rt-z28

I have a 91 z28 5.0 f code auto, my speedo reads very fast, if im in drive and am going 1mph or so it reads about 50 - 60 mph, eventually i will max it out at about 2-3 mph and it will just go back to zero and set untill i almost stop again. I have tested the wires from the vss plug to the ecm and from the ecm to the guage cluster for resistence and all seem good, with a snap on scanner hooked up to the computer i am getting a speed signal from the computer, i can see the pulses on the screen. I have replaced the vss and the computer with new delco units and replaced the guage cluster with no luck. i checked the reluctor wheel in the trans and found no missing teeth. I also have noted that 4th gear tcc is always commanded on so my torque converter is always ingaged while driving, I am at a loss here and hoping for help. I also ran a jumper wire from the ecm to the guage cluster with the same result. This may be unrelated but I also keep blowing my #9 fuse about every 2 weeks, the one that controls the vss, tcc brake switch guages etc... If anyone has any ideas please let me know, i am extremely frustrated here!

Sorry for the long post, and thanks in advance for help,
Randy

BigBadLou

iTrader: (3)

When you're moving at those low speeds and the speedometer shows that 50-60mph .... what if you put it in neutral and rev it up a little? Does the reading change?

Also, you might want to just turn the key on and put a function generator on the VSS wire from the transmission and generate freqnencies up to 100Hz (should correspond to 90mph) and see if your speedometer shows correctly. Repeat the same procedure with the engine running.

If you don't have access to an Fgen, put a scope on the wire that supplies the speed signal to the instrumetn cluster and observe the signal.

Let us know what you find, I am VERY intrigued.
Lou

rt-z28

Well there is no change by going into neutral at all, I dont have access to a function generator, but I do have the scope function on the snap on scanner and that is where I could see the different pulses - wavelengths so to speak, so I know its sending a signal, I just dont know what is causing it to be off so much. Anything else i should look into or at? Thank you for your help, Randy

6948ric

i don't know part it plays but there is a thing call a buffer small square white box under the pass. side close to the ecm brought one for my 91 but sold the car before i installed it

rt-z28

Does anyone else know anything else about this "buffer"? I have access to "mitchell on demand" at work and through their diagnostic procedures i did not see anything about it, if it has anything to do with my problem it would be about the only thing not replaced. This thing is going to have hail damage from all the parts im throwing at it! Since this has never worked since I bought it I was also wondering how to go about making sure I have the right transmission,vss and reluctor wheel package to work together, I dont know if the trans has ever been replaced or not but just a thought. I used the part number off the old vss to get the new one.

BigBadLou

iTrader: (3)

IIRC, the speed buffer box is only on older models or V6's. I don't have one on my '91. The speedometer signal should go from the VSS to the ECM and then from the ECM to the instrument cluster.

You say you can see the square wave on the snapon tool. What is the frequency of the wave at what speeds? THat would tell you a lot.

Lou

Stephen

iTrader: (12)

It reminds me of a mechanical failure in the speedometer itself. I've been through that before.

Does the needle hold steady? Or bounce a lot, when driving?

Take the car out at 10-15mph to get a look at the needle, and see what it is doing.

rt-z28

Lou - i will have to get back to you on the frequency as i dont have tools here in front of me, but it is def not a square frequency. It is more like a zig zag line from top to bottom of the screen.

Stephen - I dont think its the speedo itself I have replaced that with the same exact results. It does not bounce around it just slams over to 110 right away and then will eventually go to zero and sit till i slow back down.

I did have the scanner hooked up and at around 20 mph or so it said I was going 200mph, this is with it hooked to the vss wire at the ecm while on a lift at work. Now im really wondering if i have the wrong vss or if the trans is wrong, everything from the vss plug to the cluster seems to be fine, But I could be overlooking something small.

Brads RS

If you had a scanner hooked up directly to the vss and its said it was going 200 mph, then you have the wrong speed sensor. The ecm prob goes nutz when it see's that many pulses per mile per hour. lol The speed sensor you need is a 4000 pulse per mile per hour i think.

