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Old 09-12-2003, 10:40 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
Odd Problem: 124 fan resistor

Hi All - Was wondering if anyone has seen a problem like this before? On my new-to-me 1993 300E, one of the very few problems I identified was the aux fans not running at low speed when the a/c was switched on. It was relatively easy to trace this to the dropping resistor. One of the two wires to it was disconnected. Not on purpose, mind you, but it appeared to have overheated and burned off. The spade connector was still bolted to the resistor, but the wire had detached. The wire itself didn't look so good - the insulation was a bit charcoal-like, and the copper was greenish - looked overheated.

I cleaned things up, crimped on a new lug, and put it all back together. Worked beatifully - fans came on low after a few seconds at idle with a/c, coolant temperature never exceeded 90C, even on the hottest days.

That was early in the summer. Fast forward, and it's happened again. I noticed the coolant temp was reaching 100C in cool weather. (I should note 90F is "cool" here.) Sure enough, same problem. Both times it has been the wire on the 'supply' side, as opposed to the 'fan' side of the dropping resistor. I repaired it again last night. This time I ohmed out the resistor - 1.9 ohms, which as I recall is about right.

Anyone seen this before? Any thoughts on the matter? The hot wire seems awfully small to supply two fans - it looks to be about 18 guage to my eye. Also, the connecting post on the dropping resistor appears a bit brown and heat damaged. Think I might be getting extra resistance - and extra heat - from this?

It's not a big deal, as a/c season is winding down. However, this is the biggest problem on my new car, so it's all I have to play with!

- JimY
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Old 09-12-2003, 12:25 PM
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wjm wjm is offline
Join Date: Dec 2000
Location: Suburban Detroit, MI
Posts: 692
That happened to me once... although I attributed it more to RUST than HEAT.

Anyway, I cleaned the wire and put on a new connector, but I replaced the resistor. Have not had any problems since (over 2 yrs at least).
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Old 09-12-2003, 12:29 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Very common problem!! on the 1992 & later W124 chassis. I always rewire from the relay to the resistor with a heavier guage wire & replace the resistor. Never seen one a modified one fail.
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Old 09-12-2003, 12:43 PM
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Join Date: May 2003
Location: DC Metro Area
Posts: 365
maybe some dielectric grease

could help against oxidation / corrosion and help your problem.

I just replaced the resistor in my 1995 E420 - and will go out to check and see if it's fouled up yet. The resistor is too damn hard to get at on my car to do this job again!

1995 E420
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Old 09-12-2003, 02:39 PM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
Thanks Y'all -

I decided to dig into the 124 wiring diagrams while sitting here eating my lousy cafeteria lunch. Going to compare the fan wiring for my old 124 (1987 wagon) to my new 124 (1993 300E 2.8).

On the old car, the pre-resistor is fed by a 2.5mm wire from the low speed relay. The (single) fan is in turn fed by a 2.5mm wire from the pre-resistor. Nice and neat, it works fine after, uh, 17 years.

On the new car, the two fans are fed by two 2.5mm wires from the pre-resistor. The pre-resistor is in turn fed from the low speed relay by a *single* 1.5mm wire. Electrical design by Homer Simpson.

I'm gonna follow Donnie's path and run a new, heavier guage wire from the relay to the resistor. Should be an easy enough task, and fix it once and for all.

- JimY
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Old 09-12-2003, 02:56 PM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,768
The heavier wire should have been used OEM as the low fan not only gets much more use [ with a/c useage] than the high fan ,
[ which hardly ever is called for ] , but also draws more current than the high fan due to the dropping resistor series circuit...
Benzs mistake.....
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