I've searched the forums, but not satisfied with what I found. (probably not using the right search terms).

I have 1992 Z28, 5.7L, speedometer & odometer not working (all of a sudden just quit), all other gauges work, and only 77,000mi.

It is an electronic speedometer so what would be a good checklist to use in trying to diagnose the problem?

Thanks.

 

The wires or connector for the electronic speedo on the tailshaft of the trans might be broken or loose. Also, the speedo gears inside the tranny might be broken or slipped apart. Lastly, the speedometer in the dash might have failed.

 

Those would also throw a VSS error and light the SES light.

 

Ok, now specifically on an electronic speedometer what other functions of the vehicle would be affected if:

1. The speedometer itself is the only thing broken. Would any other functions be affected?

2. The speedometer itself is just fine and there's a problem somewhere between the transmission and the speedometer. What other vehicle functions would be affected?

I ask these questions because I have read (not sure if it's true) that a non-functioning electronic speedometer affects other components such as shift points and whether or not the TC locks.

 

Could be the vss on the tail shaft. Disconnect it and test it with an ohm meter for continuity. You can also test the wiring back to the ecm the same way. Connect the wiring to the vss and test from the female connectors at the ecm.

The connections at the vehicle speed sensor must be good and, a poor connection at the cluster can cause the speedo to quit without throwing codes (ses). If you're not seeing the ses light coming on check your connection at the back of the cluster. Carefully push the cluster and wiring harness connection together.

 

I see what looks to be a black cable attached to the passenger side of the transmission that runs up the firewall. Is that part of the speedometer or is that the TV cable?

On the driver's side there's an electrical connection. Is that the VSS?

 

The black cable on the passenger side is the tv cable. The wired connector on the drivers side tail shaft housing is the (yellow and purple wire color) electrical connection to the vehicle speed sensor.

 

I checked the VSS wiring connector at the VSS and I've got roughly 2.55 M(ohms).

Next step would be to check what????

First pic - If this is the VSS connection then (look at second pic)


This is what, and should I check it for anything???


Here's the back of my gauge cluster. Anything to check here or where it "plugs" in to the dash???

 

I have a completely rebuilt engine from the block up. The speedometer worked fine after the rebuild. However, when the tuner came in and removed the chip and plugged his laptop into the ECM the speedometer quit working. I did not realize it was an electronic speedometer until a few days ago otherwise I would have questioned him immediately as to why my speedometer quit working right after he hooked up to the ECM with his laptop.

I have a wiring diagram for my ECM. Points B9 and B10 show the VSS (purple and yellow wiring). It also shows B11 as the VSS output to the “Instrument Cluster For Speedometer” and C1 as the VSS output for the “Cruise Control”. The wiring diagram gets a little tricky to understand where the output is for the other gauges in the cluster but it looks like points E7, F7, E9, F4, and F2 come into play. If that is the case then it would make sense that if the speedometer doesn’t work, but the other gauges do then something is up with the programming.

So, is it possible to “turn off” the speedometer or program the ECM to send a constant “0 mph” signal to the speedometer? If so, then I think that is what happened. The tuner plugged his laptop into the ECM after he removed the chip and somehow messed up the signal for my speedometer.

 

I'm curious to know the answer to this one.

 

I have my fingers crossed that someone knows the answer as to whether or not the computer can be programmed to either not send a signal to the speedometer or constantly send a 0 mph to it.

 

Absolutely you can turn off the speedometer in the computer. I'm thinking the guy who made your chip did that by accident.

 

Thank you. That will be my approach when I get back with the tuner - look at the programming since the wiring checks out.

I'll come back with an update once I meet with him.

 

Ok one more thing - I got the cluster back in, dash put back together, etc. Took it out and tested the cruise control and that doesn't work either. I didn't expect it to work, but just to be sure I had to try. So, the VSS output to the speedo is either turned "off" or set to "0" to affect the operation of the cruise control.

Going to get back with the tuner soon (as scheduling permits).

 

I'm still watching this post, because I'm curious to know the outcome. Good luck, though.

-Erik

 

Ok I got back with the tuner yesterday after all the other things I needed to get worked out. It turns out that when the tuner de-soldered the chip for the first tune the connection to the pin that sends the speed signal was damaged and there's no connection now. So the signal is being sent to the speedo because he could read speed through his computer, but it can't get a signal to the speedo because the connection is severed. The tuner ordered a new "port" (I guess that's what it's called - the long thin brown piece that the chip plugs into). I expect to have it in a couple of weeks and we'll check it out then. He's also going to install a "plug-in port" component so that any tweaking to the tune can be done by just plugging in his computer to it instead of de-soldering it and plugging his laptop into it (similar to how 4th Gen F-bodies and other newer vehicles interface with an external computer).

 

OK that solved the problem! The speedometer works, the SES light is OFF, and here's the real surprise the Z runs better! Which makes me wonder if there were other signals not getting sent, etc.

Now what else is cool is that the computer module that houses the computer chip now has a special adapter plugged into it. It has BOTH the custom tuned chip and the factory settings chip. This allows me to run the custom tuned chip, and if for some reason I want to go back to the factory settings I simply unplug the "new" chip and the car will then run on the one that has the factory settings. If I want to have the new chip reprogrammed then all the tuner has to do is plug his laptop in (no soldering, etc.).
(I'll take a couple of pics to show what it looks like now).