I'm kind of stumped on this one. I have a 1991 Z28 Convertible all original. 5.0 TPI, Auto The speedo stopped working. Car performs fine. No Cruise. All other gauges function. Replaced the VSS inspected the botom all gears good. Questions: Does this vehicle have a VSS Buffer? If it does have a Buffer where is it located and what does it look like? I have read that most electronic speedometers have one, but TPI engines don't, true or false???????? Since I am in Spain it's kind of hard to go to the salvage yard and get one. Kaptinkafeen has offered kindly to sell me one. I just want to know if I need one or need to get a new instrument cluster. John

it could be the vss (vehicle speed sensor)
or the vehicle speed buffer box
or the vehicle speedometer gauge itself
or in the wiring.

the speed sensor is on the back left side of the transmission on the tailshaft
the vehicle speed buffer box is loacted directly in front of the passenger seat under the bottom of the dash remove the bottom most panel and you can see the ecm which is a big shiny metal box and then a yellow or a before charged neon plastic box almost a green tint which was the color mine had, it also could be in the wiring, mine had a bad speed sensor which i replaced installed another bad speed sensor and had bad wiring, try to figure that one out, installed new speed sensor and it still would not work, the wiring going into the speed sensor was bent at a 90 degree angle turning to go into the vss, the wiring read O.L on a multimeter but when straightend read fine, replaced wiring and got my speedo gauge working again.

check out this thread it has some nice troubleshooting guides to checking the vss, the buffer box, and the gauge, my testing stopped at the buffer box hopefully you will get lucky and only need a speed sensor.
http://www.thirdgen.org/techboard/el...90-wiring.html (wiring)

if you have just basic knowledge of these cars this is a piece of cake to follow just checking wire colors to make sure its getting the signal from the last piece to where your checking i.e from the vss to the bufferbox, from the buffer box to the gauge cluster.

also for the buffer box i believe that certain models above a 1987 i think have the buffer box and under 1987 have the buffer built into the back of the speedo gauge i may be wrong about the years, also around here in the usa if you do not buy from an indiviual for the speed buffer box we must use gmpartsdirect.com

1990+ TPI cars do NOT have a speed buffer box.

Did cruise work before the tachometer problem? If both stopped working at the same time, it is very likely that it's in the ECM, VSS or wiring.

John, do you have access to an oscilloscope? Or at least a good DMM?

Btw, hello to Spain. How in the world did you find a Z28 convertible there??

Lou

Lou,

I have already replaced the VSS. Wires fromthe VSS don't look corroded or
broken.

Before my speedometer went out. Everything worked fine including cruise control.

I searched every where under the passenger side dash. Nothing that comes close to being a VSS Buffer. I will replace the ECM since I have an extra one. I'll get a used gauge cluster off of ebay that works and replace that. Tach and all other gauges currently work.

As far as answering your questions about finding a Z28 convertible here in Spain.
I actually found it in Germany bought by the originial German owner brand new. Only time he took it out was in the spring and summers on the autobahn. Since it was manufactered for Europe it has no speed limiting program in it, full leather. It is in outstanding condition and had less than 100,000 miles on it when I bought it. I have seen 2 other ones here in Spain. The other 2 are red. One has some customization to it and the other original one is also fully loaded and the owner is trying to sell it for approx $25,000. Which he will eventually get for the car. I have lived in Europe for over 10 years and shopped and bought many American Muscle cars that are located in Europe, almost all are normally all original and untouched. They treat these cars like high end Ferraris and Lamborghinis. If there is anyway I can help you from this end let me know.

Sincerely, John

sorry about the mislead man didnt know tpi cars didnt have the buffer box i learn something everyday thats what this site is good for to teach us about these cars, just to let you know my wires on the vss didnt look broken either untill i put an ohm meter on them, when i ohm'ed them straight they showed good when connected and turned like they were they werent good, so be sure to check the wiring at the connector really good, when i re-did my wiring terminals i added about 3 more inches to the wiring to give it an easy curve unstead of a hooked 90 degree turn into the sensor.
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be carefull of the gauges you get from ebay some of them are not on the up and up, being that your speedometer and cruise went out at the same time they are both tied together im pretty sure, b/c for the cruise to work it has to know the speed of the car, which by the speedometer not working the car dosent know.

