Sunday, April 06, 2008

Thomas Buses - Air Throttle Repair

During most of the 1990's before the electronic age the engine throttle was air controlled on Thomas Bus Saf-t-liner models. The high idle was a welcome addition since it was operated with the flick of a switch on the dash.
In the photo the air line can be loosened while the engine is running where there should be no air with the high idle switch turned on. The throttle linkage is moved into high idle with spring pressure[no air]. The regular foot throttle air line is attached to the underside of the cylinder. {Click Photos To Enlarge}

The air is controlled with an electrically actuated air valve that is normally open with full air pressure and no power from the switch. The air pressure moves the internal piston to the idle position compressing the internal springs. When the high idle switch is turned on, battery power closes the switch and blocks air pressure to the air throttle allowing spring pressure to kick the throttle linkage up to high idle[around 1000 rpms].



The disassembled air throttle consists of springs pistons and seals and is easily serviced. The one thing that does fail is the 1/4 inch adjustment bolt on the end cap wearing through the washer that seats on the internal spring. Thomas buses have changed since this decade and are electronically controlled with an onboard computer but if you still own a saf-t-liner pusher these systems are reliable and easy to maintain.

19 comments :

Dizzy said...

Hi John, I'm looking into buying a used school bus to convert into a tumblebus and wanted to get your opinion on some of the best used buses that are out there. I'm in Seattle, and finding used buses in the Northwest is difficult. I have recently found a few for sale nearby that are '93 Thomas All Star and '93 Thomas Safe-T-Liner. Anything I need to be cautious of? I just had an '82 Chevy school bus lose it's rear axle in a freak accident. Thanks in advance for your advice.

John Whelan said...

I would pick the saf-t-liner. 1993 is a good year and hopefully it has a Cummins Diesel with auto trans. Check the mileage and body crossmember rust. It's a common problem, roll underneath and inspect it thoroughly. See where it came from and acquire the maintenance records, this will tell you the whole story. If you're serious about a particular unit get a reputable mechanic to put it up on the hoist and give it a good inspection.

Lisa in Carnation said...

I, too, am in the Seattle area, and I know the exact buses you reference (Northwest Bus Sales).

Some additional resources for used buses whose prices are much more affordable are the Washington State General Administration Click on "Surplus," then "EBay Auctions." The auction items are posted to EBay every Wednesday evening and run for a full 7 days. (Note, you'll be competing with bus dealers who are bidding on them sight unseen and strictly thinking wholesale, but if you think retail with a more reasonable price you're willing to pay, then winning the auction is more feasible.) In the meantime, you can preview the buses in person - most of which can be found at their Auburn facility (info in the EBay listing.) A word of caution...if you go on Saturday, the regular bus folks are not there, and the other folks are just "fill-ins" with little knowledge and no ability to jump start buses with dead batteries - and they won't let you do it, either.) We just purchased a 1988 Bluebird - 78 passenger - in great shape for $2700 at last night's auction.

Another great source is the J.G. Murphy Auctions. In fact, they have a 66 passenger (listed as a 44 passenger) '93 Thomas Vista from the Bellevue School District (practically identical to the one on the lot of Northwest Bus Sales priced at $10,700) going up for auction this Saturday in Kenmore (preview is tomorrow.) In fact, it is my understanding that NW Sales got the one they have on the lot from J.G. Murphy last year for $500 (or so I've heard.)

Finally, I also found some on Craig's List - not only in the Seattle area, but I expanded my search to Oregon and Idaho. I found several, but none that fit my criteria (we were looking for specific features, as we are using the bus for our church youth group.)

Hope this helps you find a local bus in the Seattle area.

ccrking said...

I really enjoy the site it has alot of good info...

I would like know if you can tell me if the air valve that controls the fast idle can be eliminated? I cannot find a replacement air valve solenoid for my 1992 Thomas saf t liner. The solenoid valve was so corroded that it was leaking air and dont even think it was working. I also want to verify that the valve is normally open which means I could just use a brass 1/4" couple to remove the valve and connect the two 1/4" lines thus eliminating it. I just want to make sure It is not used for anything else which would caus problems.

Your help would be great.

1992 Thomas
5.9 Cummins
Allison transmission
74k Miles
Pusher style

ccrking said...

I really enjoy the site it has alot of good info...

I would like know if you can tell me if the air valve that controls the fast idle can be eliminated? I cannot find a replacement air valve solenoid for my 1992 Thomas saf t liner. The solenoid valve was so corroded that it was leaking air and dont even think it was working. I also want to verify that the valve is normally open which means I could just use a brass 1/4" couple to remove the valve and connect the two 1/4" lines thus eliminating it. I just want to make sure It is not used for anything else which would caus problems.

Your help would be great.

1992 Thomas
5.9 Cummins
Allison transmission
74k Miles
Pusher style

John Whelan said...

The valve is available through any Freightliner dealership if you can give them your VIN number. This is much easier than re-plumbing.

The valve is normally closed with the high idle switch on, allowing spring pressure to kick in.

jack said...

Hi, I have a 1991 Thomas Saf-T-Liner with a Cummins 5.9 and an Allison AT545 Transmission. I just replaced the air modulator but am still having troubles losing the engine rpm and believe the solenoid is bad as well as the Williams throttle. My question is where do you recommend I source solenoid and the throttle rebuild kit? Thanks, Carl

jack said...

Hi, I have a 1991 Thomas Saf-T-Liner with a Cummins 5.9 and an Allison AT545 Transmission. I just replaced the air modulator but am still having troubles losing the engine rpm and believe the solenoid is bad as well as the Williams throttle. My question is where do you recommend I source solenoid and the throttle rebuild kit? Thanks, Carl

William Pontius said...

Hi John,
I have a 92 Thomas bus Saf-t-Liner which was originally a bookmobile. I rebuilt it into a bus containing 8 electronic pianos which we used in Seattle to teach group piano lessons. If you are interested I will send you some pictures. Anyway, I have a problem which I think is the fast idle switch. I can start the bus with the fast idle on but the moment i try to put it in gear, the engine shuts down. From your description it sounds like the normal air pressure is not shifting down to the normal idle mode but shutting it down. Do I need to rebuild or replace the valves? Are there other possible explanations for the behavior. Thanks for your attention

Bill

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

Hi John,
I'm working on a 99 Thomas bus with a Cummings 5.9 and a Allison auto trans. Right now im having the issue of it will not get over 10 miles per hour in gear and will not rev above 1700 rpm in neutral. It does have the sir throttle control system on it. Any help you can give me will be greatly appreciated.
Kevin

Kevin Wright said...

I'm working on a 99 Thomas bus with a 5.9 cummins engine and a Alison trans. I'm having a issue with it not going above 10 miles in gear and not going over 1700 rpms in neutral. Any help I can get with this issue would be greatly appreciated. Thank you.
Kevin

B & S Towing and Auto Repair said...

I have been looking for all over the net for an answer to my question and haven't found the answer to my question yet!
I did a swap in my 1992 International 4700 from a DT444 with a 6sp manual transmission.
I bought an air port freefer box truck that was a 1991 International with a DT360, VERY LOW MILES, had more idle hrs.and Runs like a scaled dog!
It was also equipped with a Allison transmission.
This turned out 2 be my most reliable trk in my whole fleet.
I've been trying to find out what series Allison I have so I can keep it serviced properly??
The transmission has a deap pan with groves on the bottom of the pan and and extrenal crew on filter mounted to the frame??
Can anyone pls help me to identify this trans?
Then after I find out thais delemma, I have a couple more questions, too!!
Thanks,Susan
"Women mechanics, All about our Diesels!"