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No Speedo redout, stuck on zero. I can't get the ECM to create the signal on circuit 817 to the CCM. The ECM is tested and good, I have several. The VSS is for sure inputting the signal onto the ECM. My signal generator attached to Circuit 817 shows 54 MPH on the cluster, so from the ECM to the CCM to the cluster, I am good. No trouble codes. Please help! Thanks
Have tried to check your speed though the C68 AC control, push and hold up and down the screen will go blank or 00 scroll I believe 7 there a pile of things you can do though it
The VSS is for sure inputting the signal onto the ECM
- Did you actually check that you get the differential signal from the VSS permanent magnet generator ? It's a tricky test because it requires the transmission to rotate..Have you tried changing the speed sensor ?
- Did you try to disconnect the SRC FX3 harness as it also picks up the VSS signal from the sensor. A problem on the FX3 VSS interface could drive the VSS signal low all the time.
My signal generator attached to Circuit 817 shows 54 MPH on the cluster
I see that you did the CHART1 diagnosis so CCM is out of the problem. And swap ECMs so ECM out of the problem.
I'm attaching the FSM extract to help with the discussion (see below).
- Did you actually check that you get the differential signal from the VSS permanent magnet generator ? It's a tricky test because it requires the transmission to rotate..Have you tried changing the speed sensor ?
- Did you try to disconnect the SRC FX3 harness as it also picks up the VSS signal from the sensor. A problem on the FX3 VSS interface could drive the VSS signal low all the time.
I see that you did the CHART1 diagnosis so CCM is out of the problem. And swap ECMs so ECM out of the problem.
I'm attaching the FSM extract to help with the discussion (see below).
No FX3 option and I actually have a spare VSS with pigtail hooked to a drill and connected to the purple and yellow wires on the ecm connector, excellent signal and verified on another 92 Corvette I have, So signal in just no signal out. Weird, but for some reason the ECM will not convert the signal. Today I'm going to verify and clean all the grounding spots.
No FX3 option and I actually have a spare VSS with pigtail hooked to a drill and connected to the purple and yellow wires on the ecm connector, excellent signal and verified on another 92 Corvette I have, So signal in just no signal out. Weird, but for some reason the ECM will not convert the signal. Today I'm going to verify and clean all the grounding spots.
Nice, and this is valid for the x2 ECMs you have tested, interesting.
What about the ECM harness, how clean are the contacts ? I guess that you already checked that cause you're probably injecting your test signal there right ?
Also just one (dumb) remark, if you unplug the 22 pin ECM connector to inject your VSS signal driven by the drilling tool directly on the ECM, the buffered signal that goes to the CCM is also on this very same connector, so you won't get any readings...But you can measure the output from the ECM with a scope.
Nice, and this is valid for the x2 ECMs you have tested, interesting.
What about the ECM harness, how clean are the contacts ? I guess that you already checked that cause you're probably injecting your test signal there right ?
The ECM is not throwing out a DTC 24 ?
No code 24 and tested the connector pins, they are good and clean, and cut the wires to the existing VSS connector to eliminate any short there and wired the test VSS directly in. According to the service manual, this problem does not or should not exist. Checking on my grounds and cleaning them, not sure what else to do, thanks
Spare VSS pigtailed to ECM, driven by a drilling tool :
Working on another 92 Vette
Not working on your 93 Vette
Two ECM tested and not working.
ECM harness inspected / ECM connectors inspected
Grounding under inspection (but you would have other issues, imo)
More questions :
1- Is the car running OK ? can you start it ?
2- Have you tried another EEPROM in the ECMs, did you install the same one when swapping ECMs ?
Yes on your comments and the ECMs were complete units, I have about 20 C4s here, I'm Gary's Corvettes Georgetown, TX . The engine runs perfect and every other thing on the car works great.
Yes on your comments and the ECMs were complete units, I have about 20 C4s here, I'm Gary's Corvettes Georgetown, TX . The engine runs perfect and every other thing on the car works great.
Ok, are you aware of the conditions to trigger a DTC 24, it will not show up just by starting the car :
AHHHH I figured it out, the brown connector tested good on both sides with an ohm meter but once installed it was not making contact to the ecm pins. I pushed the connector in hard as I was testing and the speedo came to life, spliced in a new connector and it's A ok, thank you all for your help!!
AHHHH I figured it out, the brown connector tested good on both sides with an ohm meter but once installed it was not making contact to the ecm pins. I pushed the connector in hard as I was testing and the speedo came to life, spliced in a new connector and it's A ok, thank you all for your help!!
There's much that C4 owners should retain from your experience for ECM's '90 - '93. I imagine there's maybe more than a few have spent many $$$ in attempts to diagnose ECM issues all of the mentioned C4 years with under-hood ECM. I had a friend that on every trip to a salvage yard he acquired all of the '88 - '91 I believe W-bodied ECM connectors that are the same as and very easily accessed .
STUMPED no speedometer reading 93 Corvette
