Bendix AD-IP Air Dryer question..

You should also have inlet check valves for the primary and secondary reservoirs. These check valves should also prevent air in those tanks from back-leaking from what ever problem is occurring. These check valves seem to have short life span and will fail after a while. Because you are seeing primary pressure drop when purging you definitely have a bad check valve there. They are usually a threaded fitting screwed right into the inlet of each tank.

 

From what I read, the AD-SP does use system PSI to purge....so I thought a drop in PSI would be expected...?

 

It is, I had an AD-SP on a truck a few years back. Your pressure drop is normal. If you had a reservoir-type and were getting that drop, there'd be issues. I bet if you bypass your dryer, you would still here the compressor cycling on and off because of the problem you're having. Just a thought, have you tried bypassing the dryer? That will let you know right quick if it's the problem. Provided you can hear when the air compressor is pumping.

 

Ok, I see we went from an IP to a SP model mid thread. You should have a slight pressure drop in the secondary system, not primary if it is plumbed correctly.

 

Yea, sorry about that....my mistake.

I see a loss in the FRONT system (green needle) , not the REAR system (orange needle) and from what I can read
thats normal for the AD-SP.

 

Green is normally primary, which is normally rear brakes. Orange is normally secondary which is normally front brake and accessories.

 

This makes sense....but the gauge is labeled GREEN (Front) and ORANGE (Rear) and anyone else with our setup sees the green needle fall after AD-SP purge.....so while I never really understood what is what, my setup appear normal.

Bendix claims that the AD-SP will draw from the FRONT tank and per the gauge, thats whats going on.
Again, not that I understand this or dispute what you say......

 

I've been thinking about this still......
HeavyD looks like he is right....

"During AD-SP Purge, System pressure will be reduced in the supply (wet tank) and secondary (front axle) reservoirs only. The primary or rear axle service reservoir will retain full governor cutout pressure.

I see the two needles at 120 then the purge and the ORANGE needle stays put and the GREEN needle falls.

Ok..with that, I understand the following:

GREEN circuit = PRIMARY AIR
ORANGE circuit = SECONDARY AIR

The gauge on the dash has 2 needles (Green and Orange) but they are labeled:

Front
Rear

If they said PRIMARY and SECONDARY it would make this much easier.
Which needle SHOULD fall when AD-SP purges?

Adding insult to injury, a year ago this gauge was replaced. I wonder if it was connected properly!?
I have three tanks on the W900:

I believe the wet tank is the small "pig" looking one and then there is one large tank directly after that and then one
large tank under the driver. I do not know which of those tanks is the primary or secondary.

 

As the schematic shows the system. I still don't see how the secondary tank should lose pressure due to the check valve. The d2 still senses wet tank psi, to cut in and to unload.

 

Yep, I bet they just simply got the air lines to your gauge backwards when they changed it back then, LOL. As for the inlet check valves, there is a special valve for the secondary tank that does allow some back flow to assist the AD-SP purge. The diagram above is just a regular generic one, AD-SP are slightly different.