When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The speedometer doesn't respond on my 1999 Corvette's dash. I need to find a way to self test it. It passes the self test at startup, sweeps the gauge needles fine. However, when I'm driving it doesn't respond to speed.
Wiring under dash from C150 connector (before firewall) to PCM is good.
Using HP tuners I can pull VSS PIDs and proper speed. However, the VSS signal to the cluster is not working.
I don't much want to probe from the firewall to the cluster, so not sure how to test that...
Question 1: Is there a VSS signal self test you can do with the PCM? To output a VSS signal w/o inputting VSS sensor signal?
Otherwise, I've pulled the PCM and am trying to bench test it as follows. Not exactly sure what I need to power up the PCM. I'm making assumption all Battery and Ground (not references) need connected per this pinout HERE
Blue: Pin 1 & 40 Red Pins 1 & 40 to ground
Blue Pin 19, 20 Red Pin 57 to Power / 12 volt
Red Pin 20 to VSS low & Pin 21 to VSS high from function generator
Red Pin 50 VSS signal output.
I've hooked everything up as such and I'm not getting any square wave out on VSS Signal when I push in a 1khrz square or any square wave for that matter wave as I'd expect from VSS sensor. I get a noisy pulse at same frequency as VSS signal input that doesn't stabilize into a square wave.
I assume the PCM is bad? Everything else works on the PCM otherwise. Car runs etc.
Last edited by weinerschizel; May 26, 2023 at 04:54 PM.
Since HP Tuners is showing speed, you know that the VSS sensor and the VSS input circuit on the PCM are working.
I understand your test setup...
When you run the function generator into the PCM, is HP Tuners showing speed? It should if the function generator is connected correctly and producing a compatible signal.
Then if you have no signal on the VSS output to the Speedometer, it does sound reasonable that the PCM is bad. Any chance this output signal got shorted to Ground or +12V in a way that might have damaged it?
I don't know if anyone has created a schematic diagram of the 411 PCM. Likely just a single component is bad and might be easy to replace since you are obviously electronically competent.
@mrvedit dang thanks for the tip! I didn't think of that... stupid me, I'll hook up HP tuners to the PCM and see what the PID reports for vehicle speed. I actually took the cover off the PCM to inspect it. It looks good, but that's not saying much. The salt contamination likely shorted out some pins. It was really bad until I cleaned it all up.
You wouldn't happen to know how I might send a VSS signal back into the dash? I took apart C150 and shot a 5Vpp square wave into the VSS signal pin... W/ function generator grounded to chassis ground. It's been a while since I've bench tested a instrument cluster to know if that's all I need... or if there's a ground / reference from the PCM on the cluster that needs tied into the function generator to?
Thanks for your generous complements. I am a mere mortal w/ an engineering degree. The real geniuses are the technicians that build and troubleshoot this stuff!
Hmmm, I would expect a 5V square wave to run the dash speedometer, it might be a much lower frequency than what the VSS sends. I would start at 100hz. If that doesn't work, I would wonder if the speedometer has a grounding problem (and the PCM is OK).
Yeah, I wonder the same. I really don't want to take the dash apart to get the cluster out and test it. I was hoping I could just patch into the PCM Red Pin 50 for VSS signal w/ the square wave. I'd tried numerous frequencies, all 5 Volt peak to peak square waves.
I do have an issue with the driver side turn lamp. The bulb will flash, but is very dim. I thought I'd checked all the grounds, at least on the front but that issue has eluded me.
My car was cherry, but then I stored it for several years in a nice cozy single car garage just for it... but all this crap happened
I don't know if I'd said, I went back and put a sine wave in on the VSS input lines.
I'm thinking there must be an isolated ground for 5 volt reference signals? That the instrument cluster must have an isolated ground for those? If so any idea what pin that is on the PCM connector?
Would explain why the instrument cluster didn't respond when I put in a 5Vpp square wave (I swept a frequencies up to 200hrz - I think equates to 200mph if memory serves me right).
Sorry that I cannot help you more now as I'm a road trip for another 10 days. (Currently in Kalispell, Montana).
Ron, you say that the VSS is a Hall Effect which puts out a sine wave. But doesn't that signal only run into the PCM which then outputs a different frequency square wave to the Speedometer. (I don't have easy access to the full schematics now.)
