The British Bus Club's Forum

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Issues when shifting gears

We would like to know if anyone has experienced the following issues with their bus:

When the bus is turned on and after it has already got to 120 psi, we used to be able to getting moving without any problems, lately the bus gives a hard time to move, like it is going to stall and then after a few feet then it works fine, also when placing the same on back gear, it is jumping very strong, I know that the gear box is controlled by the air system, do you have an idea of could be occurring? thank you all.

The bus is a 1979 Bristol

Re: Issues when shifting gears

Does you shifter look like this?¤t=shiftCAV.jpg

The alternative is a "two-position" shifter with only an "R" and "A" shift position. If it's a "two position", the entire shifter is an "add on" system to cause the transmission to shift automatically. On my bus (a '76 Daimler Fleetline, so different in some details to yours), the add-on auto system went intermittent and had symptoms like yours.

There are instructions for checking the pneumo-electric unit in the workshop manual.

If you have the 5-steed sensor, I'm mystified.

Re: Issues when shifting gears

To make a complete answer (in case someone else is looking for info and finds this), the transmissions on most 1970's buses (like Oscar's and mine) use air pressure, as Oscar mentions.

Even if you have to "shift gears" by moving the shifter knob from 1 to 2 etc., the internal parts of the transmission work like an automatic transmission. There is a series of epicylic gear sets and these are selected by a band which is pulled onto the outer drum for the desired gear set. There is a pneumatic cylinder attached to the band and air pressure in the cylinder pushes the band into engagment.

So, you have to ask yourself "is the air pressure getting to the transmission engagement cylinder?" If so, the problem is in the transmission itself.

Sometimes, a problem can be repaired by adjusting the cylinder and band within the transmission but generally an internal problem in the transmission itself means $$$$.

If you're not seeing air being applied to the transmission, the problem is most likely in the supply of air from the air system in the bus (not likely if your pressure guages read correctly and your brakes work OK) or the switching mechanism.
On these buses, there is an electrical switch (the part with the knob that you move from 1 to 2 position, etc.), wiring, and an electrical/pneumatic unit. The E/P unit has individual solenoids which, when activated, open air valves and route that air through high-pressure pipes to the transmission cylinders.

The components in the selector system can be easily evaluated by a normal troubleshooting sequence. The most effective method of checking is to detach the high pressure air line from one of the E/P solenoids and select that gear on the selector. If you have high pressure air there, the selector system is working OK. This also implies proper supply of high-pressure air. If the air pipe from the E/P unit is not damaged (a problem could easily be a clog or an air leak in the tubing itself or at a terminal), but the E/P unit is supplying air properly, a problem inside the transmission unit itself is very strongly impled.

Re: Issues when shifting gears

this actually sounds as if you have a brake hang up. try greasing the s-cams and adjusters. for the bumps in reverse check air pressure at the ep (the electric selector in the rear)80-85 PSI. this can be ajusted at the regulator near the firewall just above the trans. the stall may be unrelated to the two other problems this could be the "rack" adjustment. this is 1 of two available adjustments on the fuel pump. there is a idle aadjustment wich i'm sure you are familiar with...on the left bowl there is a nut as you turn it in and out you'll notice a idle change but this is not a idle ajustment think of it more as a stall screw.


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