Tag Axle Air Pressure Control

[Richard and Rhonda]

I have a 95 Newell. On the dash board is a rocker switch marked tag axle. You have to hold it in the up or down position to activate it. It will return to the center position automatically. It is obvious that holding the switch up dumps air out of the tag axle bags, and also obvious that holding it down reinflates the bags. However, there is no way of determining how much pressure is in the tag bag.

Also, there is only one gauge in the rear engine compartment. It reads the overall air system pressure. I do not see a regulator for controlling the tag air bag pressure.

One more tidbit. Others have described a three gauge setup in the right rear engine compartment. I only have one as stated above, however, I see a couple of screw holes where something could have been mounted and later removed.

Does anyone know how this setup is supposed to work? With what I know I feel I am operating blind with regard to tag air bag pressure, which is to say I am blind to the weight distribution between the drive and tag axles.

Thanks for any insight on this.

Richard

[fulltiming]

Richard, while my 1992 model has just a two position switch for the tag (normal/dump), many newer models had the three position switch (increase/normal/dump) allowing you to temporarily increase the pressure in the tag as well as dump the tag for tight cornering.

I suspect that the single gauge in the engine compartment is the tag axle pressure. What reading are you seeing on the single gauge in the engine compartment? If it is significantly less than the system pressure, probably around 20-30 psi, that is the tag pressure. Holding the switch in the dump position would drop that pressure to 0 while holding the switch in the increase position would increase pressure, but without having someone watch the gauge in the engine compartment while the switch was depressed, I don't know how much increase you would see. This position is needed to get onto toll roads that use inline scales to see if any axle is over 20k pounds. With a loaded coach you are probably running over 20k on the drive axle under normal circumstances. Based on my findings with my coach sitting on individual wheel scales, when I doubled the pressure on the tag from 25 psi to 50 psi, I transferred approximately 3,000 pounds from the drive axle to the tag axle.

The system pressure gauge that I have in the engine compartment is redundant since I have a system pressure gauge on the dash. The third gauge I have in the engine compartment in not needed in the coaches with the 6 speed Allison since it provides air to the air operated modulator valve on the 4 speed Allison HT-740 transmission.

Hopefully someone with a 1995+ model will chime in with the location of the pressure adjustment for the tag axle on those models.

[Richard and Rhonda]

Thanks Michael,

To answer your question. the single gauge reads between 100 and 120. It does not move when the tag axle switch is activated.

Here is the picture I am getting so far. Activating that switch causes a temporary change in the tag axle air bag pressure. I am trying to confirm that my assumption is correct. Because if it is not then dumping or increasing air in the bags changes the weight distribution and I have no idea what is going on.

If my assumption is correct then I need to look for circuitry/plumbing that controls the air bag pressure while the switch is in the default (middle) position.

On your coach, do you get a warning light or chime when you dump the tag pressure?

[fulltiming]

Based on your gauge reading, that is either system pressure OR your tag axle pressure is way too high, but I think it is system pressure. Thinking about it, a system pressure gauge there would help the service techs confirm that the system was aired up when working on the coach and using shop air.

You are correct that activating the switch only causes a temporary change in the tag axle pressure and it will go back to it's preset level when the switch to in the normal position. It would not make sense that moving the switch either direction would 'permanently' change the tag axle pressure.

Yes, I get a warning chime when I dump the tag axle to remind me to switch it back to normal before I start to accelerate. At speeds under 10 mph, the tires on the drive axle will handle the extra weight of the tag being dumped but not at normal driving speeds. However, if it is a momentary contact switch, that would not be necessary since it would start returning to normal as soon as you released it.

[Richard and Rhonda]

That's what I'm thinking about returning to normal when the rocker switch is released.

However, you know what they say about assumptions.

Hopefully, I will have time this weekend to run a few experiments, and crawl under the coach to draw some air line schematics. I will report what I find.

Thanks for your help in thinking through this.

[fulltiming]

Any adjustment to the pressure for the tag, assuming it is still 'user adjustable', should be need the solenoids for the drive and tag axle. On the early 1990's models these were in the engine compartment and the solenoids for the front axle were up under the front of the coach on the drivers side. They should be easy to spot as they are located together on a manifold with air lines running everywhere.

Let us know what you find. I enjoy finding differences in locations and functions of accessories through the years. Especially since most changes were not made at the beginning of a model year.