M936A1 Tachometer Drive Replacement

[nf6x]

I didn't find instructions for replacing the tachometer drive components internal to the fuel injection pump in the TMs, so I wonder if anybody can give me any tips. The tachometer doesn't function on my M936A1 wrecker. The cable spins freely, but the electrical RPM sender and the drive shaft coming out of the injection pump both rusted, bound up, and broke. I have the threaded part removed from the IP, but the remainder of the shaft is still in the pump and I'd like to obtain a proper clue before I break anything else trying to get it out!

The stub of the shaft will rotate a degree or two when I twist it with pliers, but it doesn't turn when I crank the engine. I have a feeling that I'll need to get inside the IP to remove and replace the shaft, and I hope that the gear that it engages will still be ok. My wrecker has the IP version with the constant-speed governor that is engaged when using the rear hydraulics. I note that it's missing the fuel pressure sender for the STE/ICE-R connector, and the 4-pin connector for it is dangling near the pump. I don't know yet if this version of the pump should have that sender (which is present on my M923), but I don't think it makes a difference if I don't expect to use the diagnostic connector.

It appears that once I fix the tachometer drive, I might optionally leave the broken electrical RPM sender out instead of replacing it, since it looks like the tachometer cable will connect to the drive without the electrical sender.

Pictures are attached. Have any of y'all had to dig out a bad tachometer drive shaft before?

 

[nf6x]

If I understand what I'm reading in the TMs, the fuel pump on the NHC250 is not timed to the engine like the injection pumps on an LDT465, 6.2L V8, etc. So, taking the pump off to work on the tachometer drive shouldn't be too hard to do, right?

 

[govt]

My pulse sender unit is bound and broken. Mine is yellow with two wires coming out of it. Is this the piece you are thinking of leaving out?

 

[doghead]

If I understand what I'm reading in the TMs, the fuel pump on the NHC250 is not timed to the engine like the injection pumps on an LDT465, 6.2L V8, etc. So, taking the pump off to work on the tachometer drive shouldn't be too hard to do, right?

Correct.

 

[nf6x]

My pulse sender unit is bound and broken. Mine is yellow with two wires coming out of it. Is this the piece you are thinking of leaving out?

Yes, that's the piece. I'll replace it if I can find one at a sane price, but getting the tachometer on the dash working seems like a much higher priority to me.

I guess I'll pull out the pump so I can pull out the bad tach drive on a workbench, clean out any cruft, and inspect the helical gear on the pump driveshaft. Would it cause problems if I used a suitable grade of RTV in place of the original gasket when I reinstall it?

 

[crasheej]

Get the gasket. They don't cost that much. Most pump placs have them. You will have fun with the bolt on the back side unless you have the right tool.

 

[nf6x]

Thanks for the warning. Maybe I should start with the bolt on the back side so that the pump isn't dangling by one bolt when I figure out that I need to order another tool.

 

[Ferroequinologist]

That bolt in the back top is a PITA, trust me! Do it first. I had to take a wrench and bend it to get back in there good on mine.

 

[crasheej]

A moon shaped one is what you need. Good luck, get the gasket.

 

[wheelspinner]

Wrench

A moon shaped one is what you need. Good luck, get the gasket.

I cut the end off of a box wrench and welded some 1/4 inch rod, then just bent to the angle I needed. Made it REAL easy.

 

[nf6x]

I got the wrench, but haven't had a chance to pull the pump yet. Maybe this weekend? I'll order the gasket along with the rest of the tach drive parts that I will need, once I get in there and find out how much is broken.

 

[seano11]

Call me I emailed you my number

 

[crasheej]

That is great. This is what makes this a great country we get the job done. I had the half moom wrench.