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International-9.0
Advanced Member

USA
394 Posts

Posted - 07/06/2013 :  2:41:06 PM  Show Profile  Send International-9.0 an AOL message  Reply with Quote
My workplace just bought a 1994 GMC Topkick with a 366 TBI engine and Allison 545 transmission. It has 261,000 miles on it with the original engine and transmission.

I'm having problems with the transmission shifting too late. The TBI controls will cut the fuel at about 4,200 (and the overspeed light will flash on and off). This is irritating as it causes much bucking and jerking before the transmission will shift. The shift itself is smooth and there is no slip, it's just that the engine has to wind very high for the shift to occur. I have to be well over 3,000 for the transmission to shift with the throttle completely at idle.

I realize they set these gas engines to wind up high as the power band occurs at high rpms. It shouldn't be so high as to overspeed the engine though. I changed the vacuum modulator and the rubber lines that attached to it. I also did a vacuum test and then lines/modulator held 20 inches of vacuum without moving for 15 seconds. So I can pretty much eliminate the modulator.

Does anybody think the governor could be bad? Or perhaps that the governor screen is clogged and there is insufficent governor oil pressure? I used to have a manual, but sold it a while back. I forget how the governor oil circuit funcitons, although I know it works in conjunction with the modulator circuit to control shift points.

The fluid in the transmission is clean (and I changed it as well) and there are no other issues present. Any ideas?

C.HARDY
Advanced Member

354 Posts

Posted - 07/11/2013 :  8:40:30 PM  Show Profile  Click to see C.HARDY's MSN Messenger address  Reply with Quote
261000 on a AT545 is impressive.

"Hardybusman"
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International-9.0
Advanced Member

USA
394 Posts

Posted - 07/12/2013 :  8:06:00 PM  Show Profile  Send International-9.0 an AOL message  Reply with Quote
I think the truck was mainly run on the highway for deliveries. It appears to be reasonably well taken care of. I have ordered a governor from the serial number stamped on the transmission, so hopefully it will be the correct one. If this does not fix the problem, I may just deal with it acting up as I don't want to get into removing the pan or messing with the valve body. There could be some issues within the valve body itself if a new governor does not solve the problem. I'm thinking that it probably won't, but it's worth a try.
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bwest
Administrator

United States
2319 Posts

Posted - 07/12/2013 :  8:20:48 PM  Show Profile  Reply with Quote
I had a 454 behind a Chevy 350 one time that was doing that and the steel vacuum line had a hole worn in it where it went past the firewall. But I guess that can't be your problem though because you said you did a vac check on all that, right?

Bryan
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International-9.0
Advanced Member

USA
394 Posts

Posted - 07/13/2013 :  09:28:41 AM  Show Profile  Send International-9.0 an AOL message  Reply with Quote
I replaced the modulator first, and then did the check (probably should have done that in reverse). There was a rubber hose at the engine end and one at the modulator end. There was a copper line in between. The hoses were leaking through a bit, so I replaced both. I then pulled the hose off the intake manifold and hooked up a hand-held vacuum pump. I pumped it up to about 20 inches of vacuum and let it sit for about 15 or 20 seconds. The needle never moved, so I was satisfied that the modulator system should be working correctly. The shift points do actually vary depending on throttle position, but the whole range is probably about 200 rpm too high.

My other option is to go through and adjust the shift points. I believe the valve body has 3 little screws you can turn and each click is supposed to be approx a certain number of rpms. (like 10 or 20) The problem is that involves taking off the pan, which is a mess I'm really trying to avoid.

I've noticed that Allison transmissions with alot of age on them usually have troubles with the 2-3 shift. It either gets really really late or really really early. I'm not sure of the cause, but I do notice that quite a bit. They also tend to slip a little on the 1-2 shifts as well, but this usually doesn't cause any issue, just some delay.

There's always the possibility that the engine governor is not working properly and is the cause of the whole thing. I've put an oxygen sensor on it that really made a difference on how it runs now. Much less missing, especially at idle when hot. I still have some stuttering when the engine is cold that I haven't figured out. At part throttle positions, it seems to miss a bit. It never misses at WOT though.
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bwest
Administrator

United States
2319 Posts

Posted - 07/15/2013 :  11:54:26 AM  Show Profile  Reply with Quote
Well, I'm still thinking vacuum. Are you sure you're getting vacuum from the engine on the port you're mod it hooked to. I'll be honest I don't know a lot about the diagnosis on these transmissions, I've just had them for 20 years so you kind of get to know them after a while. lol

Bryan
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International-9.0
Advanced Member

USA
394 Posts

Posted - 07/15/2013 :  5:24:32 PM  Show Profile  Send International-9.0 an AOL message  Reply with Quote
I'll check the port. You never know...something could be preventing the vacuum from getting to the modulator. I doubt this as the modulation actually does seem to work, but the range seems to be a few hundred rpms too high. It will shift earlier when I let off the gas some, but still at high rpms. I'll check the port next time I get under the hood.

I changed the governor and this had no effect, so it's most likely something in the valve body somewhere or perhaps the little governor screen is clogged. I must admit though that it was extremely easy to change the governor. It was a much larger assembly than I expected.

Hopefully I'll find something, but who knows what the problem is. Thanks for the help.

Jonathan
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