W201 87 190D blower fan coming on by itself??! - PeachParts Mercedes-Benz Forum

bentrod · #1 10-09-2018, 02:17 PM

Hey folks.
My W201 1987 (201.126) 190D 2.5 no turbo has at times the blower fan coming on by itself - while the push buttons are in the off position.
Comes on for just a few seconds just every few days.
Yes - you can switch the speed up and down too on command if you want.
I see the switch has some relays built into it.
Do you think I have a switch problem or something else going on.
FWIW the car has a new ignition switch in it too, just a few months old.

Diseasel300 · #2 10-09-2018, 02:22 PM

I'm not familiar with the 201 climate control system, but if you're capable of changing the fan speeds with the unit turned "off", I'd be looking squarely at the part that's supposed to be "off". Probably a crappy solder joint or fused contacts on a relay or switch.

jay_bob · #3 10-09-2018, 08:27 PM

Does the 201 use the solid state blower regulator like the 124? If so you may have a shorted transistor.

bentrod · #4 10-10-2018, 10:01 AM

I'm not sure...

Diseasel300 · #5 10-10-2018, 10:03 AM

Quote:

Originally Posted by jay_bob

Does the 201 use the solid state blower regulator like the 124? If so you may have a shorted transistor.

Even if it does, the speed control shouldn't have any effect with the PBU switched to "off". The OP states that he can change the fan speeds when the blower is acting up and the PBU is "off".

bentrod · #6 10-10-2018, 01:32 PM

Quote:

Originally Posted by Diseasel300

Even if it does, the speed control shouldn't have any effect with the PBU switched to "off". The OP states that he can change the fan speeds when the blower is acting up and the PBU is "off".

Yes that is right.

jay_bob · #7 10-10-2018, 03:25 PM

Quote:

Originally Posted by Diseasel300

Even if it does, the speed control shouldn't have any effect with the PBU switched to "off". The OP states that he can change the fan speeds when the blower is acting up and the PBU is "off".

Let me explain some more detail. The transistors are usually arranged so that the power source goes straight to the motor through the transistors. The PBU control only affects the control signal to the electronics in the blower regulator.

The way it’s done in the 124 and later models is the PBU puts out a 0-6 V dc reference signal to the regulator. The regulator takes this 0-6 V dc signal and creates a PWM drive signal for the transistors. The on-off ratio of the PWM is increased as the control voltage increases. Thus increasing the effective “on” time and the amount of energy passed to the motor.

There is one more consideration. MB design philosophy on climate control is that in the event of a control failure, the system should failsafe default to max heat, max blower, and air flow to defrost. So if the reference voltage drops all the way to 0, then the regulator drives the output to 100% on.

The motor voltage in this era came through the key switch, terminal 15X. This terminal is live only in position 2 so that the blower is stopped during cranking. The PBU has nothing to do with actually supplying power to the motor.

So if a transistor shorts, it would be 100% on constantly without ability to control. So chances are that is not the problem.

What may be more likely is an intermittent loss of power to the PBU or the control line between the PBU and the regulator, both of which would force intermittent 100% blower output due to the failsafe. I had this happen on the 87 300TD. Fuse 7 had a weak spot and crumbled when wiggled.

I just went back and read the original post again. If you can change the speed even if the PBU is off, then you have a bad PBU that is throwing a reference signal when it shouldn’t. The way it turns the blower “off” is that it really puts out a very low reference signal, low enough that you can’t tell the blower is running, but not enough to throw it into emergency mode. Even in “off” the blower still runs at very low speed to ensure there is always fresh air coming in.

The W124 style control has 3 blower buttons, low, auto, and high. There is no “blower off” even if you push the 0 button as I described. The “low” and “high” modes send a fixed reference voltage to the regulator to create a low and high speed. Auto puts the PBU logic in control and the blower speed is modulated according to the temperature sensors.

Maybe what is happening is you are commanding “off” but the reference voltage is dropping low enough to throw the blower into emergency mode. If you select the “low” or “high” blower speed, the reference voltage climbs enough that it falls back into the regulation region and obeys your speed request.

bentrod · #8 10-10-2018, 10:14 PM

Wow what a write up!
Hopefully you can see a 201.126 blower diagram.
Here is a pic of the blower motor switch.
Just to be clear.. the "I" position is the low setting.
If the blower fan comes on with the push buttons in "O" / off, I can still use the fan switch to control the speed from low to high, all fan positions work.
The W201 uses the extra odd Tempmatic system, different from, I think, all other MB cars.

bentrod · #9 10-11-2018, 10:29 AM

Picture

Mxfrank · #10 10-11-2018, 11:12 AM

Nice write up. Except that the 201 doesn't use PWM blower control. It uses an old fashioned resistor bank, located behind the battery. So nice try, but irrelevant information. It's a non-model specific forum , so misunderstandings happen. Your question is obscure and model specific, don't be misled.

Power for the blower in an '87 190 is full time through the ignition switch. The two relays that are built into the switch (idiotic design, engineered like no other car) are grounded by the head unit and switch power to the blower circuits. One is turned on by switching to defrost, and it powers the blower at high speed. The other is turned on whenever the head unit is turned on, and powers the three speed function. The head unit operates the relays by simply switching the grounds on and off. Inside the head unit, I don't think the electronics are involved at all, but rather this is done by mechanical switching.

Two possibilities here:

1) some sort of problem inside the head unit, probably a failing switch, is allowing it to ground the relays
2) One of the relays is nearly bridged with carbon, and the switch needs to be cleaned or replaced.

If the CC system is otherwise working well, I'd permanently ground the relay that allows 1-2-3 operation. better yet, just bridge the stupid relay and be done with a totally useless complication. That would keep the blower available at all times, and would take the head unit out of the equation.

jay_bob · #11 10-11-2018, 11:18 AM

Thanks, I made a big assumption (wrongly) that they used the same controls in the 201 as they did in the 124/second gen 126 since they were introduced nearly at the same time.

bentrod · #12 10-11-2018, 01:05 PM

It's a turkey shoot what to do for me.
It already has a new set of push buttons ( still does not mean they are flawless )
And the system is working great!
What is the worst that could happen if I just leave it? I can live with it for now, if the fan comes on I just have it on low and its hardly noticeable.
Really appreciate all the replies.