no voltage to blower motor (300E) - PeachParts Mercedes-Benz Forum

WANT '71 280SEL · #1 07-31-2004, 08:52 AM

Is this a symptom of having a bad PBU? I tried resoldering about 75% of the connections on the back, but it still isn't working. I checked the voltage at the motor, (wires actually in the box), and my multimeter said 12 volts. My uncle said it's just enough to show it on the meter, but not turn the motor? I don't uderstand volts vs. amps. So does that mean it isn't getting any amps? The fact that the meter is telling me 12 makes me think at least that the fuses are good. I did check the fuses of course.
Thanks
David

#2 07-31-2004, 09:41 AM

If it has 12 volts AT the motor, the motor should turn. If the motor were burned up, it would draw enough current to blow the fuse.

If there is a open wire leading to a winding in the motor, or completely worn out brushes then you could have voltage there without the motor turning.

Will the motor turn by hand? If it does, then you likely have worn brushes in the motor. If it doesn't turn by hand and still shows 12 volts that would be strange.

For sake of describing volts and amps, compare these to pressure and volume of water through a hose. The voltage is much like the pressure applied to the hose. If there is no restriction at the point where you have your pressure gauge, then the pressure (voltage) will drop because there is no resistance to maintain pressure, but the volume (amps) will be very high because the water can flow freely. If, however, you have a very small hose attached where your gauge is placed, you will have lots of pressure (volts) because the water can't go anywhere, but you will have little or no volume (amps.)

So, using this analogy, you have 12 volts at the motor. That means that the controller and associated wiring are providing a circuit to the motor. Your meter is completing this circuit, but with a very high resistance to current flow, so voltage is maintained there. If you had a short at the motor, the 12 volts would drop and would allow enough current to blow the fuse. This indicates that there is an open between the point where you are measuring motor voltage and ground. That open is likely to be brushes.

Your uncle is partially correct. The circuit is supplying 12 volts at the motor, but might not be able to supply enough current (amps.) If, however, it could not supply enough current, then this would mean that there is a high resistance upstream and you would not have 12 volts. That is, if the motor had normal resistance and the circuit could not supply enough current, you would not have 12 volts at that point.

I hope this makes at least a little sense.

Good luck,

RPM55 · #3 07-31-2004, 10:45 AM

The blower motor is controled on the gound side of the circuit by the blower current regulator. The current regulator is mounted under the blower motor, it is controled by a varying voltage from the pushbutton panel.

stevebfl · #4 07-31-2004, 11:11 AM

Just for the archives. I would seriously advise not randomly soldering pushbutton controllers after model year 1986 (including 86). From personal experience I can tell you there are multiple electronic components after 85 controllers that don't like getting hot. If you trust your diagnostics and are sure its junk, finish it off, but the electronic controllers of 86 on cars are very seldom fixed with resoldering. I'm guessing your chances of killing a controller are greater than fixing one with random solder on the electronic controllers. There are no electronics in the PBC before 86, its just a switch board.

As RPM55 stated the circuit is controlled on the ground side and most of the pertinet circuitry is easily exposed on the three wire connector next to the brake booster. Go for the yellow wire to view all that matters from the PBC. It sends a 1v-6v signal. If not there deal with the PBC. If the voltage is there and the red wire has power, you are looking at a blower or blower controller problem.

Just as doctors, proper technicians should "do no harm".

WANT '71 280SEL · #5 08-01-2004, 12:19 AM

I know the blower motor is good. I took it out and applied 12 volts to it and it spun, same with my "new" used one. Neither of them would work when plugged in. The controller under the blower, what does it look like? Is it around the blower box? In it?
Thanks
David

psfred · #6 08-01-2004, 12:24 PM

Replace the strip fuse -- it's in a little box close to the fuse box.

The strip fuse (very much like the fuse for the glow plugs but lighter) cracks with age and use, so it will conduct enough to show voltage at no load, but not enough to actually deliver any current. Your VOM will show voltage, but if you hook up a test lamp, it will be dim.

Costs about $.50.

If that doesn't fix it, it's likely the motor controller in the AC box (to the rear of the blower) or the pushbutton control unit.

Peter

WANT '71 280SEL · #7 08-02-2004, 12:45 AM

Is there a way to test the motor controller? I'll check that fuse tomorrow when I get a chance.
Thanks
David

#8 08-02-2004, 11:48 AM

Well, the ground side control explains alot.

David, TAKE NOTE of what psfred said about the fuse. Testing it with an ohmmeter is not conclusive! It costs less than a buck and can be found at any MB dealer. Just replace it.

Good luck,

WANT '71 280SEL · #9 10-18-2004, 05:48 PM

Problem solved. The blower regulator made it work again. Now, the blower won't shut off. All the buttons on the PBU work, as does the temp. wheel, but when turned off, it blows on low. I think it's outside air being blown in when off. It's definitely the blower forcing air in, not just when moving down the road.
Thanks
David