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help with purge valve (spitter)


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Ok so i have a 2006 mack mr688s vin# 032711 

always would pop air out of purge valve every 10-15 seconds all day 

Air compressor went out so installed reman from mack as well as a new air dryer, governor, and new air line from compressor to dryer still popping off every 10-15 seconds could someone help a brother out on this? i have never had one do this thanks in advance!!!

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Follow you air governor line back to the tank its sensing pressure from. That's your wet tank. Wet tank gives air to both sides of your trucks air system and never takes any back. Wet tank has no monitoring, so temporarily screw a gauge in an open port on the wet tank so you can see what's happening there. Wet tank is isolated and is a relatively small cavity, 1/2 a tank in size, usually a split tank at low position. The governor only senses your wet tank since it's the supply to trucks air system, governor can't tell you what the rest of the system pressure is, it's only function is to make sure the wet tank is always full so rest of system can be fed off the wet tank. 


1. Governor properly/accurately senses the wet tank....which (tank) is going low on air constantly - so fix origin of the wet tank leak. Your dash can't see what's going on at the wet tank either, so put a gauge in it.

2. Governor has lost its mind and is failing (AKA- Gavin Newsom syndrome). It's new, but that doesn't mean it's good. It was a topic the other day on here and I also had a new one that was bad out of the box last fall.... right from Mack dealership.

3. Any line leading to and away from the governor could be leaking off the signal governor is trying to send.

For example: Governor can't send a large volume of air to a leaking spitter valve to unload it, or a leaking sensing line, or a leaking unloader in compressor head (which yours is new, so don't worry), governor can't compensate for the leak by sending more air fast enough and then you get a "drop out"..... a repetitive cycle of loading and unloading. Get soapy water and check while running...every sensing/control line.

Note: I also only got one year out of my AD9 split valve and it blew out a seal causing wet tank leak down out the bottom of the dryer. crumby parts.


Edited by Mack Technician
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I had an AD-4 on one of my Mid-Liners and the air dryer purged five times every cycle. Strangest thing in rapid succession, psss, psss, psss, psss, psss, and then stop. Turned out to be a hairline cracked brass compression fitting on the sense line at the dryer port. Finally found it with soapy water and variable pressure shop air plumbed into the drain port of the wet tank to test the complete system.  

Dog.jpg.487f03da076af0150d2376dbd16843ed.jpgPlodding along with no job nor practical application for my existence, but still trying to fix what's broke.



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