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Old 04-25-2019, 04:57 PM   #1
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Help! Electrical Problem, Bathroom Exhaust Fan

While getting the trailer ready for summer, I found that the Ventline exhaust fan in the ceiling of the bathroom was not working. I put a voltmeter across the terminals on the motor, switched the rotary switch off and on, and got no readings. I figured the switch was bad and ordered a new one.

After installing the new push button replacement switch, the fan still is not working. This time, I unhooked the wires from the terminals and measured 12 volts on the two wires. I then ordered a new motor figuring it was the culprit. Before installing, I hooked the new motor to the circuit for the fan. It did not work! I then measured the voltage between the two wires. It was 12 volts and I could turn the electricity on/off with the new switch.

Now, I am quite perplexed. I have voltage, but neither the old nor new motor will work attached to the fan circuit, so I tested the motors. When hooked to a separate 12-volt power source (one of the cigarette lighter outlets), both of the motors worked!

I have checked and replaced the crimp terminations for the fan wires to the main wireharness (figuring I might have a high resistance connection). I still measure 12 volts between the wires, but the motors do not run on the fan circuit.

One interesting observation. When I measure across the terminals of the motors when hooked up to the fan circuit, I get zero volts. When I measure across the motor terminals when hooked to the separate power source, I get 12 volts.

Help! I am pretty good with electrical systems, but this issue is definitely beyond my understanding/experience.

Thanks.
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Old 04-25-2019, 05:40 PM   #2
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Volts don't make the motor run, amperage (power) does. You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor. Check the fuse on the circuit and if you have an ammeter put a resistive load on the circuit and see how many amps you are drawing.


Just a thought,
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Old 04-25-2019, 08:01 PM   #3
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Pat,

Thanks for your input. I have checked all of the circuit breakers. Could not find any issues there. I do not think there would be any inline fuses in this part of the body wire-harness (but who knows...).

I ordered a new meter yesterday with more capabilities. I will do some of the tests you are suggesting this weekend.

Thanks,
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Old 04-25-2019, 08:13 PM   #4
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Quote:
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I have checked all of the circuit breakers. Could not find any issues there. I do not think there would be any inline fuses in this part of the body wire-harness.
Check the wiring diagram for your unit. There should be a DC fuse block somewhere.
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Old 04-25-2019, 08:27 PM   #5
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Try using a known common/ground connection on one side of the motor.
In other words put a jumper on one of the motor leads that is connected to the skin/chassis.
Then connect the switch to the other lead on the motor.
If the motor runs. You have a bad common/ground connection.
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Old 04-26-2019, 11:41 AM   #6
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Pat - I checked the wiring diagram. There are no extra fuses shown in this circuit. It goes back to the DC circuit breaker box in the front.

TG - I tried the common ground. The motor did not run.

At this point, I am waiting for the new meter to arrive this afternoon. Let me know if you guys think of anything else.

Thanks,
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Old 04-26-2019, 12:20 PM   #7
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Quote:
Originally Posted by DaveP View Post
Pat - I checked the wiring diagram. There are no extra fuses shown in this circuit. It goes back to the DC circuit breaker box in the front.

TG - I tried the common ground. The motor did not run.

At this point, I am waiting for the new meter to arrive this afternoon. Let me know if you guys think of anything else.

Thanks,
You may have a fuse which cooked itself. You will read voltage on a meter because of the high input impedance.
Whereas the motor will cause a voltage drop across the carbon track of the cooked fuse.
Are these the modern blade fuses or the old glass units?
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Old 04-26-2019, 01:22 PM   #8
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Hi, I had the same problem when I first purchased our 25" RB. first check to see that the other devices further down on that circuit work ie. reading lights USB ports ( those are on that circuits line for my RB 25). You can check whats listed on that circuit on the breaker box or go to the Airstream wiring diagram for your Trailer.
We had a wiring harness connector loosen up that serviced that circuit. It was located behind our Microwave,it was under warranty and the tech only had to reconnect firmly and reinforce the connector with tape... Fixed

Best of luck hope this helps
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Old 04-26-2019, 08:07 PM   #9
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The system in my Classic uses circuit breakers, not fuses (although I have found a few inline fuses in different circuits. Everything around the fan seems to be working, so no easy clues...