BigBadLou

iTrader: (3)

Correct, our 3rdgen sensors send 4000 pulses per mile. Which works out to about 1.11 Hz for each mph you're driving. So at 9 mph, you should be seeing a 10Hz signal, at 90mph it would be 100Hz etc.

Brad is right, if the VSS is giving you such high frequency signal, it's the culprit. However, you mentioned that you replaced it with a new part. What was the part #? Does it fit our vehicle application?

Lou

rt-z28

The part number is 08673299. That number is a revised number, it took over the orignal part number. It was the only one I could even find that was a magnetic sensor, the rest were cable driven. Does anyone know if this is the right one, or have a different part number? When I got the new one I took the part number off the old one.

BigBadLou

iTrader: (3)

That cross-references to AC Delco 213194. It seems to be the sensor for the newer 4L60 transmission. I'm not sure but I think the new sensors give a much higher frequency (like 1000 pulses per driveshaft revolution) which would explain your situation.

Lou

rt-z28

I thought 91 & 92's did have 4l60 transmissions, do you by chance have the part number of the vss that is supposed to be with the car or know how to go about finding it? Im really leaning towards this to now after all the testing ive done. It seems like the only logical thing left. I really appreciate all the help! thanks again, Randy

BigBadLou

iTrader: (3)

To find the right sensor for you, you'd need to go to the partsamerica.com website, select the correct vehicle and then type in "speed sensor" in the search field.

Then you'll need to find the one that's right for your option code. They offer different sensors based on your RPO codes: D8M/D8G, D81/D8N, D8I, D8G).

Hope this helps.
Lou

rt-z28

By doing a search that way none of those sensors look like they will work in my tranny, it looks like the little shafts on the back of those would hit the reluctor wheel causing damage. The vss that I took out didnt have one of those shafts, its completely magnetic. I will get a picture later when I get home. Im beginning to think the trans. was swapped out and not replaced with the right one, any other thoughts? Thanks again, Randy

BigBadLou

iTrader: (3)

Look at the 700R4 pictures on this site: http://www.transmissioncenter.net/sp...n_______va.htm

Does your transmission and gears look different?

Lou

rt-z28

When you go to that page, if you hit the 4l60e link at the top and get to #70 and #70x that is how my trans is set up, thats the type of speed sensor I have and when you pull it out and look in the hole that reluctor wheel you see in the pic is right there vertically, no plastic gears no nothing, thats all there is. I hope this gives you something to go off of. Thanks, Randy

rt-z28

Any other suggestions, Im guessing I have the wrong tailhousing to work with my ecm? Does anyone know if there is a simple fix to this? Can that reluctor wheel be swapped out for a gear? It looks as though its pressed on to the tailshaft, and the gears on the above site look like they have a flat spot on them that are held in place with a clip. Can these be swapped out fairly easily? I dont know much about transmissions and all help is appreciated. Thanks, Randy

BigBadLou

iTrader: (3)

If you have the 700r4 transmission, you can easily install the correct speed sensor in it. Look at the very first picture (the cutaway) on the tranny page. They show the 700r4 tailshaft. The magnetic wheel will be replaced by one gear and the hall-effect sensor will be replaced by a 3rdgen VSS with a shaft.

Now you just need to tell us the size of your rear gears and tire size and I'll post the gear part #'s you need.

Lou

rt-z28

Does this seem like my problem though? What setup do I have? Must be out of a truck or something. Is it as easy as removing the tailhousing and installing that gear? That reluctor wheel looks pressed on. I will get my tire size and gear ratio asap and post them, thanks again, Randy

BigBadLou

iTrader: (3)

Yes, it involves tailhousing removal to get to the tailshaft to replace the drive gear.

Yes, you might have a combo from a truck or something. I remember that my 1991 Suburban had a 4l60e tranny.

Lou