1992 Jeep Wrangler with 350TPI/700R4 VSS Options

[sturgillbd]

I found a 74ls74 and wired it up on the breadboard. It works but the speed varies 6-8 mph at low speed. I uploaded a video of it and you can see why. I don't know exactly why it doesn't like the low speed but the pulse train doesn't clean up until the input frequency goes up. It was a test with an old chip. Could be the chip or just trying to use the chip at a really slow speed compared to what it is designed for. The video will show what I am describing. I'll take the divider out of the circuit and load up the new bin.
https://youtu.be/5dkDiUMUVbs

[lionelhutz]

Try the cap and resistor into the magnetic input. It'd be simpler and let the magnetic VSS pulse setting function.

[sturgillbd]

I found my issue with the divider circuit. I needed to tie the preset and clear pins high. They were in the gray area and were getting triggered sporadically causing the irregular output. The divide by 4 now works great. It has a smooth transition from 2mph through 190mph in the datastream. It also shows 60MPH with 133Hz feeding the divider.

[sturgillbd]

If anyone is interested, here is what the circuit looks like on a breadboard. This breadboard has seen better days. First pic is the circuit. Second is of dash in tunerpro with ecm running S-AUJP with signal of 133 hz feeding divider. Next is of signal gen at 133 hz and output of divider on the scope. Last is of signal gen at 6hz and what the divided signal looks like. All stable.

[JeepsAndGuns]

Just to keep track, are both of these comparisons against the the TEST1_ANHT file using optical sensor input on B21?

Anyway, here's the latest file. The VSS road speed constant is set to approximately 8000 pulses but can be changed in your favorite cal editing program. lmk.

My tests were with test1 and test2 bin files you made. The test2 bin read much faster than test1. And yes, it was inputted to B21.
I will give the new bin a try and report back this afternoon.


Another thing, we don't have to be dead set on making the jeep vss work. Remember I tested the gm truck magnetic vss and 40 tooth wheel and it worked fine on the magnetic inputs, and the output of the ecm did run the jeep speedo, it just read wrong. If it could be easier to change the output of the ecm, we could try that too. It is not really difficult to change the tailhousing on the transfer case. In fact I already have one I pulled from the JY.

[sturgillbd]

I ran a quick test on the test3 bin. It appears to be wanting a 4kppm signal on the optical input (B21 in my case 7727) . I changed the vss pulse parameter to 16024 (which Tunerpro saved as 15890) and it appears to accept the 8kppm signal. Tunerpro dash data follows my calculated input frequencies for speeds.

[lionelhutz]

Remember I tested the gm truck magnetic vss and 40 tooth wheel and it worked fine on the magnetic inputs, and the output of the ecm did run the jeep speedo, it just read wrong. If it could be easier to change the output of the ecm, we could try that too. It is not really difficult to change the tailhousing on the transfer case. In fact I already have one I pulled from the JY.

That combination of 40 tooth sensor and 4000ppm output will only work to produce a 8k output if the tire revolutions per mile times your gear ratio times 40 is equal to 2x one of those 7 values 1project2many posted way back in post 13.

[1project2many]

ran a quick test on the test3 bin. It appears to be wanting a 4kppm signal on the optical input (B21 in my case 7727) . I changed the vss pulse parameter to 16024 (which Tunerpro saved as 15890) and it appears to accept the 8kppm signal. Tunerpro dash data follows my calculated input frequencies for speeds.

Excellent! I suspected you'd end up near 16k. In the original code the optical divisor value for a 2k input is double the value used for a magnetic 4k input. I've added one more operation to double the divisor during calculations for the optical sensor. This will allow the road speed value to match the sensor output. If this file works as expected, I'll go back and remove the other files from this thread.

Another thing, we don't have to be dead set on making the jeep vss work.

It wasn't just about the Jeep VSS. This problem has manifested itself in various ways over the years and the answer most often is to give up and buy an aftermarket part which is fairly expensive. It's very cool to have two reasonable solutions available. Having an option to easily use a square wave signal on the magnetic input is very applicable to later generations of pcm which do not have the circuitry or code for a square wave input. The second solution is code based and while it can be patched into $8D easily, other code masks will require different patches to achieve the same result.

Remember I tested the gm truck magnetic vss and 40 tooth wheel and it worked fine on the magnetic inputs, and the output of the ecm did run the jeep speedo, it just read wrong.

