Is an Engine Speed sensor necessary?

Is an Engine Speed sensor necessary? Answer in post 56.

I have an issue with my 1997 1078 that I can't figure out. About a month ago, I started on a drive in the 1078 and the transmission stayed in 2nd gear before I went 15 feet with the 7 disappearing from the left side of the shift pad. I couldn't shift to neutral or reverse after stopping, it just stayed in 2nd. After doing some research and reading the codes, 25-11, I determined that the Engine or Output speed sensor was bad. Sure enough, the end was ground off of it. So I ordered the generic NAPA unit pn NHD 6569002and it seemed to work fine. I have it about a 1/3rd of a turn off of the flywheel. But after driving it a couple of times, it did the same thing again. Readjusted a bit and it's working again.
Which leads me to this question. A few years ago I owned a 1079 that I noticed had the sensor wires cut and not hooked up. I never had an issue with the truck and at the time, didn't even know what it was. So I taped the bare wires and let it go. Coffey1 just disconnected his on his 1088 and the only issue was the tach not working; it ran and shifted fine. Which leads me to believe that there has to be a way to bypass this thing. Anybody have any input?

If you have a 3126 or c-7 your going to need it. also your transmission is going to know the input speed in order to manage the shift schedule.

Oops, i was thinking it went to the trans controller...

On the A0 schematic the RPM sensor goes to the tach plug and the freq divider box. The freq divider output goes on to the STE/ICE Plug. Trans RPM appears to be sensed internally in the transmission...

So I had another friend disconnect his speed sensor from his LMTV. The results were the same as Coffey1's; it ran and drove fine. Evidently some trucks need the sensor and others don't. Anyone know the difference? I adjusted my sensor for voltage output as was recommended in another thread and I found that it was WAY off. 1/3 turn from flywheel contact gave me 12+ volts on the sensor. After screwing it out about 4 turns, I settled on around 2 volts output. I erased the 25 11 code and did the WTEC II/III Transmission and Throttle Position Sensor calibration per the TM. The truck drives and shifts fine now so I hope it will continue to do so. Question is, why is my truck different than 3 other LMTV's that I know of in needing an output speed sensor when they don't?

Snowtrac told you why. The HEUI 3126 and C7 use electric over hydraulic fuel injection. Their ECU’s may use that signal, the MUI 3116/3126 are dumb old diesels and dont need it unless you want a tach...

I think your trans issue was not related to this sensor. Quick check is to disconnect it and test drive...

From what have read in this thread and elsewhere..... it appears the 3116 A0 trucks "engine" of course does not "need" speed sensor. at all.... the tranny on these does not "need" it either, but functions better with it. Pretty sure without speed sensor input .... the tranny program goes to a default shift pattern .... yet will seek the most efficient shift pattern with speed sensor.

so in summary..... appears that anything beyond a 3116 truck "must have" it.... and the 3116 trucks only "prefer" it.

So just to make sure one more time, I unplugged the sensor, rolled about 30 feet and got this once again:



Hooked it back up, cleared the code and it drove fine. So does that mean I have a different transmission perhaps?

Got a pic of the sensor? What year is your vehicle?

Andyrv6av8r said:

So just to make sure one more time, I unplugged the sensor, rolled about 30 feet and got this once again:

View attachment 741953

Hooked it back up, cleared the code and it drove fine. So does that mean I have a different transmission perhaps?

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what are we looking at in this pic??... are you in drive??.... are you in neutral??..... are you in reverse??? Are you moving? If in drive.... looks normal to me if your foot is on the brake and not moving yet....... the truck's take off gear is 2nd. You will never see 1st gear unless your in offroad MODE (4x4)

Starting in second is correct, but If it were normal, it should have a 7 off to the left indicating speeds available up to 7... I will look over the A0 schematics again tomorrow, but the only speed sensor I see does not connect to the Allison at any point that I have found. That was why I want a pic/location of the sensor he is dealing with to make sure we are talking about the same thing...

has he EVER seen anything on the left.. could be just the display is dead there?

coachgeo said:

has he EVER seen anything on the left.. could be just the display is dead there?

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If you would read the second sentence of my FIRST post you will see that the 7 disappears from the keypad when this thing locks into 2nd gear and won't clear until the truck is stopped and shut down. I know a little more about these trucks than you give me credit. Neutral should be N N. Reverse should be R R. Normal drive should be 7 2.

Ronmar said:

Got a pic of the sensor? What year is your vehicle?

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It's a 1997 and the sensor is a new Output Speed Sensor from NAPA. Looks like the original except plastic.

The sensors are all pretty much the same. If it fits the hole, it will work.

Andyrv6av8r said:

It's a 1997 and the sensor is a new Output Speed Sensor from NAPA. Looks like the original except plastic.

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where is it physically located on the engine/transmission? I just want to make sure we are all on the same page. The one on my 3116 engined 1079 is on top of the cat bellhousing right behind the block left of center...

Mine was a bit of a basket case with a ton of electrical issues. The instrunment panel dimmer is the last one I ned to fix now, but because of this I have been spending a LOT of time in the schematic for the A0. The schematic shows no connection between that pickup and the trans. IF we are talking about the same sensor, I will try your unplug test on mine to see what happens.

Bell housing, left side about 2 o'clock looking aft.

Andyrv6av8r said:

If you would read the second sentence of my FIRST post you will see that the 7 disappears from the keypad when this thing locks into 2nd gear and won't clear until the truck is stopped and shut down.....

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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?

Andyrv6av8r said:

Bell housing, left side about 2 o'clock looking aft.

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OK, I am about ready for some test drives, I will try disconnecting mine.

Some trucks appear more sensitive to that sensor. It may be something in the programming. I have had trucks appear to drive ok with a bad engine speed sensor.

the a-1s we had in Iraq would default to second gear as I remember but it might have been 3rd. I do remember when limp home they had no reverse. the complaints on the 5988-e's all said the vehicle had no reverse as the trucks never hardly left the FOB. nobody noticed the transmission never shifted.