Auxillary fan resistor 1995 S350 Diesel - Page 3 - PeachParts Mercedes-Benz Forum

JimF · #31 11-18-2004, 12:18 PM

cynicism that you suffer from another correct fact: just because the car is moving down the highway at 80 mph, doesn't mean that " . . air is ramming through the radiator" as you said.

Since most MBs have bottom drip pans and are somewhat sealed, the air that's at the front of the radiator is not 'ramming through'. Certainly some air is but the main contributor to air getting into the engine compartment is the vfc fan. This unfortunately is mostly free wheeling (so not doing much of anything) unless you have converted as described on my page, MENU#20.

Look at it as trying to ram air into a box with 4 sides and the back that are closed. Without some "aid", the air will only bounce off!

Question 1: First, let's look at my ckt. Since that ckt is 'ideal', there are no ‘sneak’ paths to draw extra current from the battery. So the total current from the battery is 19.8 amps.

That current flows through all components. The voltage drop is due to the power dissipated by each resistor. Both R15 and R15/1 dissipate 58.8 watts each; the voltage drop from each resistor is 2.97 volts, so the voltage at the fan is 7.56 volts. Note that the fans 'resistance' is still the same; 7.56 V/19.8 amps = 0.38 ohms. Ok, so much for the non-real world.

Now to your car: you measured 8.8 amps through the fans and 9.1 amps through the resistor, so what's going on? Since this is the REAL WORLD, there's some parallel paths causing added current to be drawn from the battery. In fact go put your Fluke '36' meter at the battery's "+" terminal and measure what's being drawn. It will be greater than 9.1 amps.

oldsouth · #32 11-18-2004, 02:09 PM

OK, first I'll address the diagram. So on low speed as per the diagram, the fans will draw 19.8 amps total including power wasted by the resistors. And on high the fans will supposedly draw 35 amps. So the difference between the total draw on low and high is 15.2 amps.

Real world measurements: At idle with voltage of 12.2 with the fans on low, fans are pulling 8.8 amps plus 9.1 amps for the resistors for a total of 17.9 amps. On high speed the fans are pulling a total of 22.2 amps. The difference between high and low is 4.3 amps. For a savings on only 4.3 amps vs the output of the fans turning on low as opposed to turning on high is not worth all this effort. I mean why would Mercedes design such a system when the amperage draw is almost the same between low and high. The only thing I can think of is 1-the noise of the fans on high is greater and 2-Possibly the fans might last longer running on low (I do not believe #2 from experience of running my fans on high for 270000 miles and only one has just gone out. Anyway they will run longer on low as opposed to running on high. I disassenbled my bad fan and the bearing is what failed. The brushes still had a lot of life in them). It just seems like a waste to me having resistors dissapate energy into heat so the fans will run slower with no real big difference on how much amperage the alternator has to put out to run all this mess. When I was talking to a Mercedes mechanic about hooking my low speed back up he said "yeah, that is a big drain on the alternator when the fans are on high". Apparently he doesn't realize how much the resistors are dissapating either.

I really don't know what you mean by the cynicism statement. The only think I have done to the vicious fan is to refill it with 10,000 cst Toyota silicone and it works very good now. It locks up around 90 degrees C and has a definate roar that can be heard. When it is pulling air through the radiator at about 3000 engine rpm's, the auxillary fans look to be freewheeling as fast as they do running on low speed. And all that air seems to have no problem running past the engine and down under the car.

I appreciate all the answers and have learned a lot about the auxillary fans.

JimF · #33 11-18-2004, 07:38 PM

tiresome! Sorry but your second paragraph shows that I have not taught you ANYTHING about the fans and Ohm's law!

Last para first: The VFC is designed to lock up a LOW rpm and HIGH temperature, so it will lock in at over 90C and when it does, it IS the answer to my box analogy. Since it's 'pulling' in air, then it DOES cause lots of air to circulate.

But, the VFC is designed to cut out a 3000-3200 rpms. So when you going 80 mph, the VFC will NOT engage even if the radiator temp is 90C. Sorry but it's will cut out at high rpms. It will be active a low speeds and that's when you will need it.

