Defrost Blower Electrical Problem

Tikvah · October 09, 2014, 05:20:22 PM

Started out today, after sitting in a campground for a number of months, and I found my defroster blower isn't turning on. Not sure where to start. I have the little three speed box under the driver window. The left stud has power, but none of the others. I thought I'd put a jumper on one of the other studs but I get a short.

Thoughts?

eagle19952 · October 09, 2014, 06:21:29 PM

can you see feel turn the fan motor shaft...

Tikvah · October 09, 2014, 07:00:32 PM

Not really sure where the blowers is. I'll have to investigate in the daylight.

eagle19952 · October 09, 2014, 07:51:39 PM

if the shaft won't free spin that would be your first fix

gumpy · October 09, 2014, 09:29:37 PM

Open the panel in the center of the dash. There's your blowers.

bevans6 · October 10, 2014, 03:51:50 AM

If it's wired like my MC-5C it's complicated. There are two blower motors and they are always both on if the blowers are switch on either low or high. They are in series on low, getting half voltage each, and in parallel on high with both getting full voltage. If you have voltage at the terminal strip at all with the switch on, the drivers blower relay is probably good. With the switch on high you should have 24v power on the two outside terminals of that three terminal strip and ground on the center terminal. On low you should have 24V on one of the outside terminal and 12V on the other two. This is all controlled by the driver's heater switch. With the switch at off you should have nothing at all on two of the terminals, and one of the outside terminals will be connected to ground through one of the blower motors. Like I said, complicated.

Edit: This means that both blowers are always running whenever the driver's heater or the driver's AC is on. That means about all the time in service. I had to change the brushes in both of my blowers before they came back to life. A local electrical shop had the brushes.

Brian

Tikvah · October 10, 2014, 04:15:18 AM

Thanks for the electrical details. I'll do some voltage checking today. I'm not sure of the location of the blower(s). Seems behind that front panel is just the heat/AC coil. There are hot water lines and pressure lines on the left. I'm just going by memory. I have a gas heater attached to my front panel, so I can't open it quick. Could the blower(s) be in the spare tire bay? I don't remember seeing it there either.

bevans6 · October 10, 2014, 04:28:18 AM

The blowers will be in there. The AC coil is on the right behind the grill, the heater core and the blowers are kind of right behind your heater unit. The MC9 schematics are available here http://www.busconversion101.com/mc9_maint_manual.htm electrical is section 7, the driver's heater blower part is on page 46, kind of upper middle on the page. This is kind of working on the theory that MCI didn't change anything much in their bus electrical system until they started to go to bus-controlled computer stuff, so your bus might be like an MC-9 which is about identical to my MC-5C.

Edit: if you have a DVM with an ohm-meter, try this. With the switch off you should get some resistance to ground through the motor on one of the terminals, and no resistance to ground on the other two terminals. You should get about the same resistance as you got to ground on the one terminal between the other two terminals. That's because one motor has a direct connection to ground, while the other motor is directly connected to two of the terminals. That's how they do the series/parallel switching. I have a motor, I'll go see what the resistance might be in the range of, and if this idea will even work...

Yep, it worked. Kind of fun to see theory translate into practice! On my bus the terminal strip is behind the panel. One motor is fully inside the heater blower box, one has the motor section sticking out. Yours may well be different on a newer model of bus. On my terminal strip there are three terminals. One outside terminal has a red wire running to the motor, that one reads a few ohms directly to ground. The other two terminals have black wires running to the motor, they read infinity to ground and a few ohms between each other, exactly as I predicted. The resistance was in the 3 - 8 ohm range, and it depends a bit on where the brushes are on the commutator. If the brushes are bad, expect a high resistance or an open. I can take a picture if you like.

Brian

Tikvah · October 12, 2014, 07:59:02 AM

Brian, I don't have an ohm meter and I'm probably not smart enough to use one if I did. However I have a voltage meter. Wouldn't it stand to reason, if my blower(s) won't spin that something would humm? Or maybe overheat a wire?

I think I might tear into that from panel this week while we are parked in this campground. See if those blowers are really back there. But, sure would be nice to find something simple first like a blown fuse, or loose wire.

I've attached a pic of my three speed blower control below the driver side window.

86102A3 · October 12, 2014, 08:21:55 AM

None of the motor speeds work? Usually you will lose one of the speeds, not all of them. I would guess that they use multiple windings in the motor to get the different speeds, but would have to see the wiring diagram to see how that speed controller works.

86102A3 · October 12, 2014, 08:58:28 AM

I would leave the power wire on the controller and if you can tell the wires going to the motor, remove them from the controller. Then turn on the controller and see if you are getting power on the terminals to the motor. This will tell you if the controller is working. If it is working, then I would say you have a motor problem. A good way to troubleshoot 12 and 24 volt systems is to use voltage drop as opposed to trying to measure ohm values. What you would do is put one meter lead on the wire you identified as the hot coming in and put your other lead on the wires going to the motor. If you read close to zero that is good, there is very little reisistance, if you read a number close to 24volts that means you have a very high resistance meaning that it's shorted. This is very effective for finding loose connections and bad grounds.

bevans6 · October 12, 2014, 09:10:28 AM

If you have three speeds, then your setup is different than mine - I guess they did an upgrade. The most common failure is a stuck or plain worn out brush not making contact with the commutator of the motor. No smoke, no hum, just no spin...

Do you have a manual for your bus with a schematic?

Brian

Ed Hackenbruch · October 12, 2014, 09:32:08 AM

Had to replace the brushes in mine a few years ago, bet that is all you need to do.

luvrbus · October 12, 2014, 11:33:02 AM

There is a relay that controls the defroster and drivers heat have you checked it the rheostat is good or it would run 1 speed if bad

Tikvah · October 12, 2014, 07:53:54 PM

I have a manual for a MC9. I also have the schematic in the electrical door. Not sure where the relay would be. Rheostat?

Yes, three speed, worked last time out, this time - nothing.