So you're an E46 Cooling System Expert - Fun Quiz

Originally Posted by TerraPhantm

Emineid, I don't feel like responding to the post line by line, but the DME absolutely does use PWM. There's a map that correlates what duty cycle is necessary to attain X temperature opening. And in fact, you can hard code a desired PWM setting to override the map control of the thermostat altogether. A 12v indicator won't be as reliable for a PWM controlled function.

I've since sold my 330i and have no plans to ever own an M54 car again, so I can no longer experiment.

"... but the DME absolutely does use PWM." You sure about that? Because there are only two terminals on the tstat and when you put 12v across them the tstat opens up fully. The electric fan requires PWM and in that case you can't just apply a 12v power source to it to test the motor. You have to apply a PWM source too. At least that's my understanding. So why would the tstat be different?

Originally Posted by emineid

RayPooley:

I have an indicator LED light connected to the 12 volt wire to the thermostat.

Here is what I observe while driving:

Often-times, all it takes to get the LED light to come on is a moderate stab of the gas pedal. Meaning, increased "engine load" causes the DME to send a signal to the thermostat to heat it up.

Did you try to hook up the 12V signal to the thermostat on a "benchtop" setting?

Because during driving, the thermostat is already at or near 97C. You are not heating up the wax from room temperature. It does not take more than about 10-15 seconds or so for the thermostat to open up more when the 12V signal is sent to the thermostat. You can tell, because just a few seconds after the LED light indicator turns on (3-5 seconds on, then off), you can see the ECT temperature plummet, sometimes down to 84C.

If the LED light is on for only 2 seconds or so, the ECT goes to maybe 93C.

If the LED light is on for 4 or 5 seconds, the ECT goes to mid-80sC

These numbers are all rough estimates.



By the way, the DME always sends 12V signal to the thermostat ALL THE TIME while engine is on. But for the vast majority of the time, the signal is just a fraction of a second every second or so, and this is not long enough to heat up the wax much at all, so it has no effect on the thermostat opening.

But, if the 12V signal is 2 seconds, or 3 seconds, or 5 seconds, the wax gets heated above 97C and the thermostat opens up more. That is when you see the ECT temperature plummet.

Then the ECT temperature gradually returns to the 97C or so, as long as DME does not command the thermostat to open more again.

By the way, OBD2 tools DO NOT show when the thermostat heater is activated!!! The so-called "THERMOSTAT PWM" does NOT show any activity even when the DME sends a several-second-long 12V signal to the thermostat. At any rate, the E46 does not use pulse width modulation. There are other BMW's that use PWM signal to the thermostat.


So the bottom line is that the MAP thermostat is a very crude way to have a thermostat that works at 97C or so for the most part (low load) but acts as a lower-temperature thermostat at higher loads. The "lower temperature" can be anywhere from 84C to 90C to 93C. It all depends on how long the 12V signal is sent to the thermostat (typically 2-5 seconds).


The DME has some algorithms that make it somewhat difficult to predict mentally when the MAP thermostat is activated. Sometimes, I can drive 45 minutes and never see the MAP activated (my 12V LED indicator). Other times, I can see the LED light up almost every time I accelerate from a stop or go up hill and jab the gas pedal a little bit more.


Here is what the E46 ECT temperature behavior looks like, then, on a typical drive.


Turn on engine---ECT goes from ambient temperature (say 20C) to 97C over a few minutes of driving.

Go up hill and jab on the pedal: 12V signal goes to the MAP thermostat for about 2 seconds, say, and the ECT temperature drops from 97C to 92C, say, over about 20 seconds.

Gradually, the ECT climbs back up 97C.

ECT stays at 97C for a while.

Hit the gas pedal again, hard, and the MAP lights up for 4 seconds. ECT goes to 86C over about 20 seconds.

Gradually, the ECT goes to 97C.

Drive really hard uphill, and the ECT goes to 100C. MAP lights up for 4 seconds. ECT goes to 86C.

ECT gradually goes back to 97C and stays there.

For the most part with normal driving, the ECT will stay at 97C with dips to 84C or 88C or 93C or anywhere in that range everytime the DME decides to (mainly when you step on the gas pedal hard).


