Gauges that never rest

[Iceni John]

I have a weird situation with some of my gauges.   With the engine off and the Cole-Hersee battery disconnect completely off, why do three gauges still show readings, and what's more one is even higher than when driving?   Today the Teleflex engine temperature gauge reads 180, the Stewart-Warner transmission temperature gauge reads 270 (!), and the Teleflex tachometer reads 550 RPM.   On other days these gauges read slightly differently!   All other gauges always read zero when the bus is asleep.   When I start the engine all these gauges read correctly, so it seems the mystery three are getting some input, but from where and why?   The complete factory electrical schematic and data sheet don't indicate why they are functioning (or at least appear to be functioning) with all the power off.   My digital multimeter doesn't show any voltage at these three gauges when the engine is off.

A thread here about three years ago suggested that some gauges don't have a Return To Zero function.   Why would some gauges RTZ, and other essentially-similar ones don't?   This is