1992 Jeep Wrangler with 350TPI/700R4 VSS Options

[JeepsAndGuns]

Holy cow info overload....

I had about come to the assumption that the vss outputs from the ecm only worked with the magnetic vss inputs but wasnt sure, you however have confirmed it.
So what exactly is different about the signal needed for those inputs Vs what the magnetic jeep vss is making? The 91 only jeep vss is only two wires, so I would assume its magnetic, however hooking it to the magnetic vss inputs, the ecm will not read anything. So something must be different.

Teeing the signal from the 92 and up jeep vss would work, as that how its done in factory jeep systems, it tees off to the speedo and ecm. However that wont make the ecm happy as I can max the speed in the ecm by hand.

If you can use TP to manually edit your calibration in the hex editor, you could change $3F93 and $3F94 from 0566 to 0E67 and see how that works. If it's good we could change the XDF to allow modifications to this location by other users for pulse counts greater than two. If you'd like me to make a change to a .bin for testing, lmk.

I'm up for giving it a try. I am very uber noob with hex. I really dont understand any of it. All I can understand is how to find addresses and can change them if I have the proper value.
On the bin I was using to test with (AXXD, 91 Fcar 5.0 manual), it has the value of B6 at $3F93, and 80 at $3F94. What should I change them to?
Just for the heck of it, I looked at AUJP (90 Fcar 5.7 auto) and AXCN (91 vette 5.7 manual) the were both 05 at $3F93 and 66 at $3F94. Any reason for the different values? I actually tried all those bins in the tests yesterday, just to rule out the chance it could be a issue with only 1 bin and not others.

[1project2many]

Holy cow info overload....

Yeah... I know. I figured if I put this into one thread it would save people including me from having to look all over the 'net for resources the next time this comes up.

The 91 only jeep vss is only two wires, so I would assume its magnetic, however hooking it to the magnetic vss inputs, the ecm will not read anything. So something must be different.

I agree. But I don't know what it is. It'd take some research to come up with an answer. I have a fairly expensive aftermarket sensor that was guaranteed to work with the GM ecm but it would only read correctly to 30 mph or so. I never did figure out what was going on, although I happened to find an article the other day that talked about needing a bias voltage for some GM VSS lines so I might try and follow that up.

On the bin I was using to test with (AXXD, 91 Fcar 5.0 manual), it has the value of B6 at $3F93, and 80 at $3F94.

Neat find! AXXD has 11 additional bytes of IAC control code located at $B1D6 (shows up at $31D6 in the hex editor). This moves the location for the speed sensor ppm value 11 bytes higher in the calibration, to $3FA7 and $3FA8.

[JeepsAndGuns]

Neat find! AXXD has 11 additional bytes of IAC control code located at $B1D6 (shows up at $31D6 in the hex editor). This moves the location for the speed sensor ppm value 11 bytes higher in the calibration, to $3FA7 and $3FA8.

Interesting. So if it is in different locations in two different bins of the same mask, then I am guessing that adding a parameter to change this value is not going to work and it will have to be done manually.
Not knowing hex that good, what is the hex values I need to put in to equil the numbers you posted above?


Another question, would the vss from the transfer case of a mid 90's truck work on the magnetic input? (ACDELCO Part # 15547452) Like I mentioned in my first post, I have a tailhousing off a S10 blazer NP233 that will bolt onto my NP231 and use that vss with a 40 tooth reluctor wheel.

[1project2many]

The hex values are 0E67. So you'd enter 0E at $3FA7 and 67 at $3FA8.

The mid 90's sensor will work just fine. I believe that same sensor was used with the 7427 in some applications.

[misterpat]

Info overload is right! I'm still reading all this, but ran out of things to contribute. Worst case senario would be I have to run an aftermarket speedometer. I really do appreciate this.

[JeepsAndGuns]

The hex values are 0E67. So you'd enter 0E at $3FA7 and 67 at $3FA8.

I'm not gonna have the time tonight, but I will try to make some time in the next couple days to give this a try and report back.

The mid 90's sensor will work just fine. I believe that same sensor was used with the 7427 in some applications.