So from my observations, I would make a personal guess that the so-called "low temperature thermostat" that people talk about is not useful for the E46 BMW. The reason is that when the engine is making a lot of heat suddenly (gas pedal pressed hard), the thermostat becomes a "low temperature thermostat" very quickly (within about 20 seconds) anyway. At part-throttle (low load, not pressing the gas pedal hard), the thermostat is at 97C or so, and you are not making that much heat that would damage the valve seats, etc. So you already get the benefit of a low-temperature thermostat WHEN YOU NEED IT if you have a MAP thermostat.



Just one more observation which has nothing to do with the above discussion: The 1.4 bar pressure cap that they market for the BMW (instead of the 2.0 bar cap from the factory) is a waste of money. I have a coolant pressure gauge connected to the upper radiator hose by the expansion tank. The pressure virtually never exceeds 1.2 - 1.3 bar during normal driving. Caveat: If I deliberately fill the expansion tank almost full (way above what I am supposed to), then the pressure does climb to 2.0 bar. So if you fill the tank as you should, you don't get any benefit from the 1.4 bar cap.

All interesting stuff. I might give the in series LED a go. That sounds like a good idea. I do remember on one occasion while continuously monitoring that the temp on the display did go down to 85C for a very short period then went back up again to 98C. I can't recall anything out of character that I was doing at the time driving wise.

As far as whether the E46 325i uses pulse width modulation or not for the map thermostat...

Arguments in favor:
1. There is apparently a table of PWM values in the DME.
2. LED can't reliably tell whether the signal is DC 12 volt or PWM.

Arguments against:
1. Other forum posts show that SOME BMW's use PWM, and other BMWs use DC 12 volt.
2. The DURATION of the 12V DC signal (say 2 seconds, 3 seconds, 4 seconds, etc) clearly correlates with how much the ECT temperature drops, so I am not sure that the DME needs the additional use of PWM to control the MAP thermostat.
but most importantly:
3. My own experiments have consistently shown that even when the LED light goes on for a few seconds, the PWM percentage number (obtained by a OBD2 app such as Carly ) NEVER showed any blip or change.

Definitive proof would be if I connect an oscilloscope or a similar device to the MAP 12V line. I may get around to doing that one of these days. Circumstantial evidence makes me question whether there is any PWM at all to the MAP thermostat in my E46 325i.

Originally Posted by RayPooley

"... but the DME absolutely does use PWM." You sure about that? Because there are only two terminals on the tstat and when you put 12v across them the tstat opens up fully. The electric fan requires PWM and in that case you can't just apply a 12v power source to it to test the motor. You have to apply a PWM source too. At least that's my understanding. So why would the tstat be different?

100% positive. PWM just means the DME rapidly cycles power on and off. There is no reason you couldn't do PWM over two pins. What changing the duty cycle will do is change how much energy you're putting into the wax heater. At 100% duty cycle (i.e a direct 12v connection), the heater generates enough heat to melt the wax regardless of ambient temperature. At 0%, the heater is off and the surrounding coolant must melt the wax by reaching a high enough temperature. At partial duty cycles, the heater will not be getting enough energy to melt the wax unless the surrounding coolant is warm enough.

The electric fan has a fan controller for two reasons. 1) The fan would draw far more current than the DME can provide, so it cannot drive the fan directly. 2) Motors tend not to do well with direct PWM signals, so the fan controller will receive a duty cycle from the DME and use that to generate a varied voltage. Furthermore iirc the fan controller uses an inverted signal and treats values of 0% and 100% as error states (shorts to ground or 12v)

It just occurred to me that the 12v signal to the MAP thermostat *is* DC 12 volt, and that it also *is* pulse width modulated.

In this case, the signal appears to be DC 12 volts during the 2 to 5 seconds (or thereabouts ) that it is on, to heat the wax.

But the "width" of the signal is modulated. Meaning, the duration of the signal varies depending on the target coolant temperature.

In this case, the width is not in milliseconds, but is in the 2 to 5 second (approx.) range.

So it isn't ________-_-_-_-_-_-_-__________

But is ________----------------__________


I am curious so I will probably eventually hook up an oscilloscope to the signal, but this is what may be happening.