MCI 8 Alternator field test???

[DEMOMAN]

Hello All,

     I am having a charging problem on my 1978 MCI 8.  The alternator stopped working so I've gone to the manual and then a MCI Service Info guide on the net: http://www.mcicoach.com/service-support/serinfo/serinfo07K.htm  This has proven to be very helpful in diagnosing the problem. 

     The troubleshooting process has led me to low resistance between the field and ground, (1.6 ohms when it should be 3-3.3 ohms), which says to replace the alternator in the troubleshooting guide, or the MCI Maintenance Manual says to replace the field.  There are two different options here.

     How difficult is it to replace the field?  Is the field repairable? Dare I take the whole alternator out?

I would appreciate any input on this.

Thank you

Eric

Southeastern WI

[Len Silva]

I wouldn't jump to any conclusions just yet.  Most typical multi meters are not very accurate in those low resistance ranges.  Not saying you are wrong but I would like to know what meter you are using, and if it has a low resistance range.

[luvrbus]

LOL I am from the old school if the field terminal has power to it you take a screw driver and if you can feel the magnetic pull on the screw driver the field is good one should hear the 50D engage fwiw

[DEMOMAN]

Len, I tried two multimeters, the first one was a Fluke digital VOM with an auto adjusting reading range, and the second was a Craftsman digital VOM with several Ohm range settings.  Both were reading within .1 of each other.  I also checked the resistance on a working MCI 9 with the Fluke and it was in the correct ohm range.

luvrbus, I will try the screwdriver test but I don't know when to listen for the alternator to engage.  Is that sound when the "charging" cycle is on because mine isn't charging?

[DEMOMAN]

luvrbus, screwdriver test successful.  Power to field and the field stud had magnetic pull when bus master switch is on.  I am truly confused why the bus would not be charging if the field is good.  

All,

    I should've given more information on what I have done and seen prior to going to the troubleshooting guide.  I replaced the voltage regulator when I first had this charging issue because mine was not sending any voltage to the field.  The replacement one now sends voltage to the alternator.  This did not solve the issue.  I started up the bus and noticed the alternator was lightly smoking.  (Digital Heat gun said 165 degrees F)  I don't know if it was the oil on the outside heating up, the alternator burning up from the inside, or what.  I shut the bus down and checked the engine oil level because I know the alternator is oil cooled.  The level was low so I added some to top it off.  I the re-started the bus and the alternator seemed cooler.  (110 deg F)  Still nothing charging.

That brings us back to my first post today.

Hope that helps??

Eric

[pvcces]

DEMOMAN, I think that the field should be within specs, and if is not, it might contain some shorted turns in it's winding. That would make the field run hot WHEN the alternator is producing power, even you can't see any output.

If you had a shorted diode, it might bog down the output enough so that you wouldn't see any net output. However, I have only seen diodes fail open. When that happens, the result is low output.

DN50 alternators are supposed to be field replaceable without removing the alternator. I have not done this operation, so I don't know how it compares to removing the whole alternator from the bus.

What's bothering me is that the alternator should not be starting up if there is no output and you did not say if you were revving it up to get it to turn on.

One other thing: your reading of the field resistance happens to match the resistance for a 12 volt generator. I could see where it would cause problems if you are running a 12 volt winding.

Good luck troubleshooting your charging system.

HTH

Tom Caffrey

[DEMOMAN]

Tom,

     All of my tests have been done at idle with the bus fully aired up or with the bus not running with the master switch on.  I have not been revving up the engine on any of my tests.  I guess I didn't know that I had to do this?  Neither the Maintenance Manual or the troubleshooting guide told me I would have to do that.  Does speeding up the idle "start up" the alternator? 

Eric

[bevans6]

Random thoughts on alternators...   The field tap is an input to the alternator from the regulator, to turn on the alternator.  It varies between zero and full field voltage at high frequency to control the output current of the alternator.  If the voltage regulator senses that the alternator voltage output is lower than it's set point, it turns on the alternator by setting the field voltage to full voltage.  If the alternator has a bad diode, or is otherwise not putting out a voltage higher than the set point of the voltage regulator, the field voltage will be stuck on full voltage (usually battery voltage, whatever that is).  That means that the field winding will be getting a constant full voltage, which may or may not be good for it, and might explain the high temps you are seeing, although you can expect the alternator to be up around the oil temp anyway.  If the field coil is partially shorted, it might have a lower resistance and be pulling higher current than normal, up around 15 amps if the field resistance is 1.6 ohms, which might be harming the voltage regulator that has to supply that current.

Anyway, while it may be the field coil is what's actually bad, I think it's the just as likely the stator or the output diodes.  If it was me, the alternator would be on my bench for some serious testing.

Brian

[luvrbus]

Check it this way with the bus running
If stud 49 reads 24 v and stud 48 reads 0 then the regulator is bad
If stud 49 reads 24 and stud 48 reads 24 volts then the alternator is bad
If studs 49 reads 0 then the relay is bad

good luck

[DEMOMAN]

I have 26 volts on stud 49 and 24 volts on stud 48 with the bus running.  Troubleshooting guide points toward field.  I tested field resistance and it is low according to spec in Maintenance manual and Troubleshooting guide.  I'm guessing the field needs to be replaced. 

     I found two new fields on Ebay, one is a Delco http://www.ebay.com/itm/Delco-Remy-50-DN-Alternator-Field-Terminal-Assembly-Part-10471614-/300729769209?pt=Motors_Car_Truck_Parts_Accessories&hash=item4604e41cf9&vxp=mtr , and the other is a Kirk's http://www.ebay.com/itm/Kirks-10471614-50DN-Alternator-Field-Coil-24-Volt-/160384530224?pt=Motors_Car_Truck_Parts_Accessories&hash=item2557a9b730&vxp=mtr .  I prefer OEM over aftermarket but I heard some bad reviews on the Delco Replacements. Suggestions?

Eric 

[Zeroclearance]

Erik;

Spend the money on Genuine Delco parts.   $89 is a bargin..

[RJ]

Eric -

Is your alternator belt or gear driven?

Most MC-8s had their alternators belt driven, and, as such, had a circuit that wouldn't turn on the alternator (& HVAC) until the air pressure was over about 65-80 psi - sufficient to tighten the air-powered tensioners.

IIRC, this switching circuit is buried electrically in the fast idle circuit.

FWIW & HTH. . .

[DEMOMAN]

RJ,

     My alternator is gear driven.  The air is well over 100 psi when I have done the testing.

Eric

[luvrbus]

Eric, if gear driven remove it to do the work check the bearings and drive you will glad you did

good luck

[bevans6]

If you remove the alternator, do you have to go through the whole drive alignment process or can you just bolt the alternator back on?  Are the bearing you suggest checking the internal bearings inside the alternator?

Brian