Backup lights on MCI 9

jhaggerty · October 22, 2006, 03:23:56 PM

Hi Folks,
I am trying to get my backup lights working on my MCI 9 and need a little advice. When the transmission is in reverse, if I use a test light, I am getting power to the one contact in the socket when I ground it out. But the bulbs will not light up. The bulbs are good. I am assuming this is a grounding problem. Does the other contact go to a common ground, or what? I have a manual, but the schematic is a little blurred and I cannot tell what is going on there. Also, when I ground out the left light, the backup alarm works. Thanks for any help.

Jim H.

mikeH8H-649 · October 22, 2006, 03:30:28 PM

Jim on the backup lights one wire will be hot and the other should go to a good ground Mike

JackConrad · October 22, 2006, 05:14:29 PM

If you have the Allison 740 automatic trans,mission, one wire to the lights is hot. The other wire goes to a pressure switch on the reverse circuit of the transmission. When the engine is running and the transmission is in reverse, the contacts close allowing the circuit to complete to ground. Remember, the circuit is not complete unles engine is running, transmission is in reverse and pressure switch is functioning. Not sure about the switch on the manual transmission. Hope this helps, Jack.
PS: we re-wired ours with a switch on the dash so we can also use the lights to hook-up in a campground.

mikeH8H-649 · October 22, 2006, 06:08:45 PM

sorry Jim I guess mci's are wired different than gmc's which are wired through the reverse switch to the bulb and then to ground, Mike

jhaggerty · October 23, 2006, 10:01:37 AM

Jack,

I have the 5 speed standard, so I'm guessing there would be no pressure switch on the transmission. So, it may possibly go right to ground from the light assy.
Jim

gumpy · October 23, 2006, 10:20:43 AM

No, there will still be a tranny switch somewhere (sorry, I don't know where).

Jack is correct. It switches the ground wire.

craig

Sammy · October 23, 2006, 02:08:16 PM

Reverse switch should be on top of trans. Look at top of trans from streetside access door.
Circuit has power on it all the time from rear j box. Switch will make and break the ground for the circuit.
Might be a 16 guage blue wire going from reverse switch to stud 5 in rear j-box. Stud 5 will have 3 wires on it, all Wire 46G.
I'm going by my MC-8 book, don't have a 9 book here at home.
MCI was pretty good about not changing things up much with the diagrams.
Hope this wil help a bit.
Sammy

gumpy · October 23, 2006, 08:16:13 PM

It's almost the same in the MC9.

Power comes off 6A circuit breaker on the 24v busbar in the rear junction box to stud 16. From there it goes to remote control box horozontal block stud 4, through the two backup lights in parallel, and back to remote box horizontal block stud 6. Then back to rear junction box stud 5, and to transmisssion switch and ground.

Circuit 46 (orange-red) and 46G (blue), 16 ga wire.

craig

jhaggerty · November 06, 2006, 02:32:05 PM

Still working on those backuplights. I installed new assy, on each side, since the old ones were pretty much shot and new ones were cheap. Both contacts in the sockets have power to them. I just cut the old wires and reconnected the new assys. in the same configuration. On each side (right and left), when one of the contacts is grounded, the backup alarm will sound. The other contacts do not make it sound when grounded. This is with no bulbs in the sockets. Any ideas would be appreciated.

Jim

gumpy · November 06, 2006, 04:29:59 PM

On a dual contact 24v bulb, one contact is hot, one is ground. Don't really make a difference which is connected to which. There are two 12v filaments inside the bulb connected in series and the bulbs are keyed so they can go in the socket either way.

Your beeper is probably connected in parallel with the lights. If it beeps when you ground one contact, you are simply completing the circuit for the beeper, effectively bypassing the transmission switch. Remember, the transmission switch switches ground on the circuit. The other side of the lights and beeper is hot.

You should also see the backup tell-tale light flashing on the dash when you ground the pin that makes the beeper beep.

Again, it doesn't matter which bulb socket contact you attach the wires to. One should be hot. The other one will be hot, also, because that one is coming from the front master switch, through the tell tale light. You can easily check them with a meter.

If you trace the wires of the backup lights, one should go to stud 6 on the Horizontal Bar in the remote control box, and the other should go to stud 4 of the same bar. That doesn't mean they are still connected that way. My guess is the beeper is also connected to these two studs.

craig

jhaggerty · November 06, 2006, 05:29:13 PM

Craig,

What would your guess be when both contacts are hot?

Jim

gumpy · November 06, 2006, 05:40:59 PM

I looked at the circuit diagram again, and modified the post a bit...

Yes, they will both show hot. The reason is that one comes from the rear busbar, and the other one
comes from the front busbar via the master switch and tell tale light.

When the tranny switch closes, it grounds both the tell tale and the backup lights.

NewbeeMC9 · November 06, 2006, 06:23:58 PM

May be irrelevant, my bus originally had a stick but is now an auto. My reverse lights work off of a spring loaded switch on the side panel by the driver. It is the furthest to the rear and says reverse above it.

HTH

gumpy · November 06, 2006, 07:55:26 PM

That switch was not for your lights, it was for the reverse solenoid. You had to engage the solenoid and then put the tranny into second gear to get reverse (if I recall correctly). I think the switch that engaged the backup lights was separate and located on the tranny, but not certain. There's not a different schematic in the book for manual vs auto, so I'm pretty sure the electrical circuit for the backup lights was the same regardless of tranny.

NewbeeMC9 · November 06, 2006, 08:20:33 PM

I reckon mine was customized