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Amazing '71 Camaro Restomod Is Modern Muscle Car Under the Skin
Verdad Gallardo
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
@RonSSNova is correct. The sensor puts out a sine wave. I'm not getting the square wave from the PCM when I put in a sine wave. I think the amplitude and frequency vary on the input. Something like 40 pulses per revolution of shaft... But I don't know the amplitude part. I just threw in a 1khrz sine wave on the input and got a 1000hrz greatly attenuated sine wave on the output. I'm not sure if the grounds are isolated though. I'm using same ground for everything.
@mrvedit your suggestion was best, to rig up HP tuners on the bench and scan the PID for vehicle speed / VSS stuff. I've just not had time to do it yet :/
Got bench test to work, PCM outputs VSS signal. I had the pinout reversed and missed one of the 12V power pins. Updated my post with the pins and it works now. Seems PCM maybe fine after all. Issue must be electrical fault between speedometer and dash.
Good work and great that you understand all these signals and have the signal generator and scope to test the sensor and PCM.
Pain to fix the dash wiring, but at least you now know where to look.
This is a great example of performing proper diagnostics instead of just replace stuff. Obviously replacing anything here would not have helped anyway.
Good work and great that you understand all these signals and have the signal generator and scope to test the sensor and PCM.
Pain to fix the dash wiring, but at least you now know where to look.
This is a great example of performing proper diagnostics instead of just replace stuff. Obviously replacing anything here would not have helped anyway.
Thanks for your compliment. Probably figuring time would have been faster to play parts darts. There's a ground path I think I found I might test. It's in the Data Link connector. I need to find time to work on this more.
Is there a generic Tech 2 programmer now? I might buy one if it's reasonable.
Is there a generic Tech 2 programmer now? I might buy one if it's reasonable.
I searched on Ebay for "GM Tech 2 programmer" and see there is a dongle and phone app "simulator" for about $150, Chinese clones for about $350 and genuine used ones for $1000+.
I bought the dongle/app version years ago, but never used it as I then sold my Vette. Maybe someone has experience with the dongle/app version or a clone. I suspect some clones are very good and others are crap.
Fed a 10vpp square wave in on Red Pin 50 VSS Signal, grounded to Red pins 40 and 1. No result.
Probed Red Pin 50 VSS Signal to Red pins 40 and 1 Ground pins on Red connectors. Resistance was 5k ohms (assuming this is resistance of the circuit in instrument cluster?).
Probed grounds to battery / chassis ground, Blue: Pin 1 & 40 Red Pins 1 & 40 to ground. All were open circuits???
Probed battery positive to battery voltage on PCM: Blue Pin 19, 20 Red Pin 57 to Power / 12 volt. All were open circuits? Assuming some power is switched.
Need to be certain I have located both the S120 and G120 lactations. Can somebody verify EXACTLY where they are at? FSM isn't too specific and I thought I tested all the grounds and splice packs on the chassis.
Engine harness, approximately 15.0 cm (5.9 in) from the main harness in the branch leading to the Powertrain Control Module (PCM) connectors (CKT451)
I'm studying the wiring diagrams from the FSM and see the ground pins are not switched. They should be connected to chassis ground through splice pack S120 then to G120 on chassis. However, I cannot find G120 or S120 anywhere in the FSM or an internet search... it is a typo??
Last edited by weinerschizel; Jun 4, 2023 at 02:21 PM.
I pulled the IPC & probed the VSS Signal from the PCM connector to the cluster harness. I could find no fault in the harness. I also drove with a small oscilloscope on the VSS Signal. I got a good VSS signal / square wave.
No clue why the IPC is speedometer is not responding. I need to get setup a bench test for it now :/
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Slideshow: From wobbling harmonic balancers to failed EBCMs, these are the issues that define long-term C5 ownership and what repairs typically involve.
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Slideshow: A modern Camaro transformed into a retro icon, this limited-run "Bandit" build blends nostalgia with brute force in a way few revivals manage.
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Slideshow: Cadillac didn't just crash the high-performance luxury vehicle party, it showed up loud, supercharged, and occasionally a little unhinged...
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Slideshow: A one-off sports car that looks like a vintage Italian exotic-but hides a C6 Corvette underneath-just sold for the price of a new mid-engine Corvette.
Bench testing PCM VSS 
