Ok the short version. The trans in my car has an issue. The gear that drives the VSS is not working. I don't know why. Its an Isuzu 5spd hooked to my 4.9. I am trying to get my 3.32 4spd ready to go into the car but that has been put off till spring.

The car will run and drive without the VSS however the caddy ECM needs to see a VSS signal or it thinks the car is parked and does some wonky thing with the fuel curve.

Even if I can get a static speed this should improve drivability considerably.

I spoke to another forum member and he said that as far as he knew the VSS was really nothing more than a small AC generator sending out X pulses per mile. He was thinking if you could generate an AC pulse to the VSS leads you could rig it up to work. Your speedo would say the same speed at all times, but something is better than nothing.

I was thinking about doing it in a slightly more mechanical manor. Like putting a small brush less DC motor attached to a VSS to spin the VSS. Maybe a 12v computer fan motor.

Anyone ever came across this issue before.

I know its just a stop gap crutch to get me through the winter until I get the new trans in the car but really that is all I need. I really don't want to put much into this hole setup with the new trans in the works.

My only other option is to drive my low mileage yellow 88GT through the winter snow and salt which I would rather not do.

So any electronic guru's out there.

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85GT 5spd ,93 Eldorado 4.9 Dual O2 Custom Chip, Archie Clutch. Custom Exhaust. MSD Everything 245/50/16's Not Your Average 4.9 Capt Fiero Com --- My Over View Cadero Pics Yellow 88GT 5spd Stock.

Don't think it has to be a sine wave but someone can correct that if incorrect.

If you use this site the get the freq. you need

http://www.fieros.de/en/main.html

you could then build a timer using the post below to make an oscillator. Might work. If you need more signal strength you can add a transistor to the output.

http://home.cogeco.ca/~rpaisley4/LM555.html#1
LM555 - ASTABLE OSCILLATOR CALCULATOR

To test the Fiero speedometer I supply less than one volt AC, 60 Hz, to the input plug at the transmission and this makes the speedometer read fine. (Maybe some day I'll put the home-made tester on my web sit.) The Fiero electronics can, I believe, take up to 5 volts but I would not try it myself. If your Caddy electronics are similar to the Fiero then you could supply a low voltage sine wave from just about any oscillator to get you back on the road. With a Google search you can find a ton of 555 oscillator circuits that will run off your cigarett lighter. Here's a link to an oscillator kit made in the UK http://www.circuit-innovations.co.uk/555oscillator.html It looks like the frequency from the kit is higher than you want, but you can lower the frequency with component changes.

I certainly would not exceed 5 volts input to your speedometer unless you have reason to believe the output signal from the Caddy transmission is really stout! (Not likely.) Otherwise you are likely to blow he circuitry and have a REAL problem on your hands!

Good luck.

Paul McKibben
Norcross, GA

The caddy ECM is looking for a VSS in the neighborhood of 24,000 Pulse per mile. I don't know that the 60 hz signal is going to do it much good. You could build a custom circuit but dakota digital builds exactly what you need... and that is to convert the Fiero VSS into a signal that the Caddy ECM can understand.

http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm

The Fiero VSS will drive my caddy ECM without issue. I drove the car for a year with the caddy ECM running off of the custom chip in my caddy ECM. Its not a matter of the ECM not seeing the Fiero VSS it is a matter of the trans not spinning the VSS.

But thanks for all the ideas so far guys.

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85GT 5spd ,93 Eldorado 4.9 Dual O2 Custom Chip, Archie Clutch. Custom Exhaust. MSD Everything 245/50/16's Not Your Average 4.9 Capt Fiero Com --- My Over View Cadero Pics Yellow 88GT 5spd Stock.

I have a print of a vss signal generator circut, from the Joe Wynman cd. Never used it, but it looks about as complex as the vats signal generator circut.

Anyhow, if you want a copy, pm me a fax #

David Breeze

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Pantera Rebody Kits

You could do what I did at one point, take an extra Crank Posisiton Sensor and make a bracket that points it at the inner tripot of one of the CV shafts. Any CPS from most any GM will work, and they are about $20 new.

Its not the prettyest, but it will feed the ECM 3 pulses per tire revolution or something in the neighborhood of 4250 ppm. I used this short term until I figured out the real problem. Just mount it up and get it sitting as close to the tripot as possible, I forget specs, but 1/8" should be ideal. A CPS uses the exact same setup as a (newer) VSS, only the VSS has a internal toothed gear that it reads off of.


https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20060206-2-064905.html

There is an easier way to test if your VSS is bad or if you have a stripped gear in the trans. just pull the vss out turn your key on and spin the gear while at the same time look at your speedo. It should be moving. If not, spin it the other direction if it's still not moving try a new VSS.

quote Originally posted by 86GT3.4DOHC: You could do what I did at one point, take an extra Crank Posisiton Sensor and make a bracket that points it at the inner tripot of one of the CV shafts. Any CPS from most any GM will work, and they are about $20 new. Its not the prettyest, but it will feed the ECM 3 pulses per tire revolution or something in the neighborhood of 4250 ppm. I used this short term until I figured out the real problem. Just mount it up and get it sitting as close to the tripot as possible, I forget specs, but 1/8" should be ideal. A CPS uses the exact same setup as a (newer) VSS, only the VSS has a internal toothed gear that it reads off of. https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20060206-2-064905.html

So what you did is something similar to how ABS sensor operates? Kinda sound like it.

The pulse is actually a triangle wave as shown on my scope.

First off are you 100% sure the tranny is not spinning the VSS sender? If the gear on the sender is good, there is no reason why the gear inside of the tranny is no good - there would be some sort of damage to the sender gear as well if the tranny one is damaged.

How did you test that the sender was not working? Maybe you have a bad wire (open or shorted), or bad connection, or maybe a bad sender? The only way I know to test it is with a scope. Also assuming you have the clamp circuit installed for the speedo, are you sure this is working correctly and there is not a short in it?

Barring that:

1) You can use the VATS signal generator (555 timer circuit) to simulated the pulse. Not a good method in my opinion, I don't think the ECM would like it any better than no signal, as it would be constant (I do know that the 4.9 ecm monitors the changes in speed).

2) Try 86GT3.4DOHC's suggestion (good one) - this looks like a really good alternative.

Maybe I should have stated this in the first post. If I pull the VSS out of the trans and spin it with a drill the speedo works perfect, and ECM sees the speed perfect as well.

3.4dohc. You idea sounds like the simplest and most straight forward there is. DO you have any more details on it.

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85GT 5spd ,93 Eldorado 4.9 Dual O2 Custom Chip, Archie Clutch. Custom Exhaust. MSD Everything 245/50/16's Not Your Average 4.9 Capt Fiero Com --- My Over View Cadero Pics Yellow 88GT 5spd Stock.

Avoid all the hassle and headaches. Go to www.dakotadigital.com and get a speedometer interface. It will take care of all your problems.

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..from beautiful Caledonia, WI
87 GT 5 speed med red
87 GT 5 speed Electron Blue V8 SBC TPI

quote Originally posted by typhoon: Avoid all the hassle and headaches. Go to www.dakotadigital.com and get a speedometer interface. It will take care of all your problems.

NO, this won't work since he has NO signal coming from the sender due to it not being spun inside of the transmission - all that interface does is convert a signal you have into something else (which is not what he is looking at doing).