"Jumpy" speedo and high idle?

[DennisG01]

For years, whenever it's been really cold (below freezing) and the truck has sat for a while (at least over night), when I start it up (which is fine) the speedo will jump around (at idle, in the driveway). Today, I also noticed that the high idle was cutting in and out. After about 30 seconds to a minute, everything went back to normal (which is how it's always been with the speedo). I don't use the high idle a whole lot, so maybe it would of done this in the past, too.

It's not that big of a deal (I don't think), but just out of curiosity, any thoughts to why this is happening? My only thought is electrical connections contracting because of the cold. But, would they "uncontract" in the space of just 30 seconds to a minute?

[More Power]

I know that a problem alternator can sometimes cause a problem with how the speedo functions. The speedo is electronic, so an electrical problem can express itself visually through the speedo.

I suspect there is something that uses/generates power is causing the erratic speedo by putting noise into the electrical system - like an arcing glow plug relay, HVAC fan motor, or even a flakey alternator. Jim

[DmaxMaverick]

This condition is "normal", to some degree. I don't know at what degree it becomes abnormal. Every GM vehicle with electronic speedo, and most other brands do this, in my experience. My 2001, for example, will show "speed" to the point it will slowly rise enough to trigger the "E-brake alarm" if held to 2500 RPM for about 10 seconds. My previous 2000 Impala would do the same, if the throttle was held just below the engine speed cut-off. It's curious though, as the speedo shows speed, and the ECM sees the speed (I can read it on a scanner), but the odometer doesn't advance.

As Jim said, it could be an indicator of an electrical problem. Also, I think it has to do with the nature of the vehicle electrical system as a whole, with many common electrical conduits and ground routing. If one circuit loses a ground, the circuit may ground through another component.

[DennisG01]

Interesting, thank you. I'll take a closer look the next time this happens (and have all accessories off) to see if the jumping corresponds with the glow plugs firing. Any thoughts as to why this issue goes away after about 30 seconds? I can understand that weird things happen in the cold, but I just don't get how anything can warm up fast enough in 30 seconds.

[DmaxMaverick]

The "30 seconds" condition is more likely a timed issue (such as GP load, or alternator "catch-up"), rather than temperature related. 30 seconds isn't long enough to get anything warmed up, except the exhaust, of course, and nothing temp related will trip at 30 seconds. The IPC diagnostics are done by the time you let off the key.

[DennisG01]

No jumping today (it was 29* when I started the truck). I did have everything turned off, though. I'll keep working on taking a better note of what's going on. I'll take note to the actual time - although, I think it's a random amount of time. Meaning, I believe that sometimes it longer than others - and sometimes shorter than 30 seconds, too.