AC fan & melted fuse holder - PeachParts Mercedes-Benz Forum

JimmyL · #1 06-09-2004, 03:07 PM

Well, at least nothing broke on this car yesterday!
First off, my ACC is primitively bypassed, which it was when I bought this car 3 weeks ago ('80 300TD). I am attaching a picture of the circuit, which I drew in schematic stick-man form. I have used the AC since I've had the car, and one toggle turns on compressor, and one turns on the fan. Well, the fan was on for about 30 seconds, and then it just went off. I pulled over first chance I got, and found not only the inline 20amp fuse open, but one leg was burnt and the fuse holder melted. (wired in-line, near blower motor. What I'm trying to figure out is what would have made this fuse and holder melt. I removed the fan motor and tested it, and it works fine, however is draws 9.40 amps when running. Is this a high amount of amp draw? I know for an AC induction motor that would be tremendous, but I don't know a whole lot about DC armature fans. It is mid to upper 90's for the forseeable future, so I need to get this fixed and am hoping for some tech help. Thanks so much...
Jimmy Layton

JimmyL · #2 06-09-2004, 03:10 PM

picture of fan running with amp-draw in the background. My flash made it look like fan is not turning, but it is.

jdc1244 · #3 06-09-2004, 08:13 PM

http://www.peachparts.com/shopforum/showthread.php?threadid=67755&highlight=melted+fuse

JimmyL · #4 06-09-2004, 08:26 PM

Do yall think I need to jump up to a 30 amp fuse?? My fuse didn't just blow, but the plastic of the fuse melted, a leg burned and the holder partially melted. Also, is 9.4 amps a normal draw?
I'm nervous about a bigger fuse, as I might get a bigger melt? Or worse? Or not? Help...
Jimmy

whunter · #5 06-09-2004, 08:26 PM

Primitively bypassed?
What you did is a nice temporary work around.
Your circuit is elegant and simple.
The acid test is that it works.
I suggest going to your local parts-R-us for a 30 amp MAXI fuse and holder.
Please rewrite this and post it as a get your A/C working DIY.
There are many people who need your help.
Oh, oil the motor bearings on the blower, it will help some.
Have a great day.

JimmyL · #6 06-10-2004, 12:16 AM

Well, I went and got a 30 amp Maxi fuse and holder, and that is one manly fuse! I wasn't happy with the fuse holder, but went to 2 places and at least the 2nd place had a holder of some sort. It is wired up again, with a 30 amp rated toggle switch also. I haven't tie-wrapped everything neatly yet so I can get to the wiring quickly in need be. I put a tape flag on wire from battery and have dikes ready if I have some sort of meltdown. This may be the first car I have to have a fire extinguisher in at all times! We'll see how it does tomorrow, and hopefully beyond. This temporary fix is more permenant than yall know! I would like to incorporate the pressure switch and whatever else needs to be in the circuit at some point, but will need a smarter person than I to post that one someday. Thanks for the advice and for the link. Yall are such a help! I'm appreciative.
Jimmy

TonyFromWestOz · #7 06-10-2004, 09:20 AM

Your photo of the melted fuel showed that one leg of the fuse had overheated, NOT the fusible link.
This was probably caused by a resistive connection in the fuseholder, to the fuse - possibly due to low contact spring pressure in the socket. This caused heating of the fuseholder and the fuse leg, causing the contact to fail completely.
Have you measured the fuse for continuity? I would put my $1 on it being continuous.

The 30A holder looks good and should not give you problems.

JimmyL · #8 06-10-2004, 11:51 AM

Fuse looked OK, but was open. Must have been near leg. Your assessment I'm betting is correct. Has worked fine so far today, which is good as it is hot AND humid. Brought the kitchen fire extinguisher with me today, just in case! Now that's confidence...
thanks for your reply.
Jimmy

jbaj007 · #9 06-10-2004, 12:33 PM

What gauge wire is that?

On a 30 amp. circuit, I think 12 ga. is min., 10 ga. for power transmission, but definitely NOT sure.

Someone brighter than me, please chime in.

JimmyL · #10 06-10-2004, 08:08 PM

It is 12ga. wire. Worked well today. I'm still concerned about the 9.40 amps the fan draws. Is this normal? I understand there is a reason for the need of a 30 amp circuit, I'm just not used to fans drawing that much current. If an AC fans draws more that a couple of amps it has a problem. Are DC fans that much different?
Jimmy

TonyFromWestOz · #11 06-11-2004, 08:26 AM

Quote:

Originally posted by JimmyL
It is 12ga. wire. Worked well today. I'm still concerned about the 9.40 amps the fan draws. Is this normal? I understand there is a reason for the need of a 30 amp circuit, I'm just not used to fans drawing that much current. If an AC fans draws more that a couple of amps it has a problem. Are DC fans that much different?
Jimmy

Jimmy,
Ohms law says it all.
AC Fans are working on 110 / 220V, the fan in your car is running on 12-15V. the 10A of the 12V fan motor uses 120W, the 110V motor using 120W draws only 1.1A.
On 220V using 120W draws only 0.5A

#12 07-25-2004, 02:09 PM

PLEASE change the twist connectors to some type of crimp, such as butt or male/female crimp connectors. You'll probably have to use 6 or 8 gauge crimp connectors to fit all those wires in but it'll be a lot more secure and safe than the twist-ons. I've seen many times folks using twist-ons only to have them come loose/off with engine & road vibrations. You're asking for another problem keeping/using the twist connectors.

elusive 190E · #13 07-25-2004, 03:15 PM

"...Ohms law says it all.
AC Fans are working on 110 / 220V, the fan in your car is running on 12-15V. the 10A of the 12V fan motor uses 120W, the 110V motor using 120W draws only 1.1A.
On 220V using 120W draws only 0.5A..."

Thanks for that brief analysis...

That sure put volts/amps into a better perspective...

I hope others learned as much as I did...

Again thanks for sharing your insight...

JimmyL · #14 07-26-2004, 12:28 AM

After this worked for a while I used the solder and shrink-wrap method. I never have had any luck with butt-spices. Thanks for the heads-up Tony.
I actually spent a good deal of time in the wiring section of my Haynes manual yesterday, trying to figure out a better way to run my hot lead from the compressor clutch. I even thought about trying to hook up the ACC to see if after I soldered it (cold joint fix I saw somewhere) that it might work, but my pea sized schematic brain-pan just isn't quite up to that much wiring...

TonyFromWestOz · #15 07-26-2004, 08:16 AM

Jimmy,
Your best bet is to run a relay from the A/C clutch power. Run your 30A fused power to the relay and the normally open contact of the relay to the fan motor.
The A/C thermostat would cause the clutch to operate when cooling is needed, this then operates the relay and provides power to the fan, without needing to draw the ~10A thru the ACC control wiring or controller.
Tony