1992 Jeep Wrangler with 350TPI/700R4 VSS Options

[lionelhutz]

Well then you really can't expect to feed a 40 tooth drive shaft derived speed signal into the 7730.

This means you must feed 8000 pulses per mile if you expect the 4k output to drive the Jeep speedometer. You could also feed it 72kppm, 56kppm, 88kppm, 48kppm, 80kppm or 64kppm assuming those dividers work. You'd have to see if the driveshaft rotations per mile divides into one of those evenly, or at least close to evenly then you could build a wheel with that number of teeth.

This also means that 40 pulses per drive shaft revolution won't work unless you can set the calibration constant from 4000 to something in the 100,000 range. My guess would be that the PCM just uses a 16 bit constant which can only hold up to 65k since it only had to hold 4k.

[JeepsAndGuns]

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?

Correct.
I could max it out by spinning it with my fingers. It did not seem to be any different from a bin without those values changed.

[1project2many]

Is this easy to set up and try again? If I post a few modified .bins, would you mind trying them?

[lionelhutz]

The internal PCM speed maxing out any time that Jeep VSS puts an output into C6 is telling me that no amount of code changes will fix it. Something is causing the PCM to detect many, many more pulses than what the sensor is really producing.

Do you have 2 x 1k ohm, 1/4 Watt resistors? I so, connect one resistor from 5V to B10 and the other resistor from B10 to ground. Then connect your optical VSS output to B9. Try that and see if it works. This way, you can just use the road speed constant as 8000 instead of 4000 to get the internal PCM speed correct and the 4K output simply becomes an 8k output.

If you don't have resistors, a TPS set to 1/2 the travel could be used to create 2.5V for B10 too.

[JeepsAndGuns]

Is this easy to set up and try again? If I post a few modified .bins, would you mind trying them?

Yes, very easy to try again. I keep the harness laid out and have a full set of sensors and injectors hooked to it ready to test run a ecm. I even have a ignition module, coil, spark plug and distributor so that I can do a full running test if need be.
Post up the bins and I will give them a try.

lionelhutz, I do not have any resistors. The jeep vss is not maxing it out as soon as it is spun, but it does read way way too fast for what I am spinning it by hand. (I am comparing it to how the jeep speedo reacts when the jeep vss is hooked to it) I can max out the speed in the ecm easy if I give it a quick spin by hand.
I do have a couple spare tps sensors though.
So your saying input 2.5v to B10, and the hall effect jeep vss to B9?

[1project2many]

The internal PCM speed maxing out any time that Jeep VSS puts an output into C6 is telling me that no amount of code changes will fix it. Something is causing the PCM to detect many, many more pulses than what the sensor is really producing.

I spent about an hour yesterday looking online for scope signals or OEM information showing pulse counts for the Jeep VSS with no luck. At this point I wouldn't assume the sensor produces 8 square pulses per revolution. Nor would I assume the ecm is detecting more pulses than the sensor is producing. However, the code as written could work with up to 142,000 pulses per mile so it would be beneficial to others to determine a good way to adjust for optical or square wave sensors of various pulse counts.

Do you have 2 x 1k ohm, 1/4 Watt resistors? I so, connect one resistor from 5V to B10 and the other resistor from B10 to ground. Then connect your optical VSS output to B9. Try that and see if it works. This way, you can just use the road speed constant as 8000 instead of 4000 to get the internal PCM speed correct and the 4K output simply becomes an 8k output.

What are you trying to achieve with this? This provides a constant 6V signal to a circuit that won't detect a pulse without a zero voltage crossing. Maybe we can design a circuit to achieve what you want to try.

[lionelhutz]

So your saying input 2.5v to B10, and the hall effect jeep vss to B9?

Exactly.

[JeepsAndGuns]

I spent about an hour yesterday looking online for scope signals or OEM information showing pulse counts for the Jeep VSS with no luck. At this point I wouldn't assume the sensor produces 8 square pulses per revolution.

Only thing I can find in my FSM just says it makes 8 pulses. There is a small chart under speedo calibration that says at 20mph it should be 44.4HZ, at 55mph it should be 122.2HZ, and at 75mph it should be 166.7HZ. Not sure if that helps you or not.
If you have the equipment that you can measure it with, I wouldn't have any problem dropping this sensor and pigtail in the mail to let you test it.

[lionelhutz]

that works out to 8000ppm by my math. Find the frequency at 60mph and multiply by 60 to get ppm. 166.7*60/75 = 133.3 pulses or cycles per second x 60 = 8000 pulses per minute. At 60mph the vehicle is moving 1 mile a minute. So this is 8000 pulses per mile.