Now the biggie: NO, it's not the SUM of 8.8 amps and 9.1 amps for a total of 17.9 amps that is drawn from the battery. It's only 9.1 amps. So all of the rest of your conclusions are incorrect.

On high speed, you measured 22.2 amps: then that's the total current being drawn by the fans and supplied by the battery. All resistors should be 'bypassed' (SW1 and SW2 are closed in my diagram) and they are not dissipating any power (watts). All energy is going to the fans.

Only on LOW and MEDIUM speed are the two resistors "in" the circuit.

JimF · #34 11-18-2004, 10:42 PM

diagram based on the info that you measured. So attached are two versions; one @ IDLE and another @ HIGH.

If you look, something seems to be 'off'! The two fans on HIGH draw only 22.2 amps; that is low. That makes them have a 0.55 ohm impedance (resistance) which is too high; should be 0.35 - 0.38 ohms or so.

Thus all number that are calculated are based on HIGH speed operation since there is no resistance in the circuit; only the resistance of the two (2) fan motors.

So on LOW, then the numbers in the schematic MUST be as shown. Again it's all based on HIGH speed.

That makes the resistor(s) be about 0.79 ohms which is way too high.

You are going to MAKE me measure my fans and resistors . . . .

oldsouth · #35 11-18-2004, 10:44 PM

[QUOTE=JimF]NO, it's not the SUM of 8.8 amps and 9.1 amps for a total of 17.9 amps that is drawn from the battery. It's only 9.1 amps. QUOTE]

Ahh, finally a concrete answer! Jim, you're not a politician are you? I will feel more comfortable running on low speed now.

One more thing on the cool harness. The Ch-92 is suposed to turn the fans on low speed at around 92 degrees C. At what temperature will high speed on the fans activate?

oldsouth · #36 11-18-2004, 10:56 PM

Jim, I don't know what could be off but here is how I measured the values. I had the car idleing with the a/c on max cool-fan high (inside fan). I put a blanket in front of the fans to stop airflow where the a/c pressure would increase and turn the fans on. I had the Fluke 36 in voltage mode and the black lead grounded and the other lead checking voltage on both sides of the resistors. Came up with 12.2 and 7.4. Then I used the clamp part to check amperage on the wire going to the resistors and it read 9.1. Then I cut into the harness on both sides of the fans on the top where I could get to it with the clamp and read 3.9 and 4.9. I then unplugged the electrical connector to the a/c drier where the system reads the pressure which caused the fans to come on high speed and tested the fans at the same location as before coming up with 10.6 and 11.6. If it all don't cipher out, I am sorry. All this ohms and stuff is way over my head.

JimF · #37 11-19-2004, 06:56 PM

measure the aux fan system on my ’94 S500 Cpe with three (3) speed levels. I measured only LOW and HIGH since it was pretty simple.

Here’s what I learned:
1) The two aux fans do NOT draw 35 amps as is stated. The first ‘clue’ is the 30 amp fuse that is provided. No car designer would use a 30 amp fuse for a ckt that COULD draw 35amps. Well, rest assured that they do not draw 35 amps. The maximum appears to be around 25 amps on HIGH.

2) The resistor values that you measure OPEN ckt are not the values that the ckt uses since they are in parallel so their effective resistance is lower than you measure.

Open circuit R15 (000 158 37 45) is 0.9 ohms; R15/1 (000 158 39 45) is 0.6 ohms. The in-ckt values are smaller and are shown in the schematic.

OS, the values you measured where very close to what I measured, so you’ll be happy!

oldsouth · #38 11-20-2004, 12:52 AM

Wow, that good to hear! Thanks for all your time and effort.

Does the resistors need to be in the order shown - R15 first then R15/1 as the current flows through them? When I installed them, I did not pay any attention to the order I installed them or the flow of electricity.

JimF · #39 11-20-2004, 01:07 AM

about when the aux fans switch to HIGH; for my car it happens at a little over 100C. And that only happens when I drive the car for a prolonged period on a warm to hot (80 - 90+ F) day and then shut off the car.

After 5 - 10 minutes, upon restarting, the fans will come on high for a minute or so until the temp drops. I've NEVER seen them come on high during any form of driving.

Re the resistor order: check the drawings that myarmar posted. It appears that R15 is on the left as you look at the front of the car. As I said, my car looks nothing like those drawiings: the resistor are mounted horizontally (as opposed to vertical in the drawings) but I think R15 is the left most resistor.

