4L60E VSS signal jumpy on roll-out

[mill3833]

Am having problems with the VSS signal jumping to a very large value as soon as the car starts to roll. This happens in REV or 1st gear. After about 10 mph, it stabilizes and seems to be solid, though I haven't had it over 40 mph to date. Of course, at these low speeds, it upsets the shift table, MSShift is jumping between 4th gear, 3, and finally sets in on 1st gear during the roll-out.

I referenced the "Solving VSS Issues" page and have tried all the software options, but there are a lot of combinations, and I'm no way an expert in what it all means. I thought I had it licked by adjusting R45 (lowered the resistance) to raise the trigger threshold. That worked well with the car on jack stands, but when I put it on the ground, it acted up again.

This is a 1994 4L60E 4-speed in a Camaro. A very common tranny, and because the rest of the Megeshift SW was set-up for that tranny, well, I thought the VSS setup would be a no brainer.

Am running 4.1.3.8 code. Attached is my msq.

Before I start adding resistors in series with and/or in parallel with, in an attempt to kill the signal, I thought I'd ask the forum if there are any links to what works best for an early model 4L60E tranny, hardware and software wise. This one has the large PMA (permanent magnet alternator) style pickup.

Also, I have benn very careful with grounds. Lots of grounds, to the engine block. And all tranny solenoids have reverse diodes installed near the tranny connector, and the VSS signal is shielded.

Any help would be appreciated.

Dave

[mill3833]

The above issue was traced to a 12V to 120VAC inverter that I had powered off the same 12V power line. Turning off the Inverter, jumpy VSS goes away.

Dave

[Bernard Fife]

Dave,

Yeah, inverters can be horribly noisy things - they can be as bad as battery chargers in some instances.

Most of the early development of the trans circuits and code was done on an early 4L60e (with the large pickup), so there *shouldn't* be too many issues - in theory.

I suppose you could try adding a noise suppressor to the power line to the inverter, something like the one here: noise suppressor. This filters out noise from the car to a stereo, but it should do just as good a job filtering noise coming back from a device into the 12V supply.

Lance.

[mill3833]

Thanks Lance.

I drove the car a couple miles this evening. Drive just like stock. What a joy. Rock solid VSS, with the inverter off. Now that I know what the problem is, like you suggested, many ways to fix it. Have you ever used a blocker diode to feed the sensitive electrics (MSShift and MSPro3) and then put a filter cap (100 uf or such) down stream from the diode to filter out noise?

My original goal, which I failed at miserably, was to have separate dirty 12V power (for injectors and solenoids) and clean 12V power (for MS3Pro and MSShift and EGT). Both of these power lines have separate power wires all the way back to batt, through separate fusible links. But my failure was that the rear window defroster power feed that I had re-purposed years ago for the inverter, also shares the power wire as the new "clean" power source. Whoops.

Something for others to look out for.

Dave

[Bernard Fife]

Dave,

You can try a diode and cap, it very well might work. Circuit noise is a very complicated subject, but one that appears as if it should be simple. In the end it really just comes down to "whatever works" for your installation so you might as well start by trying whatever is simplest, easiest and cheapest for you and progressing from there if necessary.

Lance.

[mill3833]

Ok, just to help others who may be having a similar problem, thought I would explain what is working for me. My VSS circuit is using the stock design, including the 300K resistor that sets the trigger threshold. In addition, I placed a 1K ohm in parallel with a 0.1 uF ceramic cap across the VSS signal, at the circuit board. Prior to doing this, I was measuring a very strong VSS signal, aprox 80 Vp-p at 90 mph. With the cap and resistor across it, it dropped significantly (and the noise) down to 8 Vp-p around 20 mph and the voltage drops down to 3 Vp-p around 100 mph. I guess this drop makes sense with the parallel cap. I kicked it up to 140 mph (on jack stands) and the speedo was reading good, but I didn't get a look at the scope. So for now, this is what I'll be trying. VSS at low speed (2-3 mph) is very good, not jumpy, even with the inverter on and powering the laptop.

Dave

[mill3833]

Lance,

Where is the link to the MSGPIO schematics? I found the one time, but can't find them now. What to review the speed inputs circuits.

Thanks,
Dave

[Bernard Fife]

Dave,

Because there isn't one 'standard build' for most of the MShift circuits, there isn't a standard schematic either. However, all of the circuits and their optional builds are described in the GPIO docs. Look here: http://www.msgpio.com/manuals/index.htm

And in particular:

VR: http://www.msgpio.com/manuals/vr.htm (also see: http://www.msgpio.com/manuals/mshift/vss.html)
GPI: http://www.msgpio.com/manuals/gpi.htm
GPO: http://www.msgpio.com/manuals/gpo.htm
EGT: http://www.msgpio.com/manuals/egt.htm
PWM: http://www.msgpio.com/manuals/pwm.htm
VB: http://www.msgpio.com/manuals/vb921.htm

Lance.