Please help: Troubleshooting Aux Fan Short R129
 

Hello everyone/ I've got a 1990 300SL-24 with a short circuit problem in the auxilliary fan circuitry. Basically, everytime the aux fans would come on, either by A/C or water temp, the aux fan fuse will blow immediately. I checked the wiring on both fans and nothing seems to be out of place or grounding anywhere. All insulation seems fine. Im a bit challenged when it come to electrical issues so I would appreciate if someone could help me out. Does anyone have a wiring diagram for this application? I was thinking as a last resort, just to re-wire the aux fan system but I would need a wiring schematic to do so. Where would be the most likely place that a short would take place? I think that a 1990 -up 300CE or 300E would be similar since we share the same engine? If anyone has any advice or suggestion, please post. Thanks in advance

First suspect will be dropping resistor terminals ...

also, hand spin the fans for binding..

Hello Arthur and thanks for the quick reply. The fans turn freely when spun by hand. As far as your first suspect, forgive me but what is meant by "dropping resistor terminals"? Do you mean that terminals are literally hitting something metallic? Would you mind guiding me as to where these are located? I used to have a 300CE and it had a ballast resistor (ceramic) which had two terminals. Are you referring to this? I was thinking about that yesterday but could not find any ballast resistors in the engine bay. Thanks again.

Yes
Behind driver headlamp area ..check for poor connections at terminals
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1D00YU6RY1JL0RQTFV&year=1990&make=MB&model=300-SL-001&category=G&part=Aux.+Fan+Resistor

archieves -fan resistor

It should be just one resistor. Easiest access is to yank the driver's headlight out. There may be a gum wrapper in there as the terminals are exposed. Before I'd play there, hook the fan direct to a decent battery and see what kind of current draw there is with a decent ammeter. I'd suspect the fans are full-o-crud and need a good clean and lube. Check the brushes while your there and give a good spray with contact cleaner.

You might also look at the aux fan relay. It is in the relay box behind the fuse box. There are two long ones in a row, it is the back one.

Ok guys, time for an update. I took the headlight off and inspected the resistor. Nothing out of the ordinary, no frayed wires, no gum wrapper, nothing. Next, I unplugged the fans and jumped the two wires by the dryer as suggested. Checked the fan connector for current and there is current but the fuse isn't blowing. So by process of elimination, I have isolated the problem with the fans, or maybe just one fan. I spun the fans by hand and they turn freely. Assuming that there isnt any short with the fans, why would a free turning fan or fans cause the fuse to blow? Sorry if this is a stupid question but I am new to auto electrical issues. Thanks again.

Two reasons come to mind.
First, the fuse you're using may be too small to handle the load. A simple thing, sure...but I've seen it happen to folks who should know better.

Second, one or both of the fans are on the verge of failure and are drawing more current than designed. I'd go next with trying to measure the current draw of the fans.

Back to square one.
 

Well I borrowed a fan from known working system and tested my system with it...POOF! :( The fuse blew again! Back to the drawing board and will check the rest of the system. There has to be some kind of short somewhere in the wiring. But what keeps on coming to me is that when I tested the system without the fans, the fuse didnt blow. When I jumped the two wires to complete the circuit with the fans unhooked, I measure the output to connector where the fans connect and it was reading about 13.8 volts. So I suppose that assuming that there is a short somewhere in the system other than the fan wiring, the fuse would not actually blow unless there is a draw on the current? Thanks guys.

There was an upgrade on some of these 2 fan systems to up the fuse from 15 to 25 , as the 2 fan draw is 25 A
What amp are you using ???

I have schematic for 300CE, so tell me which fuse/location is blowing......
..may be able to use this schm..

Hell, I would even go as high as 30 amps. Its the starting amp draw that usually blows fuses. Most motors require up to 3 times running amps to start up. I have a DC ammeter at home, maybe Ill check to see what running amps are for the two fans on low by the resistor.......after I replace my leaking EHA valve tonight.

The fuse that is used is a flat piece of aluminum about 1" long and is held by 2 screws. On the SL (R129), there is a fuse location right next to the brake master cylinder. There are 3 fuses in a row, all the same screw-on type. The top is the blower, the middle is the Aux Fans, and I cant remember the bottom one. Needless to say, the middle is the one that keeps on blowing. Im not sure if its similar to a CE but I imagine it should as its using the same engine so I welcome your schematic.

OK

Check these and tell me what you have

use "D"
Fuse # 7

See what A. they are and what the fuse cover says they are for..
That will tell me is schm is same..

Arthur,

Under the fuse cover, "D" states "Seat Heating" and is rated 16 Amps, and Fuse 7 is for: "climate control system, heating water valves, water pump, relay auxilliary fan term.86" rated 8 Amps.
Thanks.

OK

They are using a slighly different feed for load , but the relay primary coil is the same ..
so, first thing is what Amp strip fuse have you been replacing the blown with??

I've been replacing it with 30Amp fuses.