I was traveling on the PA Pike this afternoon; cruise was set at 70mph when I felt a slight hesitation. I looked at the HUD and noticed that the MPH was flip-flopping up and down from 70 to 95. The dial on the speedometer was also erratic. I shut off the cruise and the hesitation stopped but the display was still erratic. It happened for about 20 miles, stopped for gas, and then it was ok. Anyone ever have this problem or know the cause?

 

Ouch no idea what would do that, but wouldn't that suck if your odometer started to jump up like that.

 

That sounds familiar. Didn't happen on my Vette, but did on a GTP I owned, while driving through Louisville. At first I though I had been driving too long, or I was experiencing a brain aneurysm. However, when I got home to TN, I pulled up the wiring schematics, and the schematics for this particular vehicle displayed that only two things could cause this were the power feed to the HUD or the ground.
However, the HUD on a Vette receives all its information from the DIC; whereas on the GTP the signals are received directly from the displayed systems.
Damn, I should read the entire post before making these long answers. You did say your cluster display was also acting erratic.
If only your HUD speed and I/P Cluster speedometer were acting flaky go to the following steps. If your entire gage clusters including lights, oil, fuel, etc. were acting flaky go to the ground check.
- Check the vehicle speed sensor connector (Yellow and Purple wires), located on the Top RH side of the differential. (If you have an ohmmeter, you can check the resistance in these wires from the PCM to the sensor and have someone wiggle the wires and the sensor connector to see if there is any sign of an open circuit)
- Also check the Dark Green/White wire on terminal 50 (this is the speed signal from the PCM to the cluster).
From here on in, it gets pretty tricky. You will have to pull the cluster and check the connector and wires at the cluster connector.
It could also be the speed sensor itself is going out.

Now, if all the lights and gages were flaking out, I would assume a ground was at fault. However, if confined to just the speedometer and HUD, then it sounds like a bad wire, sensor, or even a DIC.)
Check the ground in the Instrument panel harness, that is located below the battery tray, on the RH frame rail. Make sure the ground is not rusted over, and is clean, and the connection if very tight.
If this is ok, check the wire at the terminal to make sure it is not broken on the terminal stud connector. If that is ok, you can check the splice pack, located in the Instrument panel harness, on the RH side of Instrument panel. This ground splice pack is connected to the ground mentioned above.
If you need any procedures or further information, write me at whitec5vette@aol.com.

 

WhiteC5Vette;

Waht a great response

This Forum member ROCKS

 

He even sent me two more emails with more info

 

Would it be possible for you to add what was said in his e-mails to this post? Could be some additional useful info to help others out. Thanks.

Great info BTW.

 

OK here it is:

That was for the main ground wire connector to the Cluster. If this was bad, most likely the entire cluster would be acting up, since it would kill the complete circuit to the cluster. Since it is confined to the speedometer and the HUD speed (where the other indicators in the HUD flashing?), it sounds like it is confined to the speed sensor, wiring and/or connectors, or even the cluster or PCM itself. However, I would go after the sensor and the wiring/connectors first.
Conceivably, a circuit within the PCM or I/P cluster could go bad, but these are electronic devices so the likelihood is slim.
Usually, when a sensor goes bad, or there is an intermittent problem, the PCM will turn on the SES light. So, you could check to see if there are any history codes for the VSS or its related wiring. The PCM cannot actually determine if it is the wiring or the sensor, it will only see a drop in the signal. However, the drop has to be for a specific time period or a code will not set.
It definitely sounds like it was jumping quick, so the PCM may not have set a code.

Since say it goes up/down, up/down, etc., and then stopped after a while makes me believe it is wiring or a connector. If it happens again, then stops, then again (intermittently), then again, I would definitely go after a wire or connector. Especially the speed sensor wires and connector. The wires and connector itself are subjected to any type of weather you are driving through, as well as massive vibrations.


Thanks again Doug

 

Thanks for the additional info.