Folks... I’m in the process of working with a generous member mod corvette forum in getting my recently built C4 tune. Great progress and BAM... speedo is reading only zero.

I’m logging with DATA Cat and The speedo variable is reading zero there also. Cruise doesn’t work but that’s probably because it’s seeing zero speed.

my guess is VSS. I’m also getting some intermittent growling (almost like resonance at certain RPM) in the drivetrain. Checked all U-joints and no issue. Tranny was just performance built but could be something in the tranny. I’m thinking the two could be related. Anyhow, any suggestions would be helpful. Is there a VSS buffer board in the ‘93 better? If so where? Any way to test VSS?

Thanks all

 

Buffer is in cluster. VSS sends AC signal to cluster.

 

So if I’m thinking correctly with the instrument cluster AND the ECM not seeing the signal... problem should be the sensor or speedo gearing. Sound logical?

 

Your thoughts are pretty much correct! I'd say the first move is to see if you can actually get a 'compatible' drive/driven and correctly orientated VSS. They ain't all the same.

It has displayed speed previously? You hinted yes but you don't mention how long!

Post tire size and assuming you've a 3.45 it should be quite easy to give you suggestions. Tire size needs to be accurate!

You could remove the VSS and check the color of the drive gear on the out-put shaft and dependent on what you find, there's maybe solutions.Check the color and tooth count also of the driven gear, finally get all of the information off of the VSS. There's likely numeric information. Might be 35 - 39 or might be 40 - 45. Confirm the color of the VSS. White or black and if it has a center electrical connector or one that is on the outer perimeter of the VSS. You mention 4L60 built so there's no telling what it was'built with'!

 

Good place to start. You want to be sure drive gear retainer is in place.

 

Well... the speedo actually worked until today. Indicates speed properly. I bought the right speedo gears when I bought the Dana 44. I just got the car back 5 weeks ago and have been tuning with help. So... it was a bit of a pinch to the gut when the speedo read zero today after a datalog (during which it worked). I disconnected battery to adjust TB closed position , started back up to check AIC counts and “zero” speedo thereafter. I did leave the laptop connected while disconnecting battery which I usually don’t do.

 

Remove the VSS and check using a rotary tool of some sort. This might have happened, you purchased the incorrect VSS for the driven gear match! Also as mentioned - check the retainer for the drive gear on the out-put shaft. You mention having correct gears so what drive do you have? What driven? Tire size and color of the VSS and information off of it.

 

I’m making a list of things to check. Thanks very much for the help. Likely won’t have feedback until later in the week with the work week starting.

You know. I really enjoy working on this build project. This was a huge project and now nearing the end but it seems two steps forward one back!! I’m seeing the light at the end of the tunnel though. Great help from senior members in this forum!! Much appreciated!

 

Post the rear tire size and using appropriate information I can give you an idea of what to look for when you remove the VSS for inspection. You do have the 3.45 ratio?

 

In addition, you should test for resistance across the two terminals of the VSS. On the VSS in my 96 ZF, it was supposed to be 400ohms and mine was infinite. In spite of that, it was still producing an a/c signal when spinning. So you should check both things.

 

OK... As I prepare to invest a few hours on the Vette this weekend I'm hoping to gain a bit more insight and guidance to insure I optimize my troubleshooting efforts. History is this (I'm trying to separate the Transmission anomalies from the problem). First, Speedo was working fine up until this past Sunday. I had just data-logged a tune adjustment and was preparing to adjust IAC counts which were slightly high. I forgot to shutdown the data-logging and disconnect the laptop when I removed the negative connection to the battery. I completed the adjustment, reconnected the ground cable and prepared to take it for a drive when I noticed the Speedo was reading only "0". I had been observing some drone at certain Tranny RPM's at time during the tune and thought that perhaps I lost the VSS... but it would have been REALLY coincidental as I've had no issues with the Speedo in the past. So... I check speed reference with the datalogger. All signals are transmitting EXCEPT no speed indication. I checked the speed reference as accessed with the heating / AC control module... no speed reference. And obviously only "0" on the speedo.