Thank you for all the posts. This is the 5th third gen, 6th Camaro I have had. I continually learn about these cars and many others. All input is good input epecially with best intentions.
Once we stop learning we are dead.
I will look at the VSS connector again.

Actually, a DMM set to AC Voltage should detect the voltage from the VSS. I have never done it (never needed to) but since it is about 50% DC square wave, the DMM should read cca 12VAC.

The best test would be of course right on the VSS itself but that wouldn't be smart while the tires and driveshaft are spinning (with rear end on jackstands). You could test the yellow and purple wires on the ECM connector, though. Connector C1 (the smaller black one), pins B9 and B10 (purple and yellow wires - diagram here). If you detect AC voltage there while the car is moving (or spinning tires on jackstands), it's safe to assume that the VSS signal is getting to the ECM connector.

How did it go with replacing the ECM? Any change?

Ordering a replacement instrument cluster from fleaBay is a big mistake (in my personal biased opinion). If the cruise control stopped working at the same time the speedometer did, we can assume it's not a coincidence and the problem is in the ECM or upstream, as Dave suggested.

Thanks for answering my question about 3rdgens in Spain. I myself had a hard time finding a convertible Z28 here in the US (had to drive across 3 states to get the one I wanted). So I can imagine how hard it is to find one in Europe. Being from there, I remember only one (yes, ONE) thirdgen in the big metropolitan area I lived in. One beat up V8 Firebird per million people.

Let us know what you find.
Lou

the purple and yellow wires to the vss the way i checked mine was the way i was told, lift the rear wheels off the ground and check for AC alternating current not DC voltage of 1-5 volts while spinning the rear wheels by hand if the voltage is not there you have a bad wire to the vss or a bad vss, i fixed my camaro wiring and still could not figure it out, problem was the brand new speed sensor right out of the box was bad got a new one popped it in and wham the speedo has worked fine ever since, also this was said to me"when you pulled the vehicle speed sensor out of the transmission did you look to see if the plastic gear inside the transmission on the tailshaft was still good" i have heard horror stories of them breaking and falling down inside the transmission. Take your time the thirdgen family will always be here to help you.

Dave, did you mean AC voltage? AC current setting would not be a smart idea without any load in the circuit.
Did you see 5VAC on your meter? I am asking because I have never performed this test myself and while in theory it should work, I have never seen any actual results from anyone but I would like to verify that it's possible to do so.

Thanks
Lou

I will try this test with a Fluke Meter this weekend and let you guys know what I'm getting.

Fluke? Heck, that one might be fancy enough to have a frequency measurement! The frequency of the pulses can is "very roughly" adequate to the speed in mph. I believe that it's roughly something close to 100 Hz at 100 mph (160 km/h).

Lou

Well changed out the ECM. Speedometer doesn't still doesn't work.

Tested the Purple and yellow line. At idle I was reading 0.002 VAC. As I drove it increased to 0.005, 0.006 VAC. Inspected the lines and found no breaks and the bend is lose not a 90 degree bend.

Looks like a speedometer replacement job.

What do you think?

Any other suggestions or I could be Doing something wrong.

Thank you for your input

That's strange.
I would expect the AC voltage test to work (never done it myself but I'd expect it to be valid). Any chance that the VSS sensor is bad or not seated right in there? I can try the VAC test on my car and see what I get.

Lou

Thank you all for the posts. When I get a speedometer with a 145 speedometer in it I'll let you know how it will go. Just need to find one that's working and reaonably priced.

Finally got a new gauge cluster put in and the speedo still doesn't work. Help.

Did you solve your VSS problem?

I experienced the exact same problem last week (lost the speedometer and the cruise control, SES resulted in a 'code 24' in ECM) and I'm doing my research before I attempt to fix it (not in a huge hurry since my car only gets driven occassionally).

