Auxillary fan resistor 1995 S350 Diesel - Page 2 - PeachParts Mercedes-Benz Forum

myarmar · #16 11-15-2004, 06:48 PM

This is from 12/1/94 to 8/31/95, single resistor:

oldsouth · #17 11-15-2004, 09:06 PM

I did some accurate measurements today with my Fluke meter. I disconnected the wires at the resistor and did an ohm reading across the resistor and it read 1.0.

With either the 92 or 95 degree cool harness the fans turn on at 108 degrees C and turn off at 100 degrees C as per the a/c readout in diagnosis mode #6. The actual temperature is just barely below the 100 C mark on the temp guage with the 95 and very slightly lower with the 92, but still close to the 100 degree mark. They also turn on the instant 17 bar pressure shows up on the A/C panel #7 and turn off just as the panel reads 13. They turn on and off at these parametere whether I have the wires connected to the resistor or tied together with a bolt. The only difference is, through the resistor they run slow and tied together they run fast.

My main goal was to have the engine run a little cooler and I believe the cool harness will accomplish this. I am unsure whether I should bypass the resistor and let the fans run on high or use the current resistor and let them run on medium or use the parts I ordered and try for a three speed system. I think if the three speed setup would work that would be good for the fans to come on at 100 degrees C as per the a/c panel reading which is what I think Jim used in his calculations for the cool harness parameters of 92 and 95. I am pretty sure my fans are coming on higher than 92 or 95 degrees C. now. My resistor parts will be in tomorrow but I could probably return them if I do not install them. Would the three speed setup work if the resistors were wired like shown in the diagram?

I know this is a lot of rambling, but I like to know how everything works. I recently had to replace one of my auxillary fans which made me start thinking about the resistor again. The condenser had to come out to replace the fan and once out, I noticed the bottom half was completely clogged with about 4 handfulls of bugs. You cannot see or clean this section with everything installed. I steam cleaned it out and the engine is running much cooler now even before the cool harness, plus the temps are starting to fall here in Louisiana.

One other question about the resistor. Jim you said it was dissipating about 55 watts or so. This is over 4.5 amps at 12 volts of wasted electricity right? Does anyone know the difference in amp draw of the fans on medium as opposed to high. If the resistor is drawing close to the difference between high and medium, why not let them run on high all the time?

myarmar · #18 11-15-2004, 09:47 PM

Oldsouth, 1 Ohm is to high. You can't trust Fluke with that low resistance. If your voltage measurements were correct, you have 12.x-8.x voltage drop across the resistor. Lets make it 3.5 VDC. We know from MB spec the current through the motors is 35A @ 12 VDC. That gives motor impedance about .34 Ohm. To simplify things lets assume its resistive load only. Now if you had 1 Ohm resistor, at low speed total impedance would be 1+.34=1.34 Ohm. The current @12VDC would be about 9 A. That means 9 VDC drop across resistor and only 3 VDC to the motor. Doesn't look right.
Now to make it right the resistor should be about .14 Ohm. This would give you 3.5 voltage drop on it and 8.5 on the motors.
I wouldn't do high speed only. Resistor gives "soft start" by limiting current. It is good for the motors and fuse.
You can't make 3 speeds setup if K9/1 relay is not there, only 2 speeds.
Reg. Mike

JimF · #19 11-15-2004, 09:50 PM

two schematics, you first task is to figure how your system is wired. With ONE resistor, I guess it probably is the two speed system. But it would be nice for you to know before you tackle 'fixing' or re-doing it

But there's a lot of confusing information that's been presented.
1) In the '92 schematic, they show ONE resistor ONLY but in the outline pictorial, there's TWO (2) resistors, R15 and R15/1???
2) But in the late '94 to '95, only ONE (1) resistor is shown??
3) My car has a three speed system but it does not look anyting like those outline pics you added?? They are shown mounted vertically but mine are horizontal in line???

1.0 = There's no way that that measured 1.0 "ohm"??? At 35amps, the resistor would FRY!!! And do it quickly! Note: it's impossible to draw that current with a 1.0 ohm resistor.
---------------------------------------------------------------
"The actual temperature is just barely below the 100 C mark on the temp guage with the 95 and very slightly lower with the 92, but still close to the 100 degree mark." <-- Still sounds like your CTS (B10/8) sensor has drifted a bit more than others I've seen. CH-92 turns on a 92-94C on my car and most other's I've seen.

Have to go; will answer the rest tomorrow.

myarmar · #20 11-15-2004, 10:14 PM

Quote:

Originally Posted by JimF

1) In the '92 schematic, they show ONE resistor ONLY but in the outline pictorial, there's TWO (2) resistors, R15 and R15/1???
2) But in the late '94 to '95, only ONE (1) resistor is shown??

Jim, 92 has two resistors, not one. For 94 and 95 one resistor shown, but it appears in some cars two actually istalled in series without any wires in the mid point.

oldsouth · #21 11-15-2004, 10:20 PM

Forgive me, I am only partially competent on this electrical stuff.

The way I measured ohms is I set the meter to ohms and touched the black lead to the top and the red lead to the bottom and the meter read.9 then went to 1.0 and stayed there. Still beeped though.