I plan to check the VSS this weekend based on input I have already received. Is it likely that leaving the laptop running with the ALDL connected to the OBD1 port and disconnecting the ground could have ONLY taken out the Speed signal (assuming ECM or CCM modul damage)? Or is it more likely a sending unit issue (VSS). I have no feel for the communications protocol used in the C4 and whether a single signal could be affected. My feeling is NO as I'm sure all of the data being transmitted through the OBD1 is multiplexed with the exception of the ECM inputs. So, for me... I'm thinking VSS (most likely a timely failure) OR the ECM input buffer was damaged. THOUGHTS???? Thanks

 

I'm almost sure there's no way your datalogging connection could corrupt anything in the car. It's not sending any signals to the computer - just reading the data the computer is generating. Also, if I'm understanding the pre-94 system correctly, the VSS sends a signal to the instrument cluster, and the signal then gets to the ECM after that. Your data-logger is reading the ECM, but since you see 0mph on the speedo, you know the problem is upstream of the ECM (it's either the cluster itself or the VSS or wire between the two). So again, I don't think you could have corrupted it.

You said the transmission was just rebuilt, so it's possible the VSS drive in the tailshaft has an issue, such as a bad gear or a missing retainer. And it's also possible the VSS itself is faulty, especially if it accidentally got dropped or smashed during the removal/installation of the transmission. Nothing's changed from earlier posts - that's the place to start your diagnostic journey.

 

You were asked 'tire size' so that when you actually start diagnostics, VSS removal etc you could have a reasonable thought of the drive/driven combination to maybe expect. Have't seen that mentioned yet.

1st stop for '93 is ECM with splices to SRC, cluster is supplied after the ECM signal is passed through the CCM.

 

thanks for confirming the VSS signal path. It almost has to be upstream of ECM. I’m starting with tranny / VSS

 

Well... I replaced calibrated my ASR this morning successfully (I screwed up the unit with over-torque). Following that I dove into the "loss of speedo" issue. What I found is indicated in the photo's. The VSS shaft was sheared off (very strange failure location actually within and almost flush with the VSS housing). I removed the green driven gear along (broken shaft still in gear). The green driven gear shows absolutely no damage to the teeth... same for the white drive spline gear. What would cause a failure like this I have no idea unless the VSS was not pressed in correctly (cocked perhaps). Anyhow, I'll order a replacement VSS.

Thanks for everyone's help and advice.

What I found when I removed the VSS

What I found when I removed the VSS

Almost looks as if the shaft is "designed" to shear at the location of failure as the shaft appears to undercut there.

 

You mentioned buying 'the right gears' when you did the D44/3.45. If tires were stock or very close you'd need likely 15/42 and a correctly orientated VSS, I don't recall ever seeing a 'WHITE' 15 tooth for a 4L60/700R4. I might have expected a gray. What part # and brand did you buy for the VSS? Did you have any problems with insertion? A VSS for a 40+ tooth driven for a 4L60 (Corvette) is most often considered a 'hard find'!! The extension housing doesn't tolerate (I don't believe) the likely still available M6 VSS.

What information is on the VSS itself.?

 

i bought the gear change parts from exotic muscle (if I remember correctly). The transmission shop installed. The VSS was not changed. Only the green gear I believe. The speedo has
worked for 250 miles after the rebuild.

 

[QUOTE=BPontuti;1601515446]i bought the gear change parts from exotic muscle (if I remember correctly). The transmission shop installed. The VSS was not changed. Only the green gear I believe. The speedo has
worked for 250 miles after the rebuild.[

ill check info on VSS

 

[QUOTE=BPontuti;1601515457]found photos online that look like the gears I have



Photos I found online indicating green driven, wife drive.... not sure it yields 3.45 translation.

 

Confirm the color of the 'drive' gear also. Green matches what's expected for a 42 tooth.Your 'on-line' information is 'ALL FLAWED' Work with what you've got that failed!!! Pictures ain't going to correct your problem!!!