Quote:
Originally Posted by TG Twinkie View Post
You may have a fuse which cooked itself. You will read voltage on a meter because of the high input impedance.
Whereas the motor will cause a voltage drop across the carbon track of the cooked fuse.
Are these the modern blade fuses or the old glass units?
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Old 04-26-2019, 09:21 PM   #10
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Originally Posted by pdavitt View Post
Volts don't make the motor run, amperage (power) does. You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor. Check the fuse on the circuit and if you have an ammeter put a resistive load on the circuit and see how many amps you are drawing.


Just a thought,
Pat
"Volts don't make the motor run, amperage (power) does." - Partially true. The circuit voltage and the motor resistance determine the current. The motor will not draw the right current unless the proper voltage is present.

"You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor." - Partially true. Only when measured open circuit. If the circuit can only supply a few milliamps the voltage at the fan with the switch on will be only a few millivolts.

Depending on the type of fan you have there are things in the circuit other than the on/off and speed switches that can cause the fan not to run:

1. If it is motorized (to open/close) there is a limit switch that will not let the fan turn on unless the lid is open a certain amount.

2. If it is auto-close, there is a moisture sensor that runs off fan power and closes the lid. IF it is shorted the lid will not open and the fan will not run.

3. Some fans have a thermostat. If the inside temperature is lower than the thermostat setting, the fan will not run.

To get to the problem:
Measure the voltage between each motor terminal and a known good ground with the switch on.

If the plus side of the motor reads close to 0V, then you have a bad 12 V feed. Go backwards through the wiring until you find 12V. When you do you have just jumped over the bad part.

If the voltage on the plus side reads 12V and the voltage on the ground side of the motor reads close to 12V you have a bad ground connection somewhere between the motor and ground. Some of the components mentioned above may be in the ground side of the motor. Go backwards through the circuit until you find 0V. When you do you have just jumped over the bad component.


Al
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Old 04-26-2019, 10:02 PM   #11
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Oops

Quote:
Originally Posted by Al and Missy View Post
"Volts don't make the motor run, amperage (power) does." - Partially true. The circuit voltage and the motor resistance determine the current. The motor will not draw the right current unless the proper voltage is present.

"You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor." - Partially true. Only when measured open circuit. If the circuit can only supply a few milliamps the voltage at the fan with the switch on will be only a few millivolts.

Depending on the type of fan you have there are things in the circuit other than the on/off and speed switches that can cause the fan not to run:

1. If it is motorized (to open/close) there is a limit switch that will not let the fan turn on unless the lid is open a certain amount.

2. If it is auto-close, there is a moisture sensor that runs off fan power and closes the lid. IF it is shorted the lid will not open and the fan will not run.

3. Some fans have a thermostat. If the inside temperature is lower than the thermostat setting, the fan will not run.

To get to the problem:
Measure the voltage between each motor terminal and a known good ground with the switch on.

If the plus side of the motor reads close to 0V, then you have a bad 12 V feed. Go backwards through the wiring until you find 12V. When you do you have just jumped over the bad part.

If the voltage on the plus side reads 12V and the voltage on the ground side of the motor reads close to 12V you have a bad ground connection somewhere between the motor and ground. Some of the components mentioned above may be in the ground side of the motor. Go backwards through the circuit until you find 0V. When you do you have just jumped over the bad component.


Al
I misread the OP's post. For some reason I thought he was talking about a cabin vent fan. Below I have deleted the part about the other components.

I should have said:

"Volts don't make the motor run, amperage (power) does." - Partially true. The circuit voltage and the motor resistance determine the current. The motor will not draw the right current unless the proper voltage is present.

"You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor." - Partially true. Only when measured open circuit. If the circuit can only supply a few milliamps the voltage at the motor with the switch on will be only a few millivolts.


To get to the problem:
Measure the voltage between each motor terminal and a known good ground with the switch on.

If the plus side of the motor reads close to 0V, then you have a bad 12 V feed. Go backwards through the wiring until you find 12V. When you do you have just jumped over the bad part.

If the voltage on the plus side reads 12V and the voltage on the ground side of the motor reads close to 12V you have a bad ground connection somewhere between the motor and ground. Some of the components mentioned above may be in the ground side of the motor. Go backwards through the circuit until you find 0V. When you do you have just jumped over the bad component.


