Is an Engine Speed sensor necessary?

[Andyrv6av8r]

lol.. you wrote it so odds are you remember everything.... for everyone else... that info was nearly two pages back and written 3 days ago thus it is not stuck in folks heads as much as it is in yours. I wish was granted with photographic memory.. but am not probably most in here don't have it either. Thus the question asking for clarity on that point.


hmmm....so after your last test and it went back to #2 on keypad..... did you only have second gear again or is is just showing #2. That was not specified and readers should not assume.

Maybe you have a bad TCU?

Don't recall but does the tranny default to 2cnd as a limp mode? If so; maybe it is not speed sensor related but actually something else is up and your getting put into a limp mode? could the previous destroyed speed sensors have left little shrapnel in there causing problems?

Transmission will not shift out of 2nd gear if the sensor is disconnected. It shifted normally after reconnecting it and erasing the 25 11 code. It's not shrapnel from the last one; it won't shift, at least on mine, until it sees a good speed sensor.

 

[coachgeo]

Transmission will not shift out of 2nd gear if the sensor is disconnected. It shifted normally after reconnecting it and erasing the 25 11 code. It's not shrapnel from the last one; it won't shift, at least on mine, until it sees a good speed sensor.

On drivers side of tranny there is an aluminum plate with numbers on it. can't really see it cause it faces the chassis rails. I had to use my phone stuck up in there to take pics to read it. Use that to see what model tranny you have and let us know. Serial number is on this same plate. Might be a silver sticker with same info. Looks like you have a Wtech II shifter so would not expect you to have an upgraded transmission but we should not assume I guess.

 

[Ronmar]

OK, did some test drives to get a feel for the drivetrain(pulled a little to the left when braking) and did them all with the RPM sensor disconnected. Pulled the sensor and did some intital tests in my yard, forward/reverse 100’ runs AOK. Took it down the drive and out onto my road to work the speed up some. I live on a dead end rural road so I couldnt get up to too much speed but managed 6th gear, all without issue... This agrees with the A0 schematic that says that sensor output dosnt connect to the trans...

So I guess the question is what exactly do you have, and if it is an A0, why is it different than the documentation? You could hand over hand the wire. It could be that perhaps you had a transmission sensor failure and some enterprising tech patched that RPM sensor in instead of repairing the trans...

Mine is a 98 m1079A0 chassis with the 3116 engine. Here are pics of the sensor and the trans data plate... Coachgeo, my data plate was on the passenger side. Photo with an I device was a great idea, couldnt read it without a mirror...

 

[coachgeo]

there is also a RPM sensor that pretty sure is even on the 3116..... make sure you are not confusing the two sensors. Apologize I don't know specifically where each is. From my bad memory from what read in past I thought the speed sensor was closer to bottom of tranny on the side.

for getting more specific on which tranny may work with the speed sensor disconnected...... here is some info from https://fmtvtrucks.com/fmtv-history/ that gives some detail on progression of transmissions... how accurate it is I don't know

"...The Allison 3070 SP seven-speed transmission[1] fitted to A1P2 and A1R FMTVs has also evolved with the FMTV, its A1 designation being MD 3070 PT, its A0 designation being MD-D7...."

Will try to get another picture of my A0's plate for comparison

 

[Floridianson]

there is also a RPM sensor that pretty sure is even on the 3116..... make sure you are not confusing the two sensors. Apologize I don't know specifically where each is. From my bad memory from what read in past I thought the speed sensor was closer to bottom of tranny on the side.

Since I got the M915 I have forgotten to keep up on the FMTV stuff. From the TM I just looked over I believe I saw the Trans speed sensor was on the right side of trans. about 4 o'clock next to a box. RPM was left side 9.oclock on bell housing. Just found this looking around but there is a interface module with fuses / relays in side. Strange place to put fuses. Sorry my printer ran out of ink.

 

[Ronmar]

there is also a RPM sensor that pretty sure is even on the 3116..... make sure you are not confusing the two sensors. Apologize I don't know specifically where each is. From my bad memory from what read in past I thought the speed sensor was closer to bottom of tranny on the side.

That was why I asked the OP to make sure we were talking about the same sensor, on the yellow cat bellhousing, right behind the block, about the 2 O'clock position when looking toward the rear. He confirmed this...

I have no other sensors on the cat half of the bellhousing(yellow). The only sensor I find on the transmission is on the lower passenger side, about 4:30 looking forward like Floridianson suggests. It is about 2-1/2” behind where the trans and the cat housings meet, which is too far back to sense the flywheel I Think, so must sense something on the torque converter. Its line goes aft and joins the main transmission wiring harness... On My FMTV the VIM is located up behind the passenger kickplate next to the flasher module above the PCU...

