Can anyone tell me what type of speed sensor I likely have?

[desrtdog]

Replaced my ECM yesterday due to ongoing issues that are fully documented in the thread here, and the issue was fixed / bus started fine, no comm fail message. However upon starting, I now have a fault code SPN 84 / FMI 2 (Cummins code is 241). Description for this code is: Wheel-Based Vehicle Speed - Data Erratic, Intermittent, or Incorrect…seems to relate to the vehicle speed sensor on the rear of the transmission. I have yet to drive the bus to see what the impact is, but looks like this could impact max engine speed, cruise control, gear down protection and road speed governor.

As a background, when I got the replacement ECM (refurbished from GoECM) they loaded a calibration directly from Cummins that is the same as my previous - I think….I say this because my previous ECM was not readable, so I have no idea what the changeable parameters were in that program. (I am however pretty sure about the calibration code as it is the same as what was stamped in my old original ECM.) One of the adjustable parameters is the Vehicle Speed Sensor Type….from the printout I have of the programming, this is set to “Data Link - Tachograph”…. I think this may not be the actual sensor type, and that is why I get the active fault code at key-on - i.e., because the ECM is looking for a different particular type of sensor than what is actually installed on the transmission.

Cummins documentation on this fault code mentions that the speed sensor is an OEM installed component (so Blue Bird - whom I’ve already called and they can’t find any documentation), and that there are various types, including: magnetic pickup, data link, digital and tachograph…I am leaning towards an ECM setting as being the likely culprit here, as that is the first step in troubleshooting…

So here’s the Q: anyone out there have a clue as to what a 2010 BB with Cummins 6.7 ISB and Allison 2500 PTS might have? @cadillackid maybe?

 

[magnakansas]

I think I might be able to answer this. The TCM, transmission control module, reads drive shaft rpm, along with a bit of programming, there is an output for connecting to a tachometer. That is the most likely.

Some rear axles have a vss, vehicle speed sensor, that feeds into an engine control unit, then is connected to a tachometer. I am going to put my money on the transmission route.

A wiring diagram that has tachometer wiring shown would probably help.

William, 7852077600- telephone

 

[nikitis]

There's magnetic tone generators which are used on older buses like mine. A magnetic wheel spins as part of the transmissions spline, and sends pulses to the ECM to determine speed based of frequency of tones.

Then there's a more electronic version that comes out the TCM's of J1939 Transmissions like Magnakansas here mentioned. I believe your bus is the newer type, and I believe he is correct. I'm not well versed beyond how to hook it up, but if your transmission was not hooked up to your ECM you'd be getting more than just the one code, so I'm not sure why it's not getting the values. My guess would be something wrong with the ECM values. I believe you can select what type you have in the ECM if you have access to change values,

The ECM may be set up to look for tone generated type as part of the calibration it received, and isn't finding a tone because it doesn't exist, and it's just a matter of switching the value to the newer style. You may have to go back to the GoECM guys and say hey my ECM isn't reading the speed values from the trans and needs a value change. It's technically an error on their part for not testing this during install.

 

[desrtdog]

Thanks for the comments above…A few points for clarification:

1. This error is appearing at engine start - before the driveshaft is turning or the bus is ever moving. As noted above, I have not yet driven the bus.
2. GoECM provided a calibration for the ECM from Cummins - as noted, it appears to be the same code as was in my old ECM based on what was stamped on the old one. Having them recheck it will require me to remove it and bring it down to them…
3. While the Allison tech I spoke to yesterday echos some of the same comments as William (re the TCM reading vehicle speed), the Cummins TS bulletin for the error code clearly shows a link from the VSS to the ECM…first step in troubleshooting is to ensure the setting in INSITE arches the speed sensor type in the vehicle.
4. Attached is the relevant sheet from the calibration that was loaded…3rd to last line at the bottom is what I am asking about…just looking for a way to maybe visually ID the sensor type I’ve got, or if someone with a similar year/model bus knows.

 

[nikitis]

A tachograph is like an engine hour counter. Are there other options to choose from? List them all for us.

Those are used for keeping track of hours a driver drives trucks usually, and not something put on buses often that I've seen.

If there is a hour counter then it could be the right option.

 

[desrtdog]

Turns out the correct VSS choice was “tail shaft”… Finally had a guy come by a couple of weeks ago and he first changed it to “Magnetic” with no resolution of the fault code. After a few tries of not getting the ECM to accept any other option, finally got the change made and … boom … fault code cleared.

Took the bus for a drive around the storage unit lot for a bit last night, and seems to run/shift fine, so problem solved until the next one (and I’m sure there will be a “next one”) arises.

 

[Henry25]

Your issue is most likely a parameter mismatch in the ECM, not the sensor. On a 2010 Blue Bird ISB 6.7 with Allison 2500, the ECM should be set to get speed from Data Link – Transmission, not “Tachograph.” Update that setting in Insite and the fault should clear.

 

[desrtdog]

Your issue is most likely a parameter mismatch in the ECM, not the sensor. On a 2010 Blue Bird ISB 6.7 with Allison 2500, the ECM should be set to get speed from Data Link – Transmission, not “Tachograph.” Update that setting in Insite and the fault should clear.

Thanks. So I did get it resolved by having the source changed to “Data Link - Tailshaft” … at least that is what the guy said it was (didn’t actually see the screen afterwards…