Let me know what you found (if anything).

Thanks.

Still haven't fixed the problem. I believe the problem is within the prom. Everything else works but cruise. Which is also controlled by the prom with speedometer input. I need to get a prom. A little hard in Spain. Let me know if you find out anything else.

I'll be digging into mine sometime during the next week. I just put a new rear end in my car which required new speedo gears (both 'drive' and 'driven' gears) and everything worked fine for two car shows. Then, all of a sudden, the speedo and cruise quit working (code 24) and before I get started trying to find out what's wrong I was hoping to learn from your experience.

I will let you know what I find if I'm able to correct mine.

hey guys, i am having the same problem as KRUSS. Ih put 3:73's in my 1992 camaro z-28 about 500 miles ago. it was driving great, still hadn't corrected the speedometer gear, when speedometer just quit working.

i am thinking it is my vss (vehicle speed sensor) but i would really like to find some way to test it before i go by a new one. any help??

thanks

Well, I finally figured out what happened to my VSS: It would seem that I did something wrong when I re-installed the VSS on the car after having replaced the speedometer gears in the transmission. As you can see from the attached photos, the spindle shaft broke off and is galled-up at the break point (suggesting a side load on the shaft). As it turns out, there are different electronic VSS’s used on these cars (at-least in 1992) depending on which ‘driven’ gear you use. In 1992, all speedometer ‘driven’ gears with 39 teeth or fewer use one electronic VSS, and speedo gears with 40 teeth or more get another. Unfortunately, I didn’t know that, so when I removed the original 37 tooth ‘driven’ gear and replaced it with a 44 tooth gear, I just stuck in the same VSS. The reason different VSS’s are used is because the two ‘driven’ gear groups have different diameters, and the corresponding VSS’s for each group are offset slightly in opposite directions from the centerline of the tailshaft mounting hole to compensate for the difference in gear diameter (a fact that I didn’t learn until last Friday after 4 weeks of research). Even though my original VSS seemed to re-install just fine with the larger gear on it, once I tightened the screw holding it into the side of tailshaft housing, it bottomed out in its bore, which in-turn pressed the ‘driven’ gear hard up against the ‘drive’ gear on the tail shaft housing. This put a huge load on the VSS spindle shaft as it rubbed against the side of its bronze bushing inside the VSS housing. It held out for 200 miles before finally giving out! The interesting thing about this is that before I made the speedo gear change I must have looked at over 3 dozen Chevy websites researching speedo gear changes and NOT ONE of them mentioned anything about the different VSS’s. I guess this is a lesson I had to learn on my own!

when i was testing my vss i just pulled it out connected it to a power drill to get the generator to spin and checked that it was producing voltage (AC)

I jacked my car up an tested the volts in A/C at the purple and yellow wires while spinning the wheels by hand and got up to .690 volts is that enough? Should it be 5 volts while spinning by hand or do you only get that much when your driving?

You'd need decent speed (hwy speed) to see good A/C voltage reading (depends on the multimeter).

Fixed it. Thanks. It was a ground wire to the buffer box. BLK/WHT. ECM works now too.....Wierd.

Does a '92 TBI car have a speed buffer?

My speedo quit working but the cruise control still works. All the other gauges still work, too. Maybe the VSS is working and the ECM is getting the VSS signal but it's not making it to the dash?

ok i gotta question i have a 91 camaro vert. had a 305tbi and i changed it to a 350tpi(motor,harness,computer)i also changed the speedo to a 145 out of a 92..now the mph gauges doesnt work .....do i need a buffer box or to gett a different vss.not quite sure????but all the other gauges and lights seem to be working fine......i am really unhappy about this b/c i spent 200bucks on the 145speedo

Any update on this? Im having almost the exact same issue as you, I posted about it in the ECM forum. I was about to order a new ECM when I read that you replaced yours but it didnt fix the problem. Where you able to get ahold of a new PROM? You can try this site if your having trouble finding one.

http://www.rockauto.com/catalog/x,ca...,parttype,2916