A/C Low Fan not working - W140 M104 1995 S320
 

Hi All:

I especially hope Jim F and MBDOC see this post...but I will of course accept all help from everyone (thanks in advance)...

I have a 1995 S320 W140 bodied, M104 engined Benz. I had a minor overheating issue on Sunday which resulted in a small t-connector near the firewall breaking and spraying coolant out. I stopped the car and opened the hood and let it sit until the tow truck came - did not lose too much coolant. I towed it to my favorite indy shop and he replaced the part and hoses and topped off the coolant. The car now runs at 90-95 degrees C consistently (very nice).

One big problem, the electric fans only come on at 105 degrees C. When I put the A/C on, they do not run at all. I have read about the pressure switch behind the driver headlight on other posts - so I checked the a/c pressure and it turned out OK, but I still had bubbles in the glass sight. So I did a minor recharge and the bubbles are now gone (yeah).

So, the electric fans are still not coming on when the a/c is turned on. I was not sure if there is a magic relay or a way to "jump" the fans to have them come on (Jim F, are you reading this?)

Any steps to solving this problem would be appreciated.
:confused:

I think that on the 140 there is the same resistor like on my 124.

It's a big ole spring with two terminal ends. These tend to go bad at the one end. It ought to be on the drivers side, behind the headlight. Do a search on fastlane to see a pic of one.

Good luck!

first question??
Does your car have 2 white ballast resistors between the aux fans OR a skinny black tube??

IF it has the skinny black tube it needs to be modified BACK to the 2 ballast resistors.

Take a look at this diagram.

I have the tube
 

MB DOC,

Thanks for the picture - what is the tube? is it a temperature sensor? I am also a bit confused - are you saying I need to cut it open and rewire the insides with the resistors listed on the TSB? I can probably do that, but just checking...
So, to recap - is this a new assembly I have to build or can I modify the tube with the parts listed? (Thanks for your help!)

You are lucky . . . and must be a very observant person! Most stories where the infamous 'T' union broke have fried the engine! In a few minutes (or less), you could have dumped ALL of the coolant, so if you where driving and not paying attention, there would be one cooked engine!! Seen this so many times.

Other W140 owners pay attention: if you haven't replaced it on your W140, suggest that you do so.

The "key" to aux fan operation are the "temperature" and "pressures" as shown in MENU#18 in the 'table' called "Auxiliary Fans Activation".

Key: Aux fans operate under two (2) variables; temperature and pressure. Aux fans always operate with temperature when you don't use A/C. If you use A/C, then 'pressure' is the key.

Using your A/C, the fans should come on a when the pressure builds to a preset point as shown in the table. Practically, there's a difference from the tables values vs real life. My car has the three (3) speed system as shown in the table. Using A/C, my fans come on at 215psi, about 15 bar.

Without A/C, the fans come on at 102C without my Cool Harness. Spec is 100C but most come on around 105C. With a Cool Harness, my fans start at 95C.

So by recharging your A/C you have raised its efficiency and increased the pressure so the fans start when the pressure reaches the indicated point. Sounds like you got your fans working correctly.

If you want to watch the pressure while driving, use the technique described in Menu#15 under “Operational Checks”. As you drive, leave the display showing “refrigerant pressure, “#7”. As you drive you will see it fluctuate from 4-5 bar to 15 – 18 bar depending on the temperature and type of driving. Nice way to see how the A/C is operating. At the same time, you can access “#5” to see what the temperature of the evaporator is producing.

The black tube on your car was designed to be a variable resistor..didn't work very long.

You only cut the WIRES at the tube & either solder OR crimp ring connectors that can be attached to the resistors.

Thanks! I will try everything suggested...
 

Thank you MB Doc and JimF. I really appreciate your replies. I will see if Phil at Fastlane carries these parts to do the resistor change - if not, do you guys have a source?

Thanks again...Once I have time to do this over the weekend, I will post pictures of the job for others to see how to do it.

And Jim, you may be getting an order for a V2 cool harness from me in the near future.

Andrew

One last question MB DOC
 

Does it matter what sequence the resistors are wired? And what color wire they connect to?

I am assuming R15 in the diagram is part number 0001583745? (and that R15/1 is part number 0001583945)

Do you know which color wire that should be connected to R15 and to R15/1?

I just want to make sure I do this right the first time...THANKS!

Just a series circuit...connect either resistor to either wire...doesnt matter at all....good luck!

As MB Doc said, it's a series circuit. To see this 'visually', look at Menu#18 to the sketch just above the "blue" table (Aux Fans Activation).

The first sketch shows a 'three-speed' system; if you look carefully you'll see an 'extra' wire that goes to R15 resistor. There's no "jumper" bar.

The second sketch shows the 'two-speed' system. Note the jumper bar and only two wires. . . a 'series' circuit.

The resistors are switched in (shown in the schematic) with the Aux Fans according to the following;
"no-resistors-in-series" = High speed
"one-resistor-in-series" = Medium speed
"two-resistors-in-series" = Low speed

Got parts from Phil and it is done!
 

:)
I removed the old tube resistor - easy to cut and remove. The new resistors were the same length as the two bolt pattern where they are to be mounted. I removed the plastic cover in front of the fans and took out the old resistor.

Phil did not have the mounting plate from mercedes, so I went to radio shack and got a project board. I cut the board on my table saw, and then painted it black. I drilled holes to match the patterns of the resistors. I mounted the right-hand resistor to the project board using lock washer/nut and bolt. The left-hand one used the original mounting scres to mount to the board and to the frame. The connectors I made using crimped and soldered wire.

Anyway, all in - turned on the car, let it get up to operating temp, switched on the a/c and after about 1-2 minutes, the fans came on at low speed! Cool...

Thanks again MB Doc and JimF for your help. This board is the best.

Nice job . . . and good pics. MB Doc will hire you!

Nice Job! (2 thumbs up!)

Seeing those pictures brings up the following...

Since the location of my resistor is not as handy as in the ones in the W140, is it ok to relocate the resistor? In Andrew's pictures, he has them outfront...pretty much exposed to the elements. Do the resistors care if there is a downpour (we tend to get those here in FL)?

Naturally, I would use wire that would a be a gauge larger than what MB would use (just to be on the safe side).

thanks!

-Larry

It's probably obvious but the reason they are mounted in front is the high power dissipation and the 'air' is need to keep them from overheating.

I would suggest you put them in that general vicinity.

As an aside, the "pc-board" will get a brown spot . . . actually two (2) brown spots after awhile.

OVERHEATING was a problem on any ballast resistor..The one on the 1992 & later 400E's & E420"s always had problems with the wiring harness getting too hot.. that location on the W140 chass is a much better placement.