Monday, August 22, 2011

A Blog Beg.....I need some technical advice...

Not computer stuff, but a question about how to test an electrical circuit in a car.

I drive a 1990 MB 300SE that has never really had strong AC. Then earlier this summer the AC stopped blowing altogether, except that I noticed I could feel cold air when driving faster than 40mph. I figured the blower motor had expired after 20 years of near continuous use.

So. I set out to repair the motor myself. There are 4 potential items that would cause the symptoms I noticed.

1. The blower motor is dead. It is basically an electrical motor hooked to a fan, so if the motor is blown it should be a plug-and-play replacement.

2. It could be the regulator (the fan speed controller). The regulator functions by diverting some (or none) of the electrical current from the battery to a heat sink. At low speed, the fan gets about 6v of electricity, with six volts being transmitted to the heat sink. At high speed, the fan gets 12v and the heat sink gets none. The heat sink is actually a funky-looking deal that the guys on the MB World forum call "the porcupine." This is the regulator, with the three prong side connected to the dash (between my thumb and pointer finger), the two prong side connected to the blower (back of my hand) and the larger connector (near my middle finger) would be connected to the heat sink. I removed the porcupine for ease of handling.

This is the picture of where the regulator is hooked to the car body and where it hits the connection that goes to the dash selector.

I need to know how to test the female receptacle in this picture and whether I'm doing it right...

3. It could be a the dash selector, although I think i this is the case, it is a subset of 4.

4. It could be faulty wiring. I eliminated a fuse as the problem. The AC fuse on this car is NOT in the fuse box, but I found it and it's still intact.

UPDATE - After closer inspection, I rechecked my work and after breaking out a magnifying glass, I found a hairline fracture on the 30 AMP strip fuse! BINGO!!! I will have a new strip fuse installed tomorrow and that should solve the entire problem...WOOHOO.

The toughest part of this was the variable symptoms:

It started with weak fan strength me think it was a problem with the blower motor.
Then right before it died completely, the fan worked PERFECTLY for almost 30 minutes during my commute one day....which caused me to consider delving into this adventure in the first place.
Then once the fan died completely, and I removed it and tested it, it was logical to work back up the wiring diagram (to the regulator and the dashboard selector)...when I should have been more thorough with the fuse check in the first place. Ah, well, live and learn.
On top of that, the "check the simple things first" is a lesson I should have learned from my DIY mentor, my father in law...who solved a more complex diagnosis problem with his '00 S430 in much the same way...try all the hard stuff first and then back up to the simple stuff (which, naturally, solved the problem).


So, as I set out to eliminate these problems, I removed the blower motor and disconnected it from the regulator. When I hooked it up to the battery with some small alligator clips, it spun quietly and produced a strong quantity of air. It's not the blower motor. That eliminates 1.

On to 2 and the reason for my blog beg (bleg). How do I test the regulator? Or more accurately, how do I test whether I can eliminate the regulator as the problem? Initially, I left the regulator in place hooked and hooked up a multimeter to the blue and red ends that connect directly to the fan. Then I turned the car on, turned the AC on and started selecting different fan speeds looking for varying voltages. Nothing. No matter what I did, there was no voltage.

But, to make sure this wasn't a false negative, I ran the wiring back to where the regulator connects to the dash selector. Actually, I discovered a plug-and-play socket that made testing easy (or so I thought). And here's my bleg: How do I test the female side of the connector in the second picture. The "male" receptacle is connected to the regulator and is not powered in this picture. BUT, the female receptacle is supposed to be powered. It is still connected to the dash selector and (theoretically) the battery. So, I put the multimeter prongs in the 3 female sockets and repeated the AC fan speed drill. NOTHING. No voltage. No matter what I did.

So, I'm stuck. I've eliminated the fan, but I can't eliminate the regulator, nor can I eliminate the dash selector. My question is, how do I know I've tested the female receptacle properly? I can't tell why there are three live sockets? Is one a ground? Which one is it? If one is a ground, why is there no ground hooked to the blower motor?

Wednesday, August 03, 2011


I just had the greatest three-day weekend in a LONG, LONG time.

Knocked out an absolutely great hearing/mediation. Hooked up with family I hadn't seen in two years. Reconnected with the lawyer who gave me my first break in the legal world and his delightful family. Thanks Mark and Jeannette! Ate lunch and made up for lost time with a law school classmate. And spent two days tripping around one of the most beautiful (if most dysfunctional) cities in the world. AND. I found the mysterious "lost staircase"...which, to one of the coolest hidden gems of the City by the Bay.
Here are a few shots I took along the way.

Geli - this is the mysterious staircase I tried to get you guys to find when you were out in SF...but, it's *really* well hidden...and unless you know it's there, it's very easy to miss. I'll leave it at that.