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 Allison at545 reverse and drive at the same time?
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Karl87B700
New Member

3 Posts

Posted - 08/05/2013 :  7:26:55 PM  Show Profile  Visit Karl87B700's Homepage  Reply with Quote
Hi, My name is Karl and I own a 1987 Ford B700 Thomas body bus with the Detroit 8.2L and Allison AT545. I rode this bus in school and purchased it about four years ago when the county retired it. I realize that the drive train is not a very desirable one but it runs well and I just couldn't pass up the chance to buy the bus I rode to school.

I haven't driven it very much but today I had a weird experience and I am concerned to drive it until I figure out what went wrong. I drove across a very rough railroad crossing at about 40mph. Suddenly there was terrible squeeling and grinding sound, like metal on metal, which went away after about a hundred yards. The bus did not drive any differently and I stopped about 300 yards later when it was safe to pull over. Looked over the bus and nothing was broken or loose or hanging that I could see. The bus was idling while I inspected it and suddenly the reverse lights/beeper came on with the transmission still in neutral! I ran the gear selector through the gears and the lights/beeper turned off.

So I got back on the road home and 10 miles later the reverse lights/beeper came on while i was driving 45 mph! I slowed down, fiddled with the gear selector but nothing changed. I was in heavy traffic so about a half mile later I finally got to pull over but before I stopped the reverse lights/beeper turned off. I drove a couple miles home and the bus drove normally.

I searched but could not find a reference to this exact problem. I would love to have some theories on whats happening and some suggestions on what to check.

Thanks for any help and thanks for having this nice professional forum.
Cheers,
Karl

sambrutay
Advanced Member

United States
232 Posts

Posted - 08/06/2013 :  05:02:56 AM  Show Profile  Reply with Quote
The Allison at545 2/3 shift often squeals when youíre getting down on them. I am told itís a pressure problem. Not a Trans expert in any means but we have a fleet full of the same scenario. We drive them everyday like that. As for you reverse lights coming on. Maybe a wiring issue or a faulty reverse light switch.

Bruce
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mike9
Active Member

19 Posts

Posted - 08/06/2013 :  08:12:19 AM  Show Profile  Visit mike9's Homepage  Reply with Quote
40 mph over rr crossing ! good thing you own it,lol but sounds like reverse switch.
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bwest
Administrator

United States
2236 Posts

Posted - 08/06/2013 :  09:25:06 AM  Show Profile  Reply with Quote
On that old of a bus I would suspect a wiring problem. When the backup lights come on crawl under there and see if you have continuity across the backup light switch. (I am assuming you will take the necessary precautions to make sure the thing doesn't run over you.) If you don't have continuity then you need to figure out where the power is coming from. Start pulling some panels and looking for problems.

Another way to figure this out is to rig up a continuity tester that you can see from the driver's seat. Drive the thing and see if it makes the tester move. I'd use the old style analog tester for this, you'll be able to see the hand sweep much better than looking for the numbers to change on a digital one. Good luck.

Bryan
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Karl87B700
New Member

3 Posts

Posted - 08/06/2013 :  10:04:16 AM  Show Profile  Visit Karl87B700's Homepage  Reply with Quote
Thanks for the replies!
Sambrutay: I have heard the Allison shift squeal on other buses but this bus dosent do that. The sound I heard was much longer and more grinding than just a shift. I will check the reverse switch.
Mike9: I know that sounds bad but the RR crossing is on a 5 lane highway with 55mph traffic. I think the far right lane is more rough because of all the trucks crossing it. Normally I wouldn't hit one at that speed. I sure know better now. :)
Bwest: Thanks for the suggestions! I'll check out the switch and report back. And i'll be sure to chock the wheels and have my wife there when I crawl under the bus.

There is the possibility that the noise I heard and the reverse light problem are not at all related. One thing that does concern me is that the reverse switch on the at545 is pressure activated. If the switch is being activated while I am driving, how could pressure be applied to the reverse circuit while the transmission is in drive?
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sambrutay
Advanced Member

United States
232 Posts

Posted - 08/07/2013 :  06:13:02 AM  Show Profile  Reply with Quote
Usually a defective switch and not pressure while in drive.

Bruce
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Karl87B700
New Member

3 Posts

Posted - 08/14/2013 :  7:03:20 PM  Show Profile  Visit Karl87B700's Homepage  Reply with Quote
Update on my bus. After driving the bus more this week I am happy to report that the reverse light pressure switch seems to be working fine. I am getting a new one to keep as a spare for when this one does give out completely. I used this post by "bwest" as a reference: http://www.schoolbusfleet.com/forum/topic.asp?TOPIC_ID=25523 .
As far as the weird noise goes, I traced it to the rear suspension. There was metal rubbing on metal at the spring hanger and with the right set of bumps in the road it would make that awful squealing/grinding sound. Tightening a bolt and some lube fixed it.

Thanks again for the help!

Karl
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