Lift axle not working right

[PeterG]

I went to drop the lift axle on my 2007 Kenworth T800 today. With the parking brake off, and the truck in Drive, and my foot on the brake, when I activated the switch in the cab, the button would light up, but no air pressure. When I took my foot off the brake pedal, the air activated to the correct pressure and I could drive. Then when driving, every time I touched the brake pedal, the air pressure would go off, and I assume the axle would come up. Normally it would only come up by itself when putting the truck in reverse, or the parking brake. Any ideas?

 

[1693TA]

Likely a ground problem for a solenoid operated air valve to the lift axle air bags. Given the age of the truck, this is very possible.

 

[Truck Shop]

Back then there were about five or six different drop axle air control systems. None were very good, Parker
controlled was one of the worst/over designed. Kenworth had several different factory installations, not knowing
which design and brand of controls you have. Up in reverse is wired to the backup switch is all.

But for the simple compact full function of one valve that does all-Tremac Sloan makes the 401349 valve,
The Freightliner # is GT5040. Less air lines, easy to install.

 

[PeterG]

Ok, thanks guys.

 

[Tony Donahue]

I have a 2015 KW T800. Tag axles won't go down. Had a rapid dump valve go out a few weeks ago replaced it they worked for a couple weeks. Factory installed tags. Does any know where the solenoids are. There is power to the switch but nothing happens. Is it possible that both solenoids would go out at same time.

 

[Truck Shop]

Between the switch and solenoids, one wire feeding both. Solenoids are normally mounted on or near
valves. Such as the one pictured above.

 

[Tony Donahue]

No solenoid at valve. I think I found them on back of hvac box. They look like a clutch fan solenoid. I would assume the should have a fuze?

 

[Truck Shop]

Those use the same solenoid as a clutch fan, same as above. But what you have is a over thought system.
the GT5040 I posted above is the latest and greatest full function valve that eliminates many of the problems
with earlier designs. And yes it should be fused.

 

[mi042]

My lift axle would bounce sometimes when driving and also at the same time the backup alarm and lights come on too. Sometimes its nonstop which makes the truck loose air fast . I've already replaced the backup switch and shift control stalk but nothing changed. Any thoughts on this ? Truck is a 2020 Peterbilt 567 . Thanks.

 

[Truck Shop]

What condition are the tires? What condition is the road surface. Both will set that
in motion. Does it have good steering control shocks? Are the trailing arm bushings
tight?

 

[mi042]

Tires are 70% . Shocks all around were changed 6 months ago. This issue also happens sometimes when I'm parked and lift axle not in use. Lift axle lifts up automatically in reverse when in use .

 

[Truck Shop]

What pressure is regulator set at and is it keeping a constant pressure. I preset regulators on the
bench using a high quality accurate gauge, because you can't trust the gauges in the truck, those
are of cheap quality and known to be inaccurate. If pressure is bleeding off, regulator, drop axle
lift/drop valving, hose leaks, hole in air bag. Go after it with soapy water.

 

[mi042]

Regulator is at 60 psi . I don't think I have an air leak because all this happens randomly. And when it happens everything thats linked with reverse gear gets triggered like backup alarm/ backup lights and lift axle if down at that time.

 

[Truck Shop]

You have a short to solenoid at lift valve. Check your wiring.

 

[mi042]

Ok will check it out. Do you know if there's a pressure switch behind the dash or on tge firewall relating to backup mechanism ?

 

[Truck Shop]

That all depends on what valving system was installed at the time of the axle installation.
What was provided in the kit and who installed it. There are many variations of drop axle
air systems. In those years way over thought. The reverse wire comes from backup light
switch located on top of transmission routed to the dump valve solenoid, the solenoid is
the same used in many cases to operate engine cooling fan clutch.

 

[mi042]

Checked it all today and found nothing wrong on solenoids and wires . Could this be a transmission software issue ? My truck does not show any info on transmission software version like it does for engine and abs.