Jim, I looked at the schematic and after looking at the car, it is hard to determine. I removed the driver headlight in order to splice in two good wires because the brown wire was falling apart and was able to get a good view of the wiring then. The TWO wires (blue/grey and brown/green) run from the resistor into a bigger harness. This harness has two wires splitting off and going to the compressor then the wires from the a/c drier join then they run back to the fuse box and are lost in the maze of wiring. There is nothing like a relay or anything else around the headlight that is connected to the wiring. There is a small box that looks like a relay to the left of the headlight looking from the front that nothing is connected to. It has rubber covers sticking out of the bottom but I did not pull any off and look at it very close when I had the light out because I did not think it was part of the system.

This is getting pretty deep and for now I think I will run through the resistor in order to get medium speed then see how the temps compare next summer when it really gets hot. I'm thinking now why install the two resistor setup that I ordered from Mercedes.

Running on high doesn't hurt anything though because since I bought the car in '95 the fans have run on high probably 98% of the time and after 270,000 miles I just had one to go out. I should have replaced both, and would have if I had known the condensor had to come out but that's another story.

JimF · #22 11-16-2004, 01:41 AM

Fluke but when you said it "beeped", that's indicates that it's in a 'continuity' mode, useful for determining low impedance connection points but definitely no good for low impedance measurements.

If it has a 200 ohm or lower range, set it to that and make sure it's set to measure resistance NOT continuity. If it's a 'good' unit there's should be 'calibrator' button that will zero out the LEAD resistance. Remember you are trying to measure extremely low resistance, so the leads can and do contribute.

As I calculated in a few posts back, the fan's impedance is about 0.35 ohms. So the resistor CAN'T be 1.0 ohm. Maybe as both of us say, about 0.08 to 0.1 ohm is about right.

Ok bottom line: do what you are planning to do and see how it all plays in the summer. But you may want to consider changing the B10/8 sensor after you check it out.

There's only three temperature values that are important: so measure the CTS at 100C if it's close. If it's not close, consider replacing it.

CTS's temp (deg C) vs resistance.

Temp(C)--Temp(F)-------- Ohms
100 -------- 212 ----------- 310
107 -------- 225 ----------- 250
115 -------- 239 ----------- 200

JimF · #23 11-16-2004, 01:49 AM

Quote:

Originally Posted by myarmar

Jim, 92 has two resistors, not one. For 94 and 95 one resistor shown, but it appears in some cars two actually istalled in series without any wires in the mid point.

Ok, I accept your expertise. But as a design engineer, it doesn't sit right! I couldn't get away with that in my business. If there's two (2) resistors in the assy, then there's GOT to be two (2) resistors in the schematic!! Plain and simple.

oldsouth · #24 11-16-2004, 08:14 AM

My meter is a clamp meter with a 200 ohm setting. It reads only to one decimal point so it is useless in testing something very slight. It reads .1 just by touching the leads together.

My B10/8 sensor went out about 2 years ago and I replaced it (Mercedes Part - not aftermarket). As I said before, before the cool harness the a/c panel reading #6 was right on with my dash temp gauge. I think some way my car is set up for medium and high speed without any low. It is coming on right at 108 on the panel readout and going off at 100. Of course that value is inaccurate with the cool harness but it is still the accuation points. I will test the B10/8 sensor today. Thanks.

JimF · #25 11-16-2004, 10:42 AM

same Fluke meter, Model 36. It's claim to fame; measuring dc or ac current. You can use that to see how current is flowing through the fan's motors.

I NEVER use the Diag or Op mode for reading temps: the dash meter is preferred and it's much easier.

oldsouth · #26 11-16-2004, 08:31 PM

Yep, that the meter Jim.

I got my old needle type meter out and did some measurements. I set it to 1x ohm and was still unable to read any resistance on the resistor. Maybe just a hair but my meter just isn't fine enough. I set it to 100x and did some readings on the B10/8 sensor. When the car warms up and the temp guage is just barely over 80, the ohms reads just a little under 6 - which is about 590-595 I guess. According to your chart, that is too high. The part number on the sensor is 008 542 45 17. I just don't see how it could be so far off since it was pretty close to the dash gauge before installing the cool harness. I also ohm'ed the cool harness and came up with 1100.

I still have one other question: Does the resistor control the voltage to the fans on any speed but high? I received the resistor package from Mercedes today and was wondering if I hooked them up, would I still have a meduim speed with around 9 volts or would it be low speed with 7 volts? Would it depend on how I wired it according to myarmar's diagram?

If my B10/8 is off and I replaced it, I would definately want a low or medium speed because the fans would come on at a lower temp. If I leave everything as is, adding only the cool harness, the fans will come on just before 100 and I think a high speed only setup (bypassing the resistor) would be fine there.

JimF · #27 11-16-2004, 11:04 PM

Since you were able to measure the Cool Harness ok (1100 ohms), I have faith in the other readings. They are DEFINITELY high and I suspected as much since the kick-in points are close to 100C. With the CH-92 it should be closer to 92 – 94 C.

