Air/Hydraulic power unit failure?

[taylorm2]

I've got an issue with the cab tilt/spare tire/suspension mechanism that I can't find good guidance on in the TMs (TM 209-2320-365-20-2 is where I was looking). The backup manual pump works to raise/lower the cab and spare tire, however when I push in the PUMP button on the manifold nothing happens for any of the functions. I don't hear the pump going, and obviously nothing moves. Pushing it does, as far as I can tell, absolutely nothing. I should have good pressure as the cab air bags inflate/deflate and both pressure gauges in the dash are over 100 PSI. This happens with the truck running or off.

Not sure where to start here. One of the steps involves checking for 2,500-4,000PSI at one of the hoses but I don't have tooling to do that.

Any tips/advice on where to start?

 

[GeneralDisorder]

Cab air bags get their air from the wet tank PPV. Cab lift system gets primary/secondary air from a valve at the rear of the truck that disables it unless the parking brake is set. You have the parking brake set?

Disconnect the line going the pump, press the button, and see if you get air. If you do the pump needs rebuilt. If you don't then you need to start diagnosing the hydraulic control panel air supply.

 

[taylorm2]

Yes, parking brake is set, however I noticed air leaking from around the parking brake knob. Maybe it's not fully engaged?

Which air line are you referring to, the air line (507) on the power unit?

 

[GeneralDisorder]

Yes. The air line on the power unit.

The parking brake control and emergency control need to be replaced most likely. The O-Rings dry out and they leak. Upgrade to the new integrated unit used in the later A1 trucks to simplify the hoses. But leaking shaft O-Rings won't cause the issue you are having on their own.

 

[taylorm2]

Thanks for the help! Is there a good part number for that unit?

 

[Ronmar]

Is this an A0 or an A1 truck? On the A0, the remote control valve General referred to is on the inboard end of the front/secondary brake tank. It samples park air and when the park brakes are released/park air applied it disables the airflow from the pri-sec tanks to the pump(no raising the cab while driving).. I did a video when I removed mine…

if you have compressed air you can also inject air into the pump where that 507 line attaches using a rubber tipped blowgun…

they used the same type remote control valve on both A0 and A1 trucks, moving it to the rear with the other brake components simplified the plumbing a little on the A1.

 

[taylorm2]

Is this an A0 or an A1 truck? On the A0, the remote control valve General referred to is on the inboard end of the front/secondary brake tank. It samples park air and when the park brakes are released/park air applied it disables the airflow from the pri-sec tanks to the pump(no raising the cab while driving).. I did a video when I removed mine…

if you have compressed air you can also inject air into the pump where that 507 line attaches using a rubber tipped blowgun…

they used the same type remote control valve on both A0 and A1 trucks, moving it to the rear with the other brake components simplified the plumbing a little on the A1.

A0 truck I think. It is a 1998 with the cat 3116 engine and wtec 3

 

[Ronmar]

A0 truck I think. It is a 1998 with the cat 3116 engine and wtec 3

Yep, that valve is probably on the air tank then. It has a small 1/4” airline from the remote valve over to the hydraulic control valve. There are 2 air inputs and 2 air outputs on that hydraulic control valve. One supplies pri-sec service brake air from the remote control valve to the AOP control and outputs to the AOP. The second input is from the wet tank protection valve to the cab suspension control and out to the cab air springs.

 

[taylorm2]

Posting a quick update now that I’ve had some time to get parts in and troubleshoot this properly.

Firstly, thanks @Ronmar for the video and explanation! This helped me identify where do start pulling air hoses and checking for pressure.

My issue ended up being twofold. That inversion valve (circled in red) was leaking so it got replaced, but the real issue was that double check valve (circled in green) being stuck closed. Even with pressure on both sides, it was not delivering any air to the inversion valve.

I hooked that half inch air line directly to the new inversion valve and the rest of the system works! Cab tilts, spare raises and lowers. Good to go for now, seemingly.

For those reading this thread down the road, here is the inversion valve I used as a replacement:

Part Number: TWD H280758
Napa link: https://www.napaonline.com/en/p/TWDH280758

I’m still looking for a good part number to replace that double check valve. Once I get it and get it installed I’ll update this post with the details.