Hoping not to go too far off topic, but yes, thats exactly the sensor I am talking about. It was mounted on the transfer case tailhousing and read off the 40 tooth reluctor wheel. It was used in the 7427 year range trucks, as well as many others (S10's, blazers, etc..) Rock auto says it was used from 88-99 in fullsize trucks, and 89-04 in S10's.
Ok, so if this sensor could be used with the magnetic inputs on the ecm, could the speed sensor value be changed to accept the 40 pulse reluctor wheel?
I am still trying to understand the info in post #13. If the above could be done (use the 40 pulse vss) could the divisor be set to output a 8 pulse signal instead of 4?
I'm curious if the ecm output would drive the stock jeep speedo. IF by chance it did, then you could (maybe) fudge the input pulses slightly to account for different size tires/gears, thus in turn maybe making it to where you could correct the jeep speedo. Right now, with my tire size and gears, the closest speedo drive gear for my vss puts my speedo about 3-4 mph too fast. Being able to adjust it to be correct would be cool.
If only I could figure out a way to spin the reluctor wheel and hold that sensor so I could test it.....

[1project2many]

Cordless drill to spin the wheel?
The GM sensor and reluctor wheel should work fine. Speed sensor pulses value should correct sensor reading. Speed signal out depends on divide by ratios and they won't account for 2-3mph off. Try a DRAC?

[lionelhutz]

The second constant is the instrument cluster or IP pulse divisor. This constant is used for the divide by circuit which modifies the incoming magnetic VSS signal to produce a square wave out out for speedometer, cruise, or other external components. Although there are 8 bits in this value, only three are used to select the proper divide by ratio. Selecting a wrong value often results in no speedometer signal. Divide by ratios are as follows:

A B C
0 0 0 Divide by 1 (ie for a 4000 ppm VSS)
0 0 1 Divide by 9 (36000 ppm VSS)
0 1 0 Divide by 7 (28000 ppm VSS)
0 1 1 Divide by 11 (44000 ppm VSS)
1 0 0 Divide by 6 (24000 ppm VSS)
1 0 1 Divide by 10 (40000 ppm VSS)
1 1 0 Divide by 8 (32000 ppm VSS)
1 1 1 Divisor disabled, no output

This explanation doesn't make sense. Just to give examples from 2 of my vehicles, a 28" tall tire with 3.73 rear gear ratio produces 110557 pulses per mile and a 29" tall with 3.36 rear gear produces 96230 pulses per mile. There's no way to create either 2000ppm or 4000ppm signals from these values by only using these divider ratios. So, the PCM simply can't be taking the sensor input and only dividing by these constants.

[1project2many]

The output signal is separate from the internal VSS signal.

GM is known to use external buffers. I'm sure they've run into the same limitations. The DRAC in 4X4's and the yellow buffers in TBI F? cars are some examples. What are your vehicles?

[1project2many]

Info overload is right! I'm still reading all this, but ran out of things to contribute. Worst case senario would be I have to run an aftermarket speedometer. I really do appreciate this.

No problem. I think this will work although we might have some trial and error work to do first.

[JeepsAndGuns]

Ok, did some more testing when I got home from work.
I hooked everything back up with the 92+ vss and changed the values you said to change. This made no difference. I tried two different bins (AXXD with the values at $3FA7 and $3FA8, and AUJP with the values at $3F93 and $3F94) It still acted the same as it did before.

I was curious about the truck magnetic vss we spoke about above. I had no way to spin the wheel and hold the sensor at the same time, until I remembered the NP205 I just modded to take the exact same reluctor wheel and sensor. So I drug it out (dang that thing is heavy) and hook it up to the magnetic vss inputs and spun the output shaft by hand. Ta-da! It worked. Read way too fast, but worked. I changed the vss value in the bin from 4006.96 to 40000 and it read more normally. Spinning it by hand put the speed at around 4 mph. Sweet, so it looks like this sensor and wheel can be used, and the input pulses probably can be changed to what is needed. So then I got curious, I hooked the jeep speedo up to power and hooked it up to the vss output of the ecm. Hey! It works! However it was reading the wrong speed. When the datastream dash said around 4 mph, the jeep speedo was reading around 20 or more. Now I dont know if either of these speeds would have been correct for how fast I was spinning the output shaft, but it was mainly just to see if it would work, which it does.
I'm thinking, if the right number of output pulses could be sent out of the ecm, it might just work with the factory jeep speedo.