That combination of 40 tooth sensor and 4000ppm output will only work to produce a 8k output if the tire revolutions per mile times your gear ratio times 40 is equal to 2x one of those 7 values

There's also a 2k output line available but it may take removing teeth from the reluctor wheel to get a frequency that works.

[JeepsAndGuns]

Ok, I loaded up test3 bin and gave it a go. Speed looked ok in the bin and read a little more normally. However it was still off from the jeep speedo, still too fast. I checked this thread real quick and saw sturgillbd's post and changed the vss parameter to 16024 (which it saved as 15889.66) and it worked! The data stream dash reads about 2-3 mph slower than the jeep speedo, but that's close enough to work with! Nice job
What is test4?

That combination of 40 tooth sensor and 4000ppm output will only work to produce a 8k output if the tire revolutions per mile times your gear ratio times 40 is equal to 2x one of those 7 values 1project2many posted way back in post 13.

There's also a 2k output line available but it may take removing teeth from the reluctor wheel to get a frequency that works

Would adding a drac/4 out buffer/whatever they are called, like used on mid 90's trucks help any?

[sturgillbd]

Having an option to easily use a square wave signal on the magnetic input is very applicable to later generations of pcm which do not have the circuitry or code for a square wave input.

I have to back up and punt on that one because the original test was flawed on my setup. I used the normal square wave output on the generator but had offset the signal to the low side being what looked like 0v. When I zoomed in with the scope, the low was actually at -.089V. The low was actually crossing zero. I may have a solution for feeding it with a 0-5V signal but I have to use another chip. TTL is 0-5v DC. I will set up feeding a max232 chip with the 0-5v signal and the max232 output will be approximately +13 to -13 v. I will set up a test circuit and report the results in a new thread.

What is test4?

I think 1project2many was trying to correct where we were having to double the pulses per mile setting in the bin to get the proper output. I tested the new bin and had to quadruple the ppm (32000) to get the speed in the datastream to show correctly. One interesting note is now I can display 1Mph where with the other bins, I could only make it display 2Mph and above.

[1project2many]

I think 1project2many was trying to correct where we were having to double the pulses per mile setting in the bin to get the proper output. I tested the new bin and had to quadruple the ppm (32000) to get the speed in the datastream to show correctly. One interesting note is now I can display 1Mph where with the other bins, I could only make it display 2Mph and above.

Interesting. It seems like using the pulse per mile count with no doubling is the answer in this case. It doesn't make sense though. GM wouldn't write code with a dedicated divisor that's 4X larger than it needs to be.

New code posted for trial and error. I'll have to see if I can figure this out tomorrow, though.

[lionelhutz]

Would adding a drac/4 out buffer/whatever they are called, like used on mid 90's trucks help any?

Not without modified code. The DRAC has a 2k & 4k output. You might be able to re-purpose the outputs to be 8k and 4k, but the PCM still expects 2k on the optical input so you have a mismatch and need the modified code to turn the 2k PCM optical input into a 4k input. You could feed it into the magnetic input but that would require something between the DRAC and PCM to put an offset on the signal so it crosses below 0V.

The signal must be converted from 0-5V or 0-12V to go below 0V and then fed to the magnetic PCM input to use either the 40-tooth wheel and DRAC (so PCM is also feeding the Jeep speedometer) or the Jeep VSS without modifying code as-is. I suspect it'd work OK just using a capacitor and resistor low pass filter, but it might require a more complex circuit to level shift. Besides the MAX232 chip, an simple op-amp differentiator circuit would likely also work.

[JeepsAndGuns]

I should have time in the next couple days to try test5. Anything specific to look for or do?

[JeepsAndGuns]

Ok, sorry it took so long to get back with test results, I have been really busy this week.

I finally had a little time this afternoon and tested test4 and test5. Here are my results.

Test4. Read way too fast. Ended up changing vss parameter to 30000 (which tp saved as 29729.03) to make it read correct. With that value, the tp dash read correct to 1 mph slower than the jeep speedo. I did find that after the jeep speedo read 65 and up, the tp speed started reading 5 mph too fast.

Test5. It read much closer, but I had to decrease the vss parameter by 1000 (which tp saved as 7035.11) With that value changed, it read about right at 1 mph faster than the jeep speedo. However, it started reading 5 mph to fast after the jeep speedo got to 30 mph and above.

Hope this helps.

[misterpat]

I guess no one thought it would prove to be this difficult, huh? Thanks for all this work!