M35A3 tach fixed itself?

[HDN]

I actually woke my M35A3 up from its winter slumber today to move some more big stuff from my old home. The roads were dry and looked pretty salt-free due to precipitation and melting over the past week, plus I figured on rinsing the underside of the truck with my garden hose when I was done, just in case. It started with just under 10 seconds of cranking with an outdoor temperature of 36 degrees F and no anti-gel in the fuel (added that to the fuel before my run today!).

Since I bought the truck this past late October, the tachometer would always hit 3,000 RPM when I'm going above 40 MPH in 4th gear. I intended to take it to a diesel shop or the Cat dealer to get its engine speed verified and maybe get the engine speed sensor repaired. But after about an hour and a half of idling (afraid to shut down due to charging issue) then taking it down the road, I noticed that the tach started to make sense!

I watched it as I ran the truck through its gears and it would shift into 4th at about 2,600 RPM. I really learned today how smoothly the transmission shifts when I keep the pedal close to the floor, even with the motor oil in it. Previously I would quickly let my foot off the gas pedal to force a shift between 3rd and 4th gear, but now that I know what the 4th gear shift point engine speed sounds like, I don't think I'll be doing that anymore!

I'm not yet familiar with what goes wrong with these tachometers besides some kind of magnetic sensor getting out of alignment, or something like that. I noticed that the engine built up considerable heat just idling, and when I pulled out the temperature jumped to 220 briefly and the fan kicked on. Could the sustained heat at idle have done something? Or could it be something as mundane as a loose ground? After all, the truck mostly sat for 10 years before I bought it...

Of course now that I posted this thread the tach will go bonkers again in the springtime when I will next run the truck. That's probably what will happen

 

[JCKnife]

When I bought my deuce the tach jumped like a sugared-up 12 year old on a trampoline. After a few days driving it fixed itself also.

 

[HDN]

I got it out a second time yesterday to move some more big stuff and the tach still works right! I hope it stays that way

 

[DJones]

Mine doesn't work at all! I haven,t had time to look at it yet.

 

[HDN]

After driving my truck a bit more, it seems the tach is still off a little bit, by maybe 200 RPM or so. Idle speed is spot-on it seems. It just gets a little wacky when running the engine faster than that. At this point that'll probably be a next year project.

 

[HDN]

3 years later the tach finally quit working. I decided to tackle it now since I think I have a low-idle problem to troubleshoot. Running through the TMs diagnostic instructions, I determined that the tach was okay and the induction sensor was dead. I think I see why it quit working...



It was never set right from the factory! The installer just screwed it into the flywheel housing until it was touching the teeth and didn't back it off a turn and a half as specified in the TM. The jambnut locked it into this position and the flywheel ground it down over 4000 miles to the point it probably broke the inner coil wire, rendering it useless.

By the way, it was a PITA to take out. I had to remove the toe board, which itself was quite a chore. I had to free up the intermediate tunnel to slide the toe board out sideways. The shift cable and wiring harness to the shift tower allow enough wiggle room for me to do this without removing the shift tower - at least I didn't feel anything pull hard! Theoretically I can connect the new one through the engine bay. We'll see!

 

[ToddJK]

3 years later the tach finally quit working. I decided to tackle it now since I think I have a low-idle problem to troubleshoot. Running through the TMs diagnostic instructions, I determined that the tach was okay and the induction sensor was dead. I think I see why it quit working...

View attachment 901657

It was never set right from the factory! The installer just screwed it into the flywheel housing until it was touching the teeth and didn't back it off a turn and a half as specified in the TM. The jambnut locked it into this position and the flywheel ground it down over 4000 miles to the point it probably broke the inner coil wire, rendering it useless.

By the way, it was a PITA to take out. I had to remove the toe board, which itself was quite a chore. I had to free up the intermediate tunnel to slide the toe board out sideways. The shift cable and wiring harness to the shift tower allow enough wiggle room for me to do this without removing the shift tower - at least I didn't feel anything pull hard! Theoretically I can connect the new one through the engine bay. We'll see!

Probably at some point during some maintenance, one of the motor pool guys too much in a hurry or didn't care, just did the job to get it done and sent the truck on it's way. I'm actually surprised the flywheel didn't break that sensor earlier.

 

[HDN]

Probably at some point during some maintenance, one of the motor pool guys too much in a hurry or didn't care, just did the job to get it done and sent the truck on it's way. I'm actually surprised the flywheel didn't break that sensor earlier.

From what I could tell the speed sensor was never replaced before. I'm pretty sure it came from the factory like that.

There's no maintenance interval for the sensor itself - just replace it when it's not working. I'm surprised as well that it worked for as long as it did! It makes me wish I got to it sooner but for all I knew at the time it was a ground issue.

At least when I'm done there are going to be a couple less stock rubber floor mats holding moisture and a toe board that I can get up without having to free-up the mid tunnel section.

 

[HDN]

I installed the new sensor today. I managed to start it in the threaded flywheel housing hole from the engine bay, running my arm underneath the steering shaft and squeezing my hand around a ground strap to get to the spot. After I got a good bite in the hole, I went into the cab and finished screwing it in from the opening where I removed the toe board.

I screwed it in until I hit the flywheel and backed it off a turn and a half. I went back under the hood and connected the sensor to the harness. I fired the truck up and now the tach works!

I attached pics comparing the old sensor to the new one. The new one is potted differently and has a hex at the top. That made installation pretty easy as I was able to use the hex sides to count my turns for adjustment. The jambnut is a bigger hex size than the original jambnut.