So, the only question is what the output looks like. I could easily scope it but I doubt you'd want to ship it to Canada.

Try the resistors and the B9 & B10 inputs. That will put B10 at 2.5V and then hopefully this sensor should cause B9 to switch back and forth between 0V and 5V. The PCM then hopefully sees the difference voltage as AC and counts the crossings.

[sturgillbd]

If you guys can get the ecm and speedo to see the 8k pulse vss and the speed the ecm sees is doubled, as simple fix may be to use a flip-flop to divide the 8k to a 4k signal to feed the ecm. I used to use them as clock dividers. If it accepts the square wave signal, the flip-flop can divide it pretty easily. Feed the speedo with vss and tee signal to flip-flop circuit. Output of it will be half frequency. The flip-flop clock signal will be whatever the vss frequency is and the output will be half. There are probably cmos versions of them that will work at other than 5v. I'll attach a circuit so you can see how the pulsetrain outputs. http://www.electronics-tutorials.ws/...r/count_1.html

[1project2many]

Only thing I can find in my FSM just says it makes 8 pulses. There is a small chart under speedo calibration that says at 20mph it should be 44.4HZ, at 55mph it should be 122.2HZ, and at 75mph it should be 166.7HZ. Not sure if that helps you or not.

It does, thank you.

Try the resistors and the B9 & B10 inputs. That will put B10 at 2.5V and then hopefully this sensor should cause B9 to switch back and forth between 0V and 5V.

I didn't realize that you were calling out pins for a 7730 this morning. JeepsAndGuns is using a 7727. B10 is the 12V ignition feed on that ecm. So the correct pins for the 7727 would be C2 and C8.

The idea is to use the voltage divider resistors across C2 to bring VSSHi up, then use the speed signal at C8 to trick the detection circuitry into thinking it's swinging from a high on one pin to a high on the other. Does anything change in your circuit design if the A/C VSS input circuit is not isolated from the case? The pulse detection voltage is obtained at an R/C junction on VSSHi. C8 (VSSLo) is connected directly to ground. The OEM A/C VSS sensor signal works because it's a separate current generator independent of the 12V automotive circuit. To the A/C VSS, the ground connection through the ecm case is not the VSS ground, it's simply part of the VSS' circuit which allows voltage measured at the pulse detector to be positive, negative, or 0V relative to the battery ground. Connecting the digital VSS, which shares ground with the ecm, to C8 will ground the VSS output and pull the signal to 0V. The VSSHi signal will remain at a constant 2.5V once the cap to ground in the R/C filter charges. I predict the detection circuit will see no voltage transitions.

If you guys can get the ecm and speedo to see the 8k pulse vss

The ecm does see the 8k pulse VSS on the optical input. And the speedo sees the signal from the sensor as well. Many of the early GM vehicles are wired such that the vehicle speed sensor signal is delivered to both speedometer and ecm so wiring the Jeep like this would mimic an OEM GM installation.

as simple fix may be to use a flip-flop to divide the 8k to a 4k signal to feed the ecm.

True, but it would need to be further reduced to 2k to fall within the ecm's expected parameters at the optical input.

[1project2many]

Does the .adx you're using allow you to see promid? If so, would you mind trying this .bin and watching the promid. If I understand the code properly then the first number displayed will be fairly low, less than 10, when the VSS is spinning. The second number will increase or decrease with speed sensor speed.

Thanks.

[lionelhutz]

Hmmm, if either B9 or B10 is at ground potential then you can't apply 2.5V to that terminal or the 5V optical VSS signal to that terminal, which means that idea won't work. I was hoping both pins had input circuits on them and took the difference, similar to how the O2 inputs are. Got a schematic of the VSS input circuit?

[sturgillbd]

I have downloaded the bin that 1project modified and have it running on a 7727 on the desk. I have a signal generator set up feeding a square wave into the C2 and C8 pins. I have an oscilloscope connected to the output of the generator and can measure the frequency fairly accurately. I can output a sine, or square wave output and also induce a dc offset. I took a few pics of the screens and here are the results. 1project, you asked about the prom ID. It does change with speed. The whole prom ID number gets lower with frequency decreasing. It gets larger with increasing frequency. Attached are some pictures of the screen. I do not have any other sensors connected to the ecm but speed does display in the dash when connected in tunerpro. It is showing pretty much double what the speed should be.

[1project2many]

Got a schematic of the VSS input circuit?

http://www.exatorq.com/ludis_obd1/
Follow link for ecm schematics.

The whole prom ID number gets lower with frequency decreasing.

Does the first byte stay below ten? Maybe even three or less? It appears to be a pulse counter in code. IIRC (code and notes at home right now) the second byte should be the VSS.