M939A2 CTIS PCU solenoid replacement

I got a few requests for this info so here goes.

My CTIS was doing 5 blinking lights after reaching 120 psi and there were no noises coming from the PCU on the floor. The troubleshooting guide pointed me in the direction of the control solenoids.

To test f this is your issue, disconnect the 6 pin connector on the left side of the PCU. Only 5 pins are used:
Control: B – D
Deflate: A – E
Supply: A – F
You need to measure 30 to 80 ohms across each pair. No reading means that solenoid is dead. I had 2 dead ones so I ordered 3 peter paul 5C13K24 solenoids from plccenter.com. peter Paul is the original manufacturer of the solenoids in the PCU. The only difference is that these new ones will measure 75 ohms and the old ones I think measure 40 or so.

To replace you need to remove the cover from the PCU. 6 Allen screws hold it on. Also remove the but holding the 6 pin connector. The solenoids will be marked S, D, and C so you can identify the dead ones. You'll need to remove the but holding the solenoid on and slide it off the rod. Next, mark or more which wire goes to the left or right on the solenoid. They're not marked and I'm not sure if it makes a difference, but I wanted to get the same orientation. Cut the wires close to the solenoid so you have enough wire to work with. Youll be able to remove the metal cap now. Thatll go on the new solenoid. Then its just splicing in the new solenoid and put it back on the rod. Tighten down the but and you're good to go. Repeat for the next dead solenoids, if any, then put the cover back on.

You should now be thoroughly annoyed by the ctis sound in your cab.

Thanks for the info. The continuity checked OK on all of mine. They were all right around 48 Ohms, so I'm assuming they are OK and my problem must be elsewhere. Still don't know how to test to pressure transducer and if it could be causing my issue.

For the pressure transducer off the wet tank, you can disconnect the plug from the CTIS harness and short the pins. That will trick the CTIS controller into thinking there's 120psi in the tank.

I have a leaking tire and when it gets to a certain point and I start the truck the ctis will be blinking 5 lights I have to hit run flat then it will air up until all tires are the same then go to solid red hwy light.

Alexsha said:

For the pressure transducer off the wet tank, you can disconnect the plug from the CTIS harness and short the pins. That will trick the CTIS controller into thinking there's 120psi in the tank.

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That switch is OK. The system turns on right at 120 PSI. The one I'm concerned about is the little square switch that screws into the top of the PCU. It has a 3 wire plug on it.

I might have issues with that one as well. I'm thinking of adding an in-line pressure gauge to see what's going on.

On the tank its an on-off switch. The unit with 3 wires on the valve body is a transducer which reads the current tire pressure. Or at least its supposed to.

I just ordered 3 of the solenoids from PLCcenter.com. Both times I took apart the PCU the same valve was open. I know it was clean and moved freely. I'm hoping it's the solenoid for that valve that's acting up, even though it's testing OK. As soon as the truck hits the 120 PSI and the CTIS starts it makes an immediately loud, "Pfffft" then goes to 5 flash.

Anyone know if it matters which wire goes to which on the solenoid? As Alexsha pointed out they are just plain black, no markings.

I just kept the same orientation with the wires. I don't believe it matters though.

Were you able to get 30~80 ohms on each of the pin sets? If you did, your solenoids should be good. If it's running air through the PCU then going to 5 lights that should mean you have a major air leak after the PCU.

I had 48 ohms and continuity on them all. It's almost instant on the 5 light flash. When I first got the truck it worked fine, then it started making a real fast "click-click-click-click". If I pushed any mode button it would stop clicking for a little while and work normally. Then one day it just went to the 5 lights immediately and the air blast sound (in cab). Being so instantaneous is what was making me think it is an electrical fault. I might barking completely up the wrong tree, though.

And just to throw another wrinkle into it, I did have a wheel O-ring that was leaking pretty good when it was colder (below freezing). Since it has warmed up it quit leaking, so I haven't replaced the O-ring.

Carefully going through the trouble shooting guide again, I'm seeing the PCU Sensor as a possible cause. This is the little square sensor on top of the PCU. I can't find anything that says how to test this sensor without the computer or having a replacement sensor.

If it was leaking, it is still leaking. Before tearing apart the PCU I would replace the o-ring.

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where can I find part numbers and where to get parts for the ctis system. I just got my 934 and its in pretty rough shape. im sure I will be working the ctis really soon

thanks
rc

Most of the stuff you can find on eBay or surplus dealers. The ECU is hard to come by though. The PCU can be rebuilt with new solenoids and o-rings. Not much else in there to go bad. What parts do you need?

was looking for part numbers of O-rings and diaphragms and those type of things.

If you have a leaking hub, these are what you'll most likely need.

For most smaller o-rings, you can usually get a decent box of mixed sizes and match them up.

For wheel valves, you can get a rebuild kit:
CTIS wheel valve rebuild kit. P/N 599913 NSN 2530012844287