 

[Suprman]

Bottom of trans is the output speed sensor. There is a throttle position sensor on the engine it’s a box over the fuel separator with a cable that goes to the throttle. And the engine speed sensor on the bellhousing. The trans control is supposed to share sensor info with the ctis controller so it can calculate overspeed warning depending on pressure setting.

 

[Ronmar]

Yep, that same speed signal that feeds CTIS also feeds the speedometer...

I think the sensor near the bottom of the forward transmission bellhousing is another engine/input RPM sensor as it is so far forward on the housing. It is labeled “transducer, motional”. A vehicle speed sensor would have to be near the output shafts on the transfer case after all the gear ratios have modified the final output RPM’s. The diagrams in 24P show a sensor down inside the transfer, but the description dosnt give a clue as to what it does. There is a cable connection down on the transfer, but there is the word “clutch” cast into the housing next to it which leads me to believe it is for the center diff-lock and 4WD operation. But it is in the area where that other sensor is located so it may carry final output RPM/vehicle speed info. The Allison controller may also derive a vehicle speed output electronically based on input RPM and gear selected... Havent come across a description of how that signal is generated though...

 

[Suprman]

Transducer motion also is the output speed sensor. It drives the speedo. When that sensor starts going bad your speedo will act up.

 

[coachgeo]

ahhh... so question is... when someone says they removed speed sensor and truck runs great.... sounds like there needs to be some clarification on which sensor.... now that we have identified 2 or is that 3 different ones?

 

[Coffey1]

This is the sensor that has no effect on my 1088 3116 other than tac not working after unplugging it.
And tm's are calling it speed sensor.

 

[Ronmar]

Yep, but I am thinking they are meaning engine speed(RPM)

 

[Andyrv6av8r]

View attachment 742279
This is the sensor that has no effect on my 1088 3116 other than tac not working after unplugging it.
And tm's are calling it speed sensor.

But when I disconnect that one, I'm stuck in 2nd gear almost immediately.

 

[Floridianson]

Yep, that same speed signal that feeds CTIS also feeds the speedometer...

I think the sensor near the bottom of the forward transmission bellhousing is another engine/input RPM sensor as it is so far forward on the housing. It is labeled “transducer, motional”. A vehicle speed sensor would have to be near the output shafts on the transfer case after all the gear ratios have modified the final output RPM’s. The diagrams in 24P show a sensor down inside the transfer, but the description dosnt give a clue as to what it does. There is a cable connection down on the transfer, but there is the word “clutch” cast into the housing next to it which leads me to believe it is for the center diff-lock and 4WD operation. But it is in the area where that other sensor is located so it may carry final output RPM/vehicle speed info. The Allison controller may also derive a vehicle speed output electronically based on input RPM and gear selected... Havent come across a description of how that signal is generated though...

Looked today 97 MTV 3116 and there is another one passenger side close to tail shaft.

 

[coachgeo]

But when I disconnect that one, I'm stuck in 2nd gear almost immediately.

can each of you follow that wire to see where it goes? Andy maybe yours is misswired?? either by accident or as a work around before you got it ??

 

[Coffey1]

No it goes into a wiring harness with wire protection around all the wires.
Not taking all that off.

 

[coachgeo]

No it goes into a wiring harness with wire protection around all the wires.
Not taking all that off.

number(s) on the harness nearby?

 

[Floridianson]

I don't know was looking at TM 9-2320-365-20-2. Page 2-1450. and code 25 starts on page 2-1448. Code 25 sub code 11 oil level, fuse, batteries, output speed sensor. If those were good then from there it wanted pressure test or replace WTECII TEPSS. Hope I don't confuse this maybe simple thing.

Sorry had to correct TM number can't read my own hand writing.

 

[coachgeo]

I don't know was looking at TM 9-2320-363-20-2. ....

I was asking about actual harness wire numbers hooked to yours so Andy can compare with his to see if his harness has been modified or if a different harness there etc. Its just odd others can disconnect and he cant....seems somewhere something is different.

 

[Floridianson]

I was asking about actual harness wire numbers hooked to yours so Andy can compare with his to see if his harness has been modified or if a different harness there etc. Its just odd others can disconnect and he cant....seems somewhere something is different.

I can't remember what I had for dinner last night and can't see crap without my glasses and you want me to do what? OK next time I go to the property I will look.
Also sorry had to correct tm # in above post #38 the other was for 915.