JimF · #40 11-20-2004, 11:46 AM

If your aux fans operate at a two (2) level system, it won't make any difference how the resistors are mounted. For LOW speed operation, both resistors are "in" the ckt and for HIGH, both are bypassed.

I've updated my page's MENU#18 to add some of the great info from myamar and my data that was measured as a result of oldsouth's inquiry

oldsouth · #41 11-22-2004, 07:12 PM

I got my new B10/8 sensor in today and installed it. The ohm's reading is exactly like the one I took out, about 590 -595 at 80-82 degrees C. The part # is 008 542 45 17. Aux fans cut on low just under the 100 degree mark on the dash temp gauge and 108 degrees on the a/c panel readout just like before. Apparently my model is set up differently from yours Jim and the sensor is different as to the ohm's readings. Where did you get the values you posted? Did you actually read them with your meter? Anyway it is fine like it is. The fans are coming on sooner than before the cool harness which is what I wanted.

JimF · #42 11-22-2004, 09:55 PM

read, then that's what you read!

The chart I posted is from my CD and I've verified a few readings at various temps. At 100C, the resistance is very close to 310.

Remember the "CHART"??? The resistance values MUSTbe as stated to trigger the aux fans. So that's why mine comes on at 92-94C and since your varistor has higher resistance to begin with, it takes more engine temp to get to 310 ohms. This must be, as you measured, around 100C.

You need a SPECIAL Cool Harness! If you are good at soldering, I can give you new value to use if you wish. But I need you to make a chart of temps vs resistance so I can get the correct value. Need 80, 90 and 100 C readings of resistance right at the B10/8 output pins. Read the temp from the dash.

oldsouth · #43 11-23-2004, 12:43 PM

I am fairly good at soldering but not very good at coming up with the values needed. I tried measuing resistance at the three temps you needed and with my gauge, it is very hard. Looks like the best way would be to put the sensor in a pot of water on the stove with a temperature gauge. I can send the sensor to you if you would be able to fool with it. I could use the 95 degree cool harness to change the resistor out since I have two.

Just guessing here but Mercedes might have changed the sensor to what I have to be more accurate with what the a/c panel reads compared to what the dash temp gauge shows. I remember you had said your a/c panel was not on with the dash gauge and mine was right on. You might could contact Mercedes and see what the current part number to your sensor is. It might be the same as the one I have # 008 542 45 17

JimF · #44 11-23-2004, 03:30 PM

MB part man today and he said that's a valid pn for my car as well as yours. So it should work exactly as mine does. Price is $28!

Remember that the "key" to this is the RESISTANCE of the CTS (B10/8) at 100C: It must be 310 ohms! No ifs, ands, or buts!

Try the 'water' idea and measure the sensor at 100C, just as the water starts to boil. Use a good thermometer suspended near the sensor so it's reading the water temp near the sensor. Don't rest it on the bottom of the pan. Have your ohmmeter clipped to the top leads. It BETTER come out at 310 ohms or close to that number.

Why??? The N22 module that sends out the command signal to TURN ON the aux fans is PRE-PROGRAMMED to trigger at this piont. If a sensor goes bad so that 120C equals 310 ohms (rather than 100C), you will, sooner than later, fry the engine.

So your NEW sensor you just bought or my OLD sensor MUST be the same! MB can NOT change or 'update' it.

oldsouth · #45 11-23-2004, 07:56 PM

Yep, right on 300-310 at 100 degrees C!

Used a pot of water on the stove. Sensor not on bottom. Very accurate temp gauge. So-so ohm meter but I feel good about the numbers. The ohm numbers I read as the temperature was rising is higher than when it was falling after the boiling point, so I will post only the falling numbers as I feel they are more accurate. (The temp was rising pretty fast and the sensor might not have warmed up as fast as the temp guage.)

100 C = 300-305
95 C = 405
90 C = 490
85 C = 570
80 C = 640

Probably accurate within 20 points because I was looking at a swing type meter and guessing as to where the pointer was between two numbers. The 100 degree number was just over the 3 line on ohms x 100 and stayed there because the water temp did not rise any more than 100 C.