Al
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Old 04-27-2019, 05:37 PM   #12
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Quote:
Originally Posted by Al and Missy View Post
I misread the OP's post. For some reason I thought he was talking about a cabin vent fan. Below I have deleted the part about the other components.

I should have said:

"Volts don't make the motor run, amperage (power) does." - Partially true. The circuit voltage and the motor resistance determine the current. The motor will not draw the right current unless the proper voltage is present.

"You can have a voltage measurement of 12V with a circuit that can only provide a few mA of power. Not enough to run your motor." - Partially true. Only when measured open circuit. If the circuit can only supply a few milliamps the voltage at the motor with the switch on will be only a few millivolts.


To get to the problem:
Measure the voltage between each motor terminal and a known good ground with the switch on.

If the plus side of the motor reads close to 0V, then you have a bad 12 V feed. Go backwards through the wiring until you find 12V. When you do you have just jumped over the bad part.

If the voltage on the plus side reads 12V and the voltage on the ground side of the motor reads close to 12V you have a bad ground connection somewhere between the motor and ground. Some of the components mentioned above may be in the ground side of the motor. Go backwards through the circuit until you find 0V. When you do you have just jumped over the bad component.


Al
In my post I was trying to not get too technical with the OP. The below is not directed at ya'll but more to the OP.

---
Lets say the normal fan circuit with the fan running would show 12V and 5A. Which according to Ohm's Law indicates a circuit with 2.4 Ohms of resistance. In this case the Voltage still reads 12V, but the fan won't run. Assume that the fan requires a minimum of 2 Amps to start.

Back to Ohm's Law:
Assume the circuit is only producing 1A of current, so the fan won't run.
At 1A, with a volt meter reading of 12V on the circuit the resistance has gone up to 12 Ohms. Why?

DC fuses and some DC circuit breakers can blow/trip and still allow a small amount of current through but, because of high resistance, the voltage remains near nominal. This can also be caused by faulty wiring, less than a "short circuit", that adds a high resistance to the circuit. Or as you stated, it could be another device on the circuit that is raising the resistance.
---
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Old 04-27-2019, 09:21 PM   #13
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Thanks for all of the advise and suggestions.

I measured the amperage draw of the motor. It is only 0.15 milliamps, so it is definitely a weak circuit. Still shows 12 volts.

I have been digging around for where the wire from the fan goes, but not much success as it is in the ceiling behind the interior aluminum. I found where the wire bundle from the ceiling harness comes out of the wall, but have not found where the "Packard" connectors meet and I don't know which wire color I am chasing.

I am charging my bullet video camera so I can try tracking some of this in the ceiling/wall tomorrow.

BTW, the wire color in my circuit drawings do not match the colors in am seeing coming off of the fan. I think they are just trying to see if I will start screaming...
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Old 04-27-2019, 11:07 PM   #14
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Quote:
Originally Posted by pdavitt View Post
In my post I was trying to not get too technical with the OP. The below is not directed at ya'll but more to the OP.

---
Lets say the normal fan circuit with the fan running would show 12V and 5A. Which according to Ohm's Law indicates a circuit with 2.4 Ohms of resistance. In this case the Voltage still reads 12V, but the fan won't run. Assume that the fan requires a minimum of 2 Amps to start.

Back to Ohm's Law:
Assume the circuit is only producing 1A of current, so the fan won't run.
At 1A, with a volt meter reading of 12V on the circuit the resistance has gone up to 12 Ohms. Why?

DC fuses and some DC circuit breakers can blow/trip and still allow a small amount of current through but, because of high resistance, the voltage remains near nominal. This can also be caused by faulty wiring, less than a "short circuit", that adds a high resistance to the circuit. Or as you stated, it could be another device on the circuit that is raising the resistance.
---
Pat
Unless I misunderstand you, which is possible, if the OP is measuring 12V at the motor there cannot be increased resistance on the 12V feed. Resistance in a fuse or the switch will result in a voltage at the motor of (using your resistance numbers) of 2.4/12.4 * 12, or 2.32 volts. The problem must be in the ground line.

The key is to measure the voltage at the motor + and - leads with respect to real ground (battery negative). Low voltage on the + means a problem in the line feeding the + terminal. 12V on the - means a problem on the line feeding the -. If OP is measuring 12V across the motor itself, then the motor is bad or there is a bad connection between the measurement point and the guts of the motor. It is not possible for the motor to have 12V applied and not receive enough current to operate.

