses the engine is getting warmer. Without the Aux fan working, you risk overheating, when running the A/C in very warm conditions.

I would try one of the following things:

If the Aux fan is on it's on fuse, I would just unplug the fuse and see if that solves your problem.

The other thing I would possibly do is unplug the Aux fan from the wire that brings it electricity. On the drivers sidedd there is a short wire that comes off the Aux fan and pugs in to a female plug, just under and behind the edge of the radiator. You will have to get on your back at the front drivers side of the car to do this. This connection is attached to the fram by a mental clip that the connection will side out of.

Final suggestion. I have found over the years that a bad battery contact can cause lots of problems. Whenever I have a starting problem, or think I have a battery related problem, the absolute first thing I do is clean my battery contacts. I take the cables off and clean the inside of the clasp and then clean the battery posts. I have had problems that I would have sworn were not a bad battery contact (i.e. thinking I had a bad starter motor, hard to diagnose electrical gremlin) that were fixed by cleaning the battery contacts.

Please keep us informed of your progrss, or lack there of.

Pike

Thanks again for the advice. I am trying to solve my problem while being TDY for 3 months in the military. My brother has my car for these 3 months and trying to relay info back and forth is very hard, especially when either of us have a good working knowledge of cars.
This 300D is my joy and I get nervous when something goes wrong.

I do know it has its own fuse for the fan. I will pull the fuse. Can I turn on the AC, just not the AC Compressor?

Pike

sph17

The A/C will work w/o the Aux Fan working. In the Winter, when you use widow defrost mode the compressor works to de-humidify the air, but since it ususally is cold outside the Aux fan dosn't kick on. As I mentioned, I ran my 84 W123 for a few years w/o the Aux fan working. I just had to keep an eye on the Enging Temp guague, to turn the A/C off if the engine temp started going into the red.