Jumping Aux. Fan Resistor, be careful
 

Hi friends. I want to share an experience that c an help others prevent an accident. Looking for improvement in the air flow at the A/C condenser, I heard some rumors of jumping the Aux. Fan resistor R-15 in order to get high speed during normal A/C operation. I did it and yes there is an improvement specially if you are using R-134a as I am. The problem was that the wiring (1.5 mm wire) of that cicuit is for a max of 16 A, and the 2 fans in a 126134 draw 23A. The 16 A fuse before burning, it burned the fuse information card and started a small fire inside the fuse box that melt part of the external light failure unit and the fuse box cover. If the car has 2 aux. fans, do not jump the resistor. I did it using the high power circuit which is designed with 2.5mm wire and a 40 amp relay. I hope this comment can prevent an accident.

Good luck to the DIY's.

Darrel
350SD 1991.

Confusion
 

Although I appreciate the warning, something here doesn't ring true. Both fans are controlled by two circuits. The A/C slow speed uses a 30 amp circuit through a ballast type resistor, the high speed uses a 15 amp circuit. I was always under the impression that a dc motor uses maximum current at stall. So...jumping the ballast resistor putting full voltage to the fans should lower, not raise, the current flow.

I think you have something else going on inside your auto!

response to tkamiya
 

So if that is the case, why does mb put a 30amp fused circuit with the resistor in series and only a 15amp fused circuit without the resistor in series. (Hint, the answer is in the whole circuit load, not the load of the fan itself ie. power requirements.)

Maybe a bit more. As I understand it. Putting the fans to a lower speed essentially makes the windings of the motor behave more like an electric heater. As the fan slows, the windings behave more and more like a direct short because you have not removed the energy via motion of the fan. Energy cannot be created or destroyed, so it heats up, effective motor winding resistance decreases causing the current load to go up. So...the massive resistor in series works as a current limiter. In the worst case, if the fan is stalled, all the energy is dissipated as heat in the resistor, not in the electric dc motor. No fryed motor....

I am sorry I made a general statement
 

I'm sorry I made a general statement regarding the resistor. This information might be true for the late model W126 only. My car have 2 circuits, the low power uses 1.5 mm wire with a 16 A fuse through a 30 amp. relay, activated by grounding through the refrigerant pressure switch (close at 225 psi, opens at 150 psi). The high speed circuit uses 2.5 mm wire. the circuit starts on the X 4/10 connector and it goes through the 40A relay to the fans. It is activated by grounding the relay with the high temperature coolant switch. I believe it closes around 105 degrees C. I did test the amperage with a DC Clamp-on Fluke meter and read 22.8-23A in the circuit without the resistor. Again I am sorry of making a general statement, this might be limited to the W126 late diesel model.

DHILLMAN
 

Please HILLMAN, observe that the MB original disign has a 15

ampers fuse for two auxiliary fans. It means that each fans has

to consume no more than 7 ampers. At the contrary case the

fans has a high current consume because of friction in the

bearings or electrical motor problems.


Thanks


Mario Farias
laprefar@cantv.net

DHILLMAN
 

pLEASE Mr. Hillman tell about this "DC clamp on FLUKE meter"

How many ampers in DC can you read with this clamp?

What is it the model and price?


Thanks
Mario Farias
laprefar@cantv.net

Electric diagram does not show 15 amp. fuse
 

Hi Mario Farias. I have the MB of North America Electrical trouble shooting manual of this model (350SD-1991 Chasis #126134), and there is no 15 amp. fuse in the circuit neither in the fusebox of my car. There is a 16 amp fuse for the low speed circuit through the pre-resistor, and there is another circuit that goes from the battery to the 40 amp. relay (high speed circuit). I believe the fuse for the high speed circuit is the relay itself. Why would MB designed the low speed circuit with 1.5 mm wire diameter and 2.5 mm diameter for the high speed, as illustrated in the diagram. High speed draw around 23-24 amps. as shown by the instrument. The instrument is a Fluke AC/DC clamp-on ammeter. I think is model 36 but I have to check the correct model #.

MR. DHILLMAN
 

tHANKS DHILLMAN FOR YOUR REPLAY.

I wanted to say that one of fans consume 18 amperes in low

velociti, while the another one consume only 8 AMPS. The total

consume for the two fans is 24 AMPS, in low celocity, and the 16

fuse is burned.

Therefore I think that the highter consume fan has problem in its

bearing or in its motor.

This is the question : Can the fan be fixed or is neceesary to buy

a new brand?. Do These electrical motor use bearing? Why this

high ampers consume?

If its possible how to fix it?

Thanks

Mario Farias

laprefar@cantv.net

When I read posts like this it helps me to understand how DIY are able to improve upon engineering designs by just shorting or bypassing unneeded or in-the-way electrical and other componants.

For some, a better name that do-it-yourselfers is screw-it-up-yourselfers

According to the diagram (1986 300E) the wire that connects from the relay to the resistor and then to the fan is THINNER (rated at a lower current- AC pressure) than the other (temperature-high speed fan). If the resistor is jumped, more current will be drawn, hence frying the wire. (if the fuse doesn't blow first)

W126 #5 A/C fuse blowing
 

This post might be right on target (or thereabouts). Yesterday while driving my '86 420SEL at about 65mph the A/C blower quit. I saw the fuse was blown and replaced it with the same (#8 white ceramic). Blew again. Replaced it. Fuse only blows when A/C is on. Running blower w/o A/C does not trip it. Would a failed or failing compressor do this? I'm asking 'cause it doesn't seem so and I'm just waiting for the shop to say the standard "you need a new compressor" when it seems to me its something else electrical. Thanks for your help!