I recently replaced the Bendix PR-2 air pressure protection valve with a proper air regulator for my HT740's modulator, and found that the PR-2 has been letting full air pressure through to the modulator, maybe for years. As a result, the modulator's diaphragm is leaking, allowing transmission fluid to back up into its air line. When I keep the now-70 PSI air flowing, I think I can also hear bubbling inside the transmission, so I know the modulator is kaput.
So, do I need to partially drain the fluid to replace the modulator? It's located just above the top of the transmission's oil pan - is the fluid level higher than the top of the pan? If it isn't, I may be OK to replace it without draining some fluid out.
Is the modulator a Williams WM775B / 117830 ?
Thanks, John
PS - Bendix's Service Data sheet for the PR-2 valve shows the delivery and supply ports mis-labeled on their drawing! The supply port actually is underneath, the Delivery port is to the side. Oops!
safest way is to drain off 3 gals the oil level is above the pan and you take chance of dropping the pin in the pan .I don't recommend it but when in hurry I start the engine to drop the level and replace the modulator lol and I have lost a few pins over the years too so careful with the pin.There are lot of different models of the modulator and parts number, it needs to match the air throttle. I use Brake Systems of Portland Or for parts 503-236-5005 they own Williams now and great to deal with .You probably know the hose coming from T on the supply has to be a certain length for better and smother shifts
OK, I'll drain a few gallons out first. Will the pin stay in the old modulator, or does it want to stay in the valve body? If I carefully remove the old modulator, keeping it as straight as possible, will that help?
The air hose is longer than it needs to be, so I assume it's sized correctly for good shifts. The hose now has fluid in it, so I'll need to blow it all out before reconnecting it.
I've ordered a WM775B - several folk on Newell forums and elsewhere say it's the right one for HT740s, and my old one looks identical to it.
John
Slow and steady because the pin will stay in the body most of the time, I forgot you have the fly by wire throttle with your ddec ,Gary Hatt has that crazy modulator setup on his series 60 with the 740 and Van's Prevost with the 8v92 DDEC with a 740 has it too
Today, on Luvrbus's advice, I sucked about 3 gallons of transmission fluid out through the dipstick tube, replaced the modulator, then pumped the fluid back into the transmission. Easy. The old modulator's diaphragm had ruptured because it was receiving full air pressure instead of the 75 PSI maximum that Williams specifies. However, it still works, sort of, even though it allows air into the oil pan. The original setup had a Bendix PR-2 valve instead of a proper pressure regulator, and it was jammed fully open due to corrosion inside, hence the blown modulator. I've installed a decent regulator with its own gauge, so now I know exactly how much air pressure the modulator gets. I test-drove the bus for about 30 miles, but the transmission is still shifting lazily from 2nd to 3rd when the fluid is cold - I thought a new modulator may fix that, but it needed to be replaced anyway.
John
The lazy shift from 2nd to 3rd are usually cause by the shift point from 2nd being to low next time you have the pan off to change the filter give the trimmer valve on 2nd 1 click and it will raise the shift point 50 rpms and the few extra rpms helps increase the fluid pressure for a faster shift
Quote from: luvrbus on August 01, 2025, 06:22:44 PM
The lazy shift from 2nd to 3rd are usually cause by the shift point from 2nd being to low next time you have the pan off to change the filter give the trimmer valve on 2nd 1 click and it will raise the shift point 50 rpms and the few extra rpms helps increase the fluid pressure for a faster shift
OK, will do, but it won't be for a while because I have new fluid in it now.
Thank you for all your help with this.