The FAILURE mode of a VARISTOR (the CTS sensor, B10/8) is precisely what you measured: the readings get higher and higher for the same temperature. When I drove Cadillacs (ugh!!), I regularly replaced the CTS about every 1-1/2 years. BION!

The resistance value is KEY in the operation of the aux fans: In fact, MB uses a precision resistor substitution box to check that the trigger points are as shown in the table in MENU#18.

Switching Points -------1st ------ 2nd ------ 3rd
Eq CTS res (ohms) -----310 ----- 250 ------ 200

The N22 control module reads these values and at those points the aux fans are switched on DEPENDING on the resistance read. Normally, the aux fans will come on LOW at the CTS varistor reaches 310 ohms. If the car continues to heat, the CTS resistance will continue to decrease and at 250 ohms, the fans will switch to medium.

That’s why after you've driven the car on a warm (or hot) day, shut it off for 5 – 10 minutes or so, then start, the fans will ALWAYS come on HIGH for a few minutes until the resistance changes from 200 ohms to the next point, 250 ohms. Then they switch to medium. You get the picture.

So the CTS sensor is critical for proper operation of the aux fans without A/C. With A/C on, the resistance operation is ‘swamped’ by the A/C pressure sensor, B12 (I think). The fans now respond to high a/c pressure and are controlled via B12.

Personally, I think that LOW speed operation is a MUST since the fans will come ‘earlier’ than normal. If you wait until 100C, and your in stop-go driving, you will HEAR the fans while at the “stop” part in stop-go operation. The aux fans on high while waiting at a stop light, are very easy to hear.

The Cool Harness enables the fans to come on a little early so that the higher fan speeds are not needed. That’s its claim-to-fame.

oldsouth · #28 11-17-2004, 12:49 PM

OK a few questions and I'll be done here. First off I have ordered a new CTS sensor. I will see what the kick in temps are with that one when I receive it. I installed the new resistors from Mercedes. It turns the fans on low speed. Voltage going in at idle was 12.2 and out was 7.4. I guess the one resistor I had only allowed medium and these two allow low. I measured amp draw on the fans and on high the new one draws 11.6 amps and the old one 10.6. On low speed they draw 4.9 and 3.9 amps respectively. I also measured the amp draw through the resistor wires and it measured 9.1 amps.

My questions are:
1-When the fans are on low speed and are drawing a total of 8.8 amps, is the resistor also drawing 9.1 amps for a total of 17.9 amps or is the voltage from the resistor going straight to the fans and I am just reading the amperage twice?

2-Say you are going down the interstate at 80 mph and the car reaches 92 degrees C. and the fans come on low; are they really going to help cool the engine off any since 80 mph wind is already ramming throught the radiator?

3-Why would my fans wait to come on until I reach almost 17 bar of a/c pressure when I apparently have a two speed setup? Shouldn't they be coming on at 14? The CTS sensor doesn't have anything to do with that does it?

JimF · #29 11-17-2004, 08:43 PM

like the new B10/8 sensor plus the two (2) new resistors.

Question 2: the fans will come on if the car's engine temperature exceeds 92 C (assuming that's the trip point) because that's what it's been programmed to do.

Your assumption is not really valid: A few years ago, we went to Napa Valley. It was in the HOT summertime and the temps were 108F as we proceeded. On some hills, the car exceeded 92 and so the fan's came on. Your ASSUMPTION that they don't NEED to come on is based on a faulty conclusion: that there's already ENOUGH air going through the radiator: NOT TRUE. The fans will INCREASE it and lower the engine temperature.

Question 3: Now we are talking about the A/C pressure and it has nothing to do with B10/8 sensor (I know you know this but just repeating to be sure). It SHOULD come on at 14 or 15 bar; 17-18 bar is for second stage. My car comes on at 15 bar for 1st stage. Just as the CTS sensor goes bad, so does the A/C pressure sensor (B12??). They don't trip at the required pressure so they need to be replaced.

Question 1: Now we are into a famous electrical "law" called OHM's law. Simply stated; Volts divided by current (amps) equals resistance (ohms). In a circuit, the SAME CURRENT FLOWS THROUGH ALL COMPONENTS!

The attached ckt drawing shows a simplified version of the fan ckt. R15 and R15/1 are 0.15 ohms and will drop about 3 volts across each with the SAME CURRENT FLOWING through each resistor and fans. It's that simple.

oldsouth · #30 11-17-2004, 11:12 PM

Good to hear that the fans will increase airflow at highway speeds. I would hate for them to be turning for nothing.

I'll get a A/C sensor to have on hand for the next time I check my freon. (unless it starts tripping higher than 17)

I think I understand your answer to question 1 but maybe not. I have a good knowledge of amp draw, basic wiring, relays, converting amps to watts and such. But ohm's and resistance, I am kinda foggy on. Let me ask it this way: Suppose I have a battery (no alternator) and the auxillary fans come on on low speed drawing a total of 8.8 amps according to my Fluke clamp meter (3.9 + 4.9). I also measure 9.1 amps at the resistors with the Fluke. What is the draw from the battery if I was able to put the Fluke there assuming no other power accessory was on?