Please, any help would be appreciated, I have wracked my brains and asked some smart people and each has had an idea or two but without results.

The problem.
Two years ago in the fall, the heater/AC would stop and restart all on its own. I ignored it and decided to look at it in the winter, well I never did and never bothered really last year as I hardly used the car, less than 500 miles all last year and this year to date.

Well, this year I decided to solve the problem, while I was just looking at the fan regulator the fan suddenly went up to top speed and was only stopped by pulling fuse #37. The fan starts as soon as the key is turned to the on position.It has got to be something simple, at the moment I can vary the heat and AC temperature but I cannot vary the fan output speed which is always at full speed.

I have tried another new cheap fan regulator (resistor), another used BCM and another used heater control module, all three parts at the same time, each individually and combinations of the three in conjunction with the original parts The BCM has the same part, software and hardware numbers on it, the only difference is the manufacturing date.

No configuration of components eliminates the constant 12-volt output at the centre pin to the regulator which should be variable from 2 to 7 volts approximately.

There are three pins to the regulator which should supply a ground, a constant 12 volt and a variable voltage at the centre pin.

The problem lies upstream on the centre pin somewhere, the trouble is the wires coming from the regulator and controller disappear behind the dash.

For the next step, I shall try to trace the centre pin wire and reinstall the original parts which will be a pain in the ar$e.

What made the system go from normal but intermittent operation to a constant full blast from the fan while the temperature selected still operates correctly? Could there be a shorted circuit somewhere or a faulty part I am not considering?

Knowing squat about electronics I am at a complete loss.

I take plenty of rest for my back so this could take some time.

Thanks for your time reading this post.

 

I think your problem lies in the blower motor resistor. I know you have changed it for a cheap one but I have gotten bad ones out of the box before. I have looked over the schematics on page 24-60 in the service manual and you have changed everything else but the motor and the wiring itself.
https://www.crossfireforum.org/forum...-resistor.html
This was an amazing write up that you turned into a PDF about 8 years ago it has the wire color and everything and the voltage you should have at them.
Good luck and keep us posted.

 

Thanks for the reply but the centre pin should give a voltage varying from 0 at position #0, approximately 2 volts at position #1, increasing incrementally to approximately 7 volts at position #5. Instead it gives a constant 12+ volts from position #0 to position #5 resulting in maximum revs from the fan. I get the same result from the original and the new regulator. The manual calls it a regulator not a resistor so I have followed the manuals name.

 

Where that center pin wire exits the A/C control module cut that wire a couple inches from the controller connector If it is bundled cut back that wrapper. Now measure the voltage at the wire. Does it change with the rotation of the ****?? Yes, run a new wire to the fan by soldering in a replacement. No, it doesn't change, but is 12v then the A/C control has a problem.

 

This has been suggested before and is a good logical step in solving the problem, it will be tried first.
Thanks for the reply.

 

After carefully testing voltages from the controllers, the original and the one from eBay, the original controller either puts out 12 volts or very low volts at the socket to the regulator.
I think in my tests of the various components in the various configurations possible I omitted to include the eBay controller. It produces the correct voltages at the various settings for the fan speed.
There are at least two parts that are faulty, the original controller that caused the intermittent fan operating, and the original regulator which may have failed when the fan started to run at full speed only when I was starting to look into the problem. The original controller puts out a constant 12 volts now, maybe when it suddenly went from working intermittently to 12 volts it blew the regulator, who knows. I think the regulator normally sees about 7.3 volts max.
The cheap regulator may have only been tested using the original controller or it was NFG when I got it some years ago.
The BCM I got seems to be fine and I will leave it in the car. I shall order a a Behr regulator which is the brand my original one is.
I will use the eBay controller and the Behr regulator when it comes as the regulator requires the antics of a gymnast to install.
I will report back.

 

Update
Not totally convinced my tests were thorough enough I did them again and logged each reading instead of relying on memory. That was a good decision on my part. I made special leads with pins that fitted the socket holes in the wiring harness socket that fits to the regulator, the alligator clips on the other end held the multimeter probes.
The BCM from eBay was no trouble, just a bit awkward to put in. The three controllers gave almost the identical voltage out puts at the regulator socket as the one tested and logged in Jeffery Alisons bench test on the Heater/AC components.
The two regulators were both faulty, the cheapo eBay one put out no voltage at all through the range from zero to five. The original one put out .06 volts from position zero to position #1 and 7.8 volts from there to position #5.
I am now waiting for the Behr regulator to arrive and hopefully solve my problem.
Those people who said the regulator was at fault were correct, top marks to you all.

 

Congrats. That's why I try to buy parts with the manufacturer Mercedes used originally. The Bosch crankshaft/camshaft sensors are a good example of why. The Behr regulator at Autohaus is less than half of what I paid 6 or 7 years ago.

 

I got the Behr/Hella regulator from German Parts Supply, it came to Canada from California via USPS in nine days, this was the earliest date given in their delivery window. The Pony Express is my favoured shipper as all the others are more expensive, less reliable and collect import duties on every little item.
Support your postal service.

The regulator is installed and everything is working again.

Hella part# 89192 (original part on the Crossfire, possibly an obsolete number)
Behr Hella part# 351321011
Mercedes part# 202 820 73 10

 

Could you please list the part number of the regulator you installed for the future.

 

Good point, I have added it in post #9.

 

Thank you!