Originally Posted by
Jon Wehrenberg
Elliot,
There are several possibilities such as the selector switch (my guess) or a defective solenoid valve.
The selector switch is a rotary switch not likely to fail, but it could have a loose connection. I would start there and check it for tight connections and continuity.
My second choice, but easiest to check is the second solenoid down on the solenoid manifold under the driver's seat. It is a stack of five, mounted on the rear bulkhead and is a row of black solenoids identifiable by the red caps. This can be checked by listening for the valve to "click" when you select the front position on the rotary knob and then alternately press the up and down. In the "UP" position you will hear that valve and the one above it click, and in the "DOWN" position the second one down should click.
These solenoids are opening the air circuit to the five port Norgren valve which should be located below or near the solenoid valve manifold. If they appear to be working loosen the small end air connections to verify the valve is getting air signals.
If neither click it is likely the rotary switch, if the second one down fails to click it is likely the valve. If they all work and the Norgren is getting air, it may be the culprit.
As near as I can determine the Norgren five port valve, rotary switch and the second solenoid down are the only common parts to making the front go up or down.
Hector created a wonderful tool which I used to make the analysis and it is available to POG members. It is the Prevost Suspension Demonstration power point and if you can access it through the POG site or get Hector to provide it to you it will help you sort out this problem. It is not going to be difficult or expensive for you to tackle, but an inexperienced mechanic that does not understand your system will burn through your money at an alarming rate.