[lionelhutz]

The output signal is separate from the internal VSS signal.

GM is known to use external buffers. I'm sure they've run into the same limitations. The DRAC in 4X4's and the yellow buffers in TBI F? cars are some examples. What are your vehicles?

It doesn't matter what the vehicles are. They are examples of the pulses per mile when using a 40 tooth reluctor wheel and sensor. You posted that the PCM takes the output from the 40 tooth reluctor wheel and divides by one of those constants to produce the 4000ppm signal and the math doesn't work.

There are no DRAC or yellow buffers in the sensor -> PCM -> 4K output circuit.

[1project2many]

Ok, did some more testing when I got home from work.
I hooked everything back up with the 92+ vss and changed the values you said to change. This made no difference. I tried two different bins (AXXD with the values at $3FA7 and $3FA8, and AUJP with the values at $3F93 and $3F94) It still acted the same as it did before.

Ok, just to get this straight...
You had the 92+ speed sensor connected to B21 and the "magnetic VSS installed" box was unchecked, you changed the values in the bin, and the vehicle speed reading in the TP dash maxed out?

It doesn't matter what the vehicles are. They are examples of the pulses per mile when using a 40 tooth reluctor wheel and sensor.

Why the attitude? When a good question comes up it only makes sense to get as much info as possible and start fact checking.

You posted that the PCM takes the output from the 40 tooth reluctor wheel and divides by one of those constants to produce the 4000ppm signal and the math doesn't work.

Not exactly. I did say the 40 pulse wheel should work but when I said "should" I'm not absolute. I was thinking of it in terms of voltage and signal shape... whether or not the signal would trigger a speed reading in the pcm. I don't happen to know of a confirmed application with the 40 pulse ring and wasn't really thinking of it in terms of count. I might even have assumed that your comment in post #10 was right...

Remember, this PCM will also produce the 4000 pulse output when using the 40 pulse AC sensor.

As I sit here thinking about it, I don't actually remember a 40 pulse per rev wheel used with the 7730/7727 in rear drive. As I remember, they first appeared in trucks then moved to cars as the LT1 was installed. Front drivers use a reluctor wheel but they produce a lower frequency signal because they're connected to the axle.

I'm confident the information regarding the handling of the speed signal and the divider is correct. And I'm confident that the internal VSS signal and the 2k / 4k output are separate. If you have an application I'll do my best to follow up and maybe we can figure out how GM made a 40 pulse input work.

[lionelhutz]

It doesn't matter what vehicle or PCM since all RWD vehicles with 40 tooth wheels on the trans output produce the same signal where pulses per mile = tire revolutions per mile x rear gear ratio x 40. So, if you use a 40 tooth wheel and GM sensor connected to terminals B9 & B10 then those are examples of the signal the PCM is starting with.

The schematics for a '7730 TPI F-body shows this speed sensor configuration. The schematics also show the 4K output to the speedometer and the 2K output to the cruise control. The sensor -> 2K and 4K outputs need more math than just that 1 of 7 divider to produce these outputs correctly.

I have not used a '7730 PCM but your explanation of how the PCM creates the 2k and 4k outputs just doesn't make sense. There has to be more to it than a simple divider.

[1project2many]

It doesn't matter what vehicle or PCM since all RWD vehicles with 40 tooth wheels on the trans output produce the same signal where pulses per mile = tire revolutions per mile x rear gear ratio x 40. So, if you use a 40 tooth wheel and GM sensor connected to terminals B9 & B10 then those are examples of the signal the PCM is starting with.

Ahh. I understand. All RWD vehicles do not use a 40 tooth reluctor. Corvettes and Camaros with the 7730/7727 use a gear driven speed sensor like this one from Eckler's:

Correcting a tire or gear ratio change on these vehicles means changing the speedo drive to driven gear ratio, just like a speedometer. This sensor produces a 4kppm signal. You can see more sensors here:
http://www.jagsthatrun.com/Pages/Spe...eedometer.html
GM sensors are about 1/2 way down the page.