Tachometer quit working when crankshaft and camshaft sensors replaced

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Tachometer quit working when crankshaft and camshaft sensors replaced

October 31, 2023, 09:44:51 am

On my 1999 U270, a faulty crankshaft sensor caused me to put it in a shop and they replaced both it and the camshaft sensor. I did not notice until getting it home that the tachometer quit operating. I have power to one of the terminals on the back of the tach with the engine running but that is as far as I have gotten in diagnostics. The compressor had to be be removed to replace the sensors so I am afraid something happened there to cause the problem. It took the diesel shop a lot of hours to diagnose and repair the slow starting issue so I am hesitant to take the coach back there.
Has anyone had this happen or have any suggestions ?

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #1 – October 31, 2023, 11:14:22 am

I have replaced both of my sensors and no issues with the tach. I did not remove the compressor, although it would make the job easier. Do you have any check or stop engine lights on the dash?

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #2 – October 31, 2023, 12:53:43 pm

On a non-computerized 8.3 engine, Foretravel counts engine revs with a sensor that is bolted to top of engine bell housing that is visible from an open bed. This sensor is known to get loose and prevent tach from showing rpm's. Even with computerized engines where the computer may have RPM output signals, Foretravel is known to simplify and still use add-on sensors like the bell housing tach sensor.

Dash tach gets ignition voltage to turn tach on with 2-terminals. Another terminal receives pulses from engine tach sensor. Tach is adjustable as to how many pulses make up one RPM.

Look on top of your bell housing for a tall narrow round sensor bolted on with a wire connection.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #3 – October 31, 2023, 03:09:49 pm

Thanks Bruce ! No the check engine lights is not on.

Thanks Barry ! I will look for the counter on the top of the bell housing from below the bed. The tech that did the sensor replacements may have disturbed it.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #4 – October 31, 2023, 09:50:41 pm

I don't recall any sensors in my bellhousing. I'm fixing to take a look at the wiring schematic for some othe issues. But I think your ISC picks up tach signals different than 8.3. I just glanced at the wiring diagram looks like the tach signal comes from the engine interface (ECM)

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #5 – November 08, 2023, 12:16:59 pm

Thanks! Yes I did not see a sensor on the top of the bell housing.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #6 – November 08, 2023, 01:30:46 pm

My bad mine has a sensor in the d/S of the bell housing

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #7 – November 13, 2023, 03:37:58 pm

I am still trying to chase down the problem with my tach haven quit working on my 1999 U270 with 8.3 ISC 350 after the crank and camshaft sensors were replaced. Perhaps the addition of my engine and model might spur additional diagnoses suggestions or pictures.
Thanks in advance.

Mark

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #8 – November 13, 2023, 09:39:44 pm

Quote from: bbeane – November 08, 2023, 01:30:46 pm

My bad mine has a sensor in the d/S of the bell housing

The d/S?

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #9 – November 13, 2023, 09:45:36 pm

Drivers side

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #10 – November 14, 2023, 01:03:42 am

Thanks Richard as I was confused on the d/S.

Mark

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #11 – January 07, 2024, 09:22:08 am

Further to this post and unresolved problem, I have tried actuating both black switches behind the dash instrument panel, with the engine running and off. The one on the left is an on-off toggle and the one on the right seems to be a momentary switch. The tach needle pegs (full on) when I crank the engine but then returns to '0'.
I took a picture of a sensor on the fore+end of the transmission bell housing. Is this the rpm sensor discussed in a previous reply or a temperature sensor or other ? It appears to be undisturbed and intact.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #12 – January 07, 2024, 11:22:11 am

Have no experience with computerized ISC, but I am quite certain that the photo of the bell housing shows a different kind of sensor that feeds information to the engine computer, not to the dash tach.

Also assume the crankshaft sensor also has engine RPM info, and since they were worked on, the problem lies somewhere else. Foretravel may know where the dash tach picks up the pulse count and I assume it is from the engine computer.

Seems like who ever worked on the engine sensors may be the place to go. Getting a diagram of the engine computer will go a long way to helping diagnose your tach.

BTW, the bell housing photo shows corrosion that may have come from driving on salted winter roads.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #13 – January 07, 2024, 02:29:49 pm

There have been a few of us on the Forum who have had there tac quit with no luck in figuring out
hot to fix it. As I have a Silverleaf and the Tac work on it, so it hasn't been a priority. The bell housing
sensor may be a speed sensor for the transmission.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #14 – February 08, 2024, 06:45:19 am

Further to my previous posts on this unresolved issue - upon engine start up the tach needle does go rotate from full peg (right) back to zero, up again. Once recently it stopped around 2,000 rpm but was static. Next start up it went from full peg back a couple of times then stopped again at full peg. I need to look again at the bell housing with good lighting where the problem probably is. Thanks for everyone's help !

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #15 – February 08, 2024, 09:46:21 am

When I was working on my ECM I checked and the tach is feed from my ECM. I also checked
and I have a voltage change with revving up the engine. The tach also swings back and forth
with turning on the ignition.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #16 – February 08, 2024, 10:18:37 am

I dont know if u have the test switches under the dash cover. One was depressed on mine which disabled the tach. Pushed the button to release it and tach works now

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #17 – February 08, 2024, 01:16:00 pm

Yes I have tried changing the position of those. Keith Risch showed me where they are located when I was at his place in Nac last month. Thanks for posting that as many are unaware of their existence, as was I.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #18 – February 08, 2024, 04:33:58 pm

On some coaches that has a 2 way momentary switch (as 1 of the 2 ) it is used to raise/lower idle speed set point. Seems like for every push it moves the idle set point about 75rpm. This is totally different than using the cruse control for raising idle speed.

Mike

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #19 – February 09, 2024, 11:34:00 am

Thanks Mike!
That is valuable to know about the momentary switch. If I get brave I may try it.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #20 – February 09, 2024, 12:29:27 pm

On my coach Mike the momentary switch moves the RPM up or down 25 RPMs.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #21 – February 09, 2024, 03:54:38 pm

Mark,

If you get to happy and get the idle to high the trans. won't go into gear.

@ Peter,
I should have reread that as ours it about 25rpm like you have.

Mike

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #22 – February 28, 2024, 07:38:21 am

Further to this unresolved issue, I looked again for the RPM sensor for the tach from under the bed. I did not see anything on the top of the bell housing. If anyone with this engine could post a picture of it this will be appreciated. Now sometimes when I start the engine the tach needle goes from full to zero peg and then stops at about 80% full, whereas the engine is running at idle. The tach needle remains in this spot when driving. I have again tried toggling the two position switch under the instrument panel cover and actuating the momentary switch next to it with still no change to the non operating tach.

Re: Tachometer quit working when crankshaft and camshaft sensors replaced

Reply #23 – February 28, 2024, 08:56:23 am

Do you have a picture of the wires going into the back of the tach and or a wiring diagram to see where it's getting it's signal? Since this issue occurred after the shop replaced those sensors, my first guess would be that they bumped something next to the sensors and disconnected the tach sensor. Or the tach is getting the signal from the engine ECM and has just died. I have no clue if they are using CAN for the tach or a hardwired sensor. If the tach is bad, you should be able to replace it with a CAN based J1939 tach and just read the speed from the engine ECM.

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