MCI MC5B Generator Wiring Diagram - "Not Gen" Light On

Michael Mc · May 17, 2009, 06:12:28 AM

Well, I finally did it. I have entered the fun world of conversion bus ownership. This is the first bus/rv we have owned. We have enjoyed camping in our travel trailer, but I am excited to be a bus owner and am looking forward to hitting the road.

We purchased a very nicely converted 1972 MCI MC5B for a really good price. Everything is good except the "Not Gen" light is on. I have traced down the wiring as much as I can without a diagram, but a diagram would help.

I have no power at the voltage regulator. I have power coming from the generator to the rear junction box. I have power on terminal 3 on the field relay, but not on any others. I'm mostly confused about the pressure switch and how it works in conjunction with the field relay. I can trace a wire back from terminal 4 on the field relay to the pressure switch, so I know they work together, but without a wiring diagram, I'm stuck.

I'm not discouraged by this. I knew it when I bought the bus. But, I got such a good deal on the bus, this is worth it. I just need a little help to get past it, and we will be on the road.

JackConrad · May 17, 2009, 06:24:25 AM

First, Welcome to the madness!!. You really need to get your hand on a service manual. The "pressure switch" is to prevent the alternator from charging until you have air pressure (alternator belts are tensioned by an air operated tensioner). Jack

Michael Mc · May 17, 2009, 07:16:47 AM

Thanks for the welcome Jack. I think this is going to be fun!

I have a Maintenance Manual for a MC5A. It has interior and exterior wiring diagrams, but not the generator circuit. Do you have any idea where I can order a service manual? (This is all totally new to me)

By the way, I did not mention that the bus has been re powered with a dd 6v92 engine and an automatic transmission. The alternator is on the back of the engine now (belt driven), but the air operated tensioner still operates to tighten the cooling blower belt.

I know at least one set of contacts in the pressure switch is working because the "low air" tel-tale will stop flashing. I really feel like the field relay is bad, but I would like to prove it before I start swapping parts.

Thanks again for the reply.

junkman42 · May 17, 2009, 07:48:40 AM

Hi Michael, welcome to the bus world. Just curious if You have checked across the battery to see what the voltage is. It is possible that the unit is charging. You never know when the unit has been modified. Good luck with the problem. Regards ,John

Michael Mc · May 17, 2009, 07:57:01 AM

Junkman42

What voltage should I expect to get at the battery terminals if the unit is charging?

As I was trying to work thru the problem, I had to charge the batteries with a charger (after starting the bus several times). This is what led me to believe that it is really not charging.

I thought that I would have voltage on the voltage regulator if it was charging. Is this true?

Thanks for the help.

JackConrad · May 17, 2009, 08:43:26 AM

Quote from: Michael Mc on May 17, 2009, 07:16:47 AM
Thanks for the welcome Jack. I think this is going to be fun!

By the way, I did not mention that the bus has been re powered with a dd 6v92 engine and an automatic transmission. The alternator is on the back of the engine now (belt driven), but the air operated tensioner still operates to tighten the cooling blower belt.

If you look up in there, I think you might find another air cylinder, similar to the one on the blower drive bely. Manuals are available from www.coachinfo.com and sometmes on Ebay. If you get your hands on a service manual, you will find many pages of wiring diagrams. The one you want is Starting & Charging Circuits. Jack

Michael Mc · May 17, 2009, 09:00:52 AM

Thanks for the link Jack. I saw that they had a Maintenance manual, but no service manual. The maintenance manual that was in the bus when I bought it is for a MC5A, but it is probably similar. It doesn't have the diagram I need. I'll keep looking.

Thanks again,
Mike

junkman42 · May 17, 2009, 09:42:36 AM

Michael, depending on temerature and state of charge of the batteries 13.75 for 12 volt system and 27.5 for a 24 volt system. If You have the orignal charging system Your life is a little easier as the bus prints should help. When turning on the master switch with the engine running and the low air light out You should have voltage applied to the regulator. You can jump from the battery to the regulator bat terminal to test to see if the charge system is operational. You can also jump to the alternator field terminal which should cause max charge. If you do not have battery voltage at the regulator the system will not function. If you do full field the alternator do it only for a short time as in 10 to 20 seconds. The relay that controls the charge circuit on My mc7 is located on the street side in the box to the left when you look in the engine access door. If You check to see if it is energized that will tell You alot about where to start. If it is energized You should have voltage on the bat terminal of the regulator. Hope I have not caused total confusion. Regards John

Michael Mc · May 17, 2009, 09:52:44 AM

Thanks John. That will give me some things to look at. I do know that the relay has power on terminal 3 but I have nothing on any of the other terminals when I check between ground and the terminals. Is there any way that I can jump around the relay to see what I get? I don't really know the internal wiring of the relay, (not sure which terminals are the coil and which are the switch legs) so I don't want to try the jumper and short something out. I do know that at least one of the wires coming from the relay (on terminal 4) goes to the pressure switch. I would think that is one side of the coil. Any ideas?

Michael Mc · May 17, 2009, 12:06:26 PM

I found the problem!! I dug around in the bus and found a maintenance manual for a MC5C and it had the wiring diagram I was looking for.

Luckily it was a broken wire! I put a new end on, re terminated the wire and she started generating. It's amazing how much help a drawing can be.

At least now, I know a lot more than I did about the wiring on the bus.

Thanks again guys for the help. Maybe I will be helpful to you some day in return.

Thanks again,
Mike

junkman42 · May 17, 2009, 02:15:21 PM

Mike I was going to tell You to remove the cover and close the contacts but You did one better, amazing what a print will do! Glad You found it. Regards John

gfcgfc1 · May 17, 2009, 06:48:38 PM

Hello Michael Mc , I have a '72 MC-7 it just started the NOT GEN thing also. Could you please email or snail mail me a copy of the diagram? Thanks , Glenn

Michael Mc · May 17, 2009, 08:36:15 PM

Quote from: gfcgfc1 on May 17, 2009, 06:48:38 PM
Hello Michael Mc , I have a '72 MC-7 it just started the NOT GEN thing also. Could you please email or snail mail me a copy of the diagram? Thanks , Glenn

Glenn,

I sent you an email with an attached diagram. I hope it helps.

Mike