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Old 06-29-2017, 07:41 PM   #111
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the next chapter!!! alas I poured in the transmission fluid, started the bus and no leaks! feelin pretty good yeah... put it in D and the wheels in the air started to turn.. yeah!! cool!! oh wait.. the dashbaord computer says VSS fault.. well drats i mustve wired it backward... reverse repeat.. and error code 141 you have become my next challenge!!

so after probing around with a meter and hitting the books heavy I learned that Navistar has a fairly elaborate system of making sure your Vehicle speed sensor is there and works..

first off, they have a sensing ciruit that when the speed isa reported at zero.. looks to see if there is 2-3 volts DC returned to ground on each leg of the sensor.. WTF? wierd.. esp since the sensor is an AC circuit... nevertheless the original sensor has an internal resistance of 1400 ohms.. .. leave it shorted or open and it triggers a code as one or the other leg shows as LOW.. this means the speedo and the cruise control are defunct.. it turns out that the resistance between the 2 legs is very forgiving.. it ships out about 3 volts on a pullup open drain type circuit and expects to get at least 2 back.. any more than 5 and it triggers a high (shorted to power) condition..

so enter my TCM speedo output.. it is a normally high 12 volt open drain circuit.. so of course when I connected it to the + on the Navistar ECM circuit.. it triggered a code 142.. VSS circuit high.. spin the tires and clear the code.. all is well the speed signal was received and appeared to be correct.. stop the wheels and that checking routine goes in effect and see the over-voltage and BAM! ..

so i installed a Dakota digital SGI5-E VSS sensor adjuster / recalibrator.. that one provides a low out.. and appears to have much more bias and holds down the circuit when there is no signal present (wheels stopped).. so it tripped a code 141.. VSS circuit LOW..

welcome to the jungle baby.. im deep in it now!!



time to grab the transmission scan tool and connect it.. because if I go over 140 degrees F trans temp im screwed.. it enables the learning mode.. and that we dont want to happen till we want to teach it.. plus i needed to monitor the Output shaft speed on the trans.. the other piece of making this work is making sure that while my dash can see the signal so can the transmission!.



my god its full of wires!!! yes.. resistors and wires... a lot of meter / measure/ calculate and try again.. since i have no idea the values of navistar's bias resistors on their end i have to just do a bit of guesswork and trial and error..



by george i think i got it!!! I still set a code occasionally when I restart the truck.. the values of these resistors saeem awfully high so i may adjust them a bit. with Pots and see if i can come up with something thats not on the edge literally... nevertheless I came up with the schematic below that so far works on all accounts.

this is leaving the Dakota box out and letting the ECM pullup voltages do the work.. i felt that was better than iontroducing extra voltage against an ECM.. even though the ECM fully expects it as theres AC present when the speed sensor is sensing..



next is to play with the values of the resistors a bit, put it all together nicely and see if it continues to work and drive it..

I MAY have to put a 555 timer circuit in for when I first start the bus until the ECM and TCM boot fully before letting the VSS signal apply...

-Christopher
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Old 06-29-2017, 07:53 PM   #112
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Originally Posted by cadillackid View Post

I MAY have to put a 555 timer circuit in for when I first start the bus until the ECM and TCM boot fully before letting the VSS signal apply...

-Christopher
Great thread Christopher!!

Thank you for sharing you hard work and learning experience with us.

One thing..... I had you pegged as being quite a bit younger than I am until you made that comment. I don't think anyone under 45 know what a 555 is.....

Thanks again for sharing this with us. I can't wait to see you on the road with the 2000.
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Old 06-29-2017, 08:06 PM   #113
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Am I correct in stating that the TCM wants to see a VSS of 2 to 3 volts DC and it flips an error message when it does not?

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Old 06-29-2017, 08:13 PM   #114
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dag! you are good!.. 48 here stayed un-married all my life so im still young and kickin..

and of course there are alot newer things to use than a 555.. but really its super simple and easy to use.. for what I need it for its perfect.. or i could just buy something like this

https://www.amazon.com/Timer-Delay-R.../dp/B00PD65UGA

wire it up and be done.. LOL



i can feel it now!! i can taste that bus ride now!! Hot summer night road trips in the skoolie!! my favorite times to travel!.

i noticed with the new firmware update in my ECM that my engine runs quieter.. less injector cackle.. not sure why I kept ignoring the firmware updates till now..


-Christopher
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Old 06-29-2017, 08:19 PM   #115
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Quote:
Originally Posted by Tootalltechie View Post
Am I correct in stating that the TCM wants to see a VSS of 2 to 3 volts DC and it flips an error message when it does not?

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the ECM. (the navistar bus ECM). wants to see 2-3 volts DC between each leg of the VSS circuit and ground. when the VSS AC speed signal is zero.. ie when the bus is stopped..

the Transmission has the VSS sensor.. (40 tooth tone ring), which is fed to the TCM (transmission computer).. the TCM provides a VSS Output which is designed to feed into the speedometer circuit.. or in this case into the original navistar VSS circuit where the sensor for the AT545 went.

in the factory Allison 1000/2000/3000 busses, there is a comm link between the TCM< and the ECM and this data is shared across that.. so the ECM shares some data with the TCM and vice versa.. since my transmission is run standalone I dont have that comm link.. in fact the pins werent even wired in my ECM, so my ECM may be too early for the factory electronic allisons.. my bus was built right around the time they were introduced.
-Christopher
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Old 06-29-2017, 08:19 PM   #116
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The 555 is the cats meow. I use to use them for everything.

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Old 06-29-2017, 08:26 PM   #117
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Oh my mistake my brain was thinking VSS was voltage steady state. DOH!

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Old 06-29-2017, 08:32 PM   #118
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The 555 is the cats meow. I use to use them for everything.

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I have probably purchased a few hundred of them for one off & prototypes. Thousands for production use. But... That was 25 years ago.

I was at the local Makerspace and offered a 555 solution to a "how do I" question and everyone looked at me like I was from Mars....
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Old 06-29-2017, 08:44 PM   #119
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ive seen two VSS over the years.. most common is an AC sine wave.. there is a type that produces an all above zero square wave.. most common is AC. but having an ECM actually care enough about the VSS to use detection code when its in zero state is something ive not run into before.. perhaps a commercial truck thing so the DOT could check for VSS faults in the event people try and disco the sensor to fake their logs.. i dont know.

the nsvistar ECM doesnt have huge requirements on the amplitude of the signal.. i think they count it as a pulse anytime zero is crossed. they say 2 volts AC which most sensors put out double that.

here is the manual page that talks about testing the VSS circuits..





-Christopher
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Old 06-30-2017, 12:16 AM   #120
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Did I accidentally reset the language here somehow???

You guys are talkin WAAAY over my head.
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