hello, so I have finished up a 1993 Corvette LS2 swap. I have all the gauges, working, Tach, oil, temp. All that stuff. But the issue I am having, is that the Speedometer is stuck @ 0. So I first found out that when the trans was rebuilt, and the adapter was used, and put into the 4l65e, the VSS Sensor was the old style, and wasn't generating the signal. So I swapped out the driven/drive gear setup with the Magnetic style VSS. This started working. I am using the Holley Dominator EFI, to generate the 4K PPM signal to the CCM. I hooked up my O'Scope to the 4KPPM signal in the cabin, and found out that the 4KPPM@5 Volts is being generated by the Holley ECM, going to the CCM. But I am not getting any speed. So does the CCM need any other signal to display speed on the LCD Display. Any Help would be appreciated.

 

Oh, and all the other things on the LCD seem to work (Gas Gauge, sys indicator). And yes I have DTC's being display.

 

I doubt you'll get much help on this forum for such a custom installation.

I'm assuming you've got a service manual? It should show the CCM signal input/output traces to the speedometer.

I'm not too familiar with the function of the CCM with respect to the speedo, but if you can figure out what kind of signal the CCM is sending to the speedo, you may be able to go direct to the speedo with the signal by use of a Dakota Digital signal converter or something similar.

Sounds like a cool project. Good luck.

 

1993 C4 Corvette Restomod Speedometer issue when using a holley ECM(Solved!).







About 2 years ago I started on a restomod, pulling the LT1 engine out of the Corvette, and putting in the All aluminum LS2 from a 2005 GTO. At that time, I didn’t know how much of a project this would turn out to be. Well Long story shot, I got all the features working in the Corvette, with just one thing giving me problems. The speedometer. The Holley documentation states you can drive an electronic speedometer using one of the General Purpose Input/Output(GPIO) pins. Now I am sure, that this may be true in most cases, but alas not in my corvette. It doesn’t use a standard Electronic Speed-o, but the signal needs to be driven to the CCM (Central Control Module). GM now calls this the BCM. So the CCM senses the 4KPPM (Pulses Per Mile), and then drives the Digital LCD speed display. So the 4KPPM signal does not go to the Dash at all.










No matter what I did, the dash always displayed a big fat ‘0’, no speed was ever sensed. I tore apart the harness numerous times. Tore the interior apart. And ohm’d out the harness to the CCM. But I just couldn’t figure out what was going on. I got to the point where I knew I had to get out the big guns’ to figure this out.




First I want to send a big thank you to Bryan Thompson at batee.com. He answered a number of questions I had about what the CCM was expecting. Which started giving me hints on what to look for.







I setup the holley to put out the standard 4KPPM on pin J2B pin 2, which is a PWM+, Speed signal. So when I hooked up my O-Scope to the speedo output, I could see that the signal was biased @ 6 volts above ground, and then started pulsing @ 4kppm. This is not adjustable. You can adjust the frequency, but not the voltage. Nothing I did, would make the dash work. So I unpinned the speed signal from the car. And bought a Frequency generator.




With the frequency generator, I found that the CCM, would display speed, when I generated a 1 HZ to 200 HZ signal at anything from -5 volts dc starting bias, to +5 volts dc. And then adding a ppm signal that was about 5-10 volts. So I took a Divider Network using resistors (130 ohms, 1 Watt). And This changed the signal from the Holley to start @ ground, and then the pulse magnitude, was 7 volts, so it went from ground to 7 volts. And wala I now have speed being generated by the CCM to the Dash display. After about 6 months of work, the fix was 2 40 cent resistors. I know I have seen numerous issues on the web with the Holley, feeding the OEM speedometer’s. I didn’t want to spend 80 Dollars on the SGI-5 from Dakota digital. I have taken a number of pictures of the Holley software, the O-scope, and the Frequency generator.

 

Very cool. I always liked the 90-96 speedo. Feels good to solve issues like that without spending any real money.

It's also funny that we never seem to put any value on our free time.... LOL. 6 months for $80! I do the same thing all the time with this car. Congrats on the fix.

 

Nice job figuring it out. Do you have any pictures of the install that you can post. Are you still using the 93 ECM also?

 

I'll post some pics once I button it up.

 

Please post some install pics

super cool for ls2

 

where is the follow up of this, I am willing to pay for the info if you are willing to share it.