M939A2 Fuel Pump Shut Off Wire......Motor wont Die, Help

[osagehargraves]

Ok guys I need a little guidance. Our Fire Department has a 939A2, we have had it a few years and have randomly had problems when we attempt to kill the motor by flipping the switches off, and it would continue to run. Therefore we would pull the emergency kill cable to shut it off.

The problem has now got to the point that we have to use the cable to kill it every time. I talked with a local department and was told to find Wire 54 because it was the wire to cutoff the fuel pump and they had a similair problem with theirs.

So here is my question, I have searched the TM for wiring diagram in an attempt to trouble shoot the wiring. I dont know if I am over looking it or just cant find it. Could someone tell where to find the diagram or explain what I need to be attemping to locate as for this wire and how the fuel cutoff works

Thanks in advance

 

[Rmtaunton]

If it's an A2 it's just the solenoid sticking as the default is off

 

[WillWagner]

Put a DVOM on the fuel shut off. With the engine running, there should be battery voltage at the terminal. When you shut it off the voltage should go away. If it stays there, there is an issue in the chassis wiring, if the voltage goes away and it is still running, the shut off solenoid is junk. It has expanded and is holding the plate/disc open.You will need a new one. Pay attetnion to the order of parts as you take it apart. 10 min repair.

 

[osagehargraves]

Ok that a perfect explanation I understand thanks WillWagner, I'll check that out and let ya know

 

[osagehargraves]

Ok, so I finally got to put a DVOM on the terminals. It appears I have no voltage to the solenoid on either wire, not the one that is suppose to energize when I engage the starter nor the one the that should stay energized with the battery switch. If just one was not work I would just trace that circuit, but with it being both wires I'm a bit lost, any suggestions where to star? I would assume they get there voltage from completely separate circuits due to them being activated by separate switches.

Further more the emergency kill cable broke on the one the guys on a fire (just my luck). I have the solenoid manually wired open, and now have to raise the hood and untie to kill it and then retie to be ready for the next call out.

I have ordered a new kill cable, but if I am unable to trace my electrical problem couldn't I just put a spring on the
mechanism to hold the solenoid open and kill it with the new cable?

Make it to when I pulled the cable to kill it would just return to open after the motor dies with the spring?

Thanks for the patients guys you all are agreat

 

[The HUlk]

Wire 54 goes to the ignition switch RUN circuit. There should be 24V on this wire when the ignition iswitch is in RUN.
Wire 82 goes to the starter motor. There should be 24V on this wire when the starter is engaged.
The other wire (94?) goes to ground.

 

[The HUlk]

I have ordered a new kill cable, but if I am unable to trace my electrical problem couldn't I just put a spring on the
mechanism to hold the solenoid open and kill it with the new cable?

Make it to when I pulled the cable to kill it would just return to open after the motor dies with the spring?

Your proposed solution would seemingly work, however it may not be as reliable as the original design. I would guess the electric shutoff solenoid valve is rated for a high number of actuations being the device that is primarily responsible for shutdown. However nothing lasts forever and it's obvious that component is beyond it's life expectancy. Since the emergency shut off valve is not intended for actuation each time the vehicle is shut down, it's lfe expectancy may be far less if used in this manner. The cable may also fail more frequently when used alot which already seems like it may be a problem. Installng a new shut off solenoid and solving any associated wiring problems may be a more reliable fix.

 

[osagehargraves]

Wire 54 goes to the ignition switch RUN circuit. There should be 24V on this wire when the ignition iswitch is in RUN.
Wire 82 goes to the starter motor. There should be 24V on this wire when the starter is engaged.
The other wire (94?) goes to ground.

That is perfect, thanks a million that is the schematic I need but apparently wasn't smart enough to find, lol. Hopefully I can test it out and fix it correctly, verses my other option.

Thanks for the help Hulk.....By the way I love the color of yur truck its very close to our departments color

 

[The HUlk]

That is perfect, thanks a million that is the schematic I need but apparently wasn't smart enough to find, lol. Hopefully I can test it out and fix it correctly, verses my other option.

Thanks for the help Hulk.....By the way I love the color of yur truck its very close to our departments color

Thanks, we bought it that color. My wife seen it on the side of the road and had to have it lol.

The manuals can be found at the link below and they are filled with good info. The wiring diagram is at the back of field maint. manual part 5...

http://www.steelsoldiers.com/showthread.php?121487-TMs-for-M939-Series-Trucks

 

[74M35A2]

It is a 3 wire connector because the solenoid has 2 separate coils within it, just like a starter solenoid. It has a strong "pull" coil (start), and then a weaker "hold" coil (run). Third is ground for both. A starter solenoid does this coil switching inside its case. These solenoids would burn up in minutes if held in their stronger pull coils due to heat generation. Just FYI, it sounds like you found your issue.

 

[Csm Davis]

Ok, so I finally got to put a DVOM on the terminals. It appears I have no voltage to the solenoid on either wire, not the one that is suppose to energize when I engage the starter nor the one the that should stay energized with the battery switch. If just one was not work I would just trace that circuit, but with it being both wires I'm a bit lost, any suggestions where to star? I would assume they get there voltage from completely separate circuits due to them being activated by separate switches.

Further more the emergency kill cable broke on the one the guys on a fire (just my luck). I have the solenoid manually wired open, and now have to raise the hood and untie to kill it and then retie to be ready for the next call out.

I have ordered a new kill cable, but if I am unable to trace my electrical problem couldn't I just put a spring on the
mechanism to hold the solenoid open and kill it with the new cable?

Make it to when I pulled the cable to kill it would just return to open after the motor dies with the spring?

Thanks for the patients guys you all are agreat

If you have a M939A2 series truck with the 8.3L turbo engine then I would take the electric shutdown solenoid off permanently it will not harm anything to pull the emergency kill as it actuates the fuel pump arm in the same manner and the springs should already be in place so when you push the cable back in it is ready to crank and run. I and many other people think this solenoid is a bad design and I believe unsafe to use in anything a fire department might use it for. The deuce used a pull kill for 60 years or so without any problems that I have heard of.

 

[osagehargraves]

Ok just to follow up and let you all know what I figured out, I got the problem resolved. The solenoid tested bad, therefore I removed it and replaced my broken kill cable and have the truck running again.

I was under the impression due to my lack of knowledge, or lack of paying attention to detail lol that I would have to install a return spring once the solenoid was removed to get the mechanism to return to the run position. I found that it removed automatically to the run position once the kill cable was pushed in, due to what appears to be an internal spring or return.

Needless to say the truck is back up and eating fire again, and that pesky solenoid will no longer be a headache.

Thanks for all you guys help