The only thing I can think of that might exhibit the described symptoms (assuming OP is measuring 12V on a known good motor and it is not running is that it has a diode in it and he is connecting it backwards, i.e. + wire to - terminal and - wire to + terminal.

Al
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Old 04-27-2019, 11:57 PM   #15
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Sometimes the devil is in the details.

To my point, consider the figure:

Click image for larger version

Name:	Fan Circuit.png
Views:	26
Size:	15.5 KB
ID:	339263

V1+V2= 12 volts. The only way that V2 can be 12 volts is if V1 = 0. The only ways that V1 can be 0 are 1. I=0, but if I=0 then V2 = 0 and we know it isn't, and 2. R must be zero. In the case of finite, non-zero R, 12 volts divides across the two resistances in the proportion of 2.4/(2.4+R).

These simplistic analyses (mine and others) assume the "resistance" (actually the effective resistance) of a DC motor is constant. It is not. It is very low when not turning and rises with speed. I'll not provide the technical explanation for this for the sake of simplicity. For the curious Google users the term is Back EMF.

Al
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Old 04-28-2019, 08:16 AM   #16
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Hi

Ok, let's get the motor out of the way.

Grab a 12V lightbulb plus socket. You want a real bulb, not one of the LED gizmos. Something in the 10 to 20W range would be ideal. Socket needs wire leads on it.

Put the bulb in where the motor goes. Does it light? Probably not.

Try one side of the bulb to a known good ground (even if that is far away). Does it light? If so you have a bad ground. If not ...

Try the bulb at the breaker. If it still does not light, you have a bad breaker. If it does light, there is a loose connection in the wire going to the motor. You now work your way from the breaker to the motor checking each connection along the way.

The idea is that the bulb pulls an "real" load current and you can spot marginal connections with it.

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Old 04-28-2019, 09:31 PM   #17
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I decided I could either spend the whole day trying to track down the fault in the circuit or I could fix the fan. I decided to fix the fan.

I knew where to find a 12-gauge positive wire in the wall between the bedroom and the shower (the lighter outlet circuit behind the TV). I tapped into that wire, pulled a wire up through the wall and into the ceiling right at the fan. The ground wire was good (and already there). The fan works just fine now. I taped off the old wire and may it rest in peace.

Thanks for everyone’s help.
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Old 04-29-2019, 02:50 AM   #18
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Congratulations. Lucky that power line was there.

What I draw from this fix is that you did not have 12V under load at the fan.

Airstream uses several harnesses that join with connectors. There must be a corroded connector or bad splice somewhere in between the inner and outer skins. You probably couldn't get access to it to fix it anyway.

Al
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Old 05-05-2019, 10:12 PM   #19
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Emf

Electromotive Force. As the motor turns it will developed a induced pushback voltage on the hot side that in turn limits current. The fan motor becomes a generator. When started current is always the highest. Why when you turn off the bath fan as it spins down the led ceiling light will flicker. That fan is back powering the circuits.

I hope the OP has taked the motor out and tested it on a good battery. Hooking hot to hot and completeing a ground. That takes out all the wiring and resistance problems.

Most likely it is a voltage drop problem on the hot side. If it would light a simple 12 volt test light with the motor removed from the circuit. There is a problem in that circuit. Plug? Connector? Ground? Fuse ot Circuit Breaker? Green internal corrosion in the Hot wire. Easy to say hard to find.






Quote:
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Sometimes the devil is in the details.

To my point, consider the figure:

Attachment 339263

V1+V2= 12 volts. The only way that V2 can be 12 volts is if V1 = 0. The only ways that V1 can be 0 are 1. I=0, but if I=0 then V2 = 0 and we know it isn't, and 2. R must be zero. In the case of finite, non-zero R, 12 volts divides across the two resistances in the proportion of 2.4/(2.4+R).

These simplistic analyses (mine and others) assume the "resistance" (actually the effective resistance) of a DC motor is constant. It is not. It is very low when not turning and rises with speed. I'll not provide the technical explanation for this for the sake of simplicity. For the curious Google users the term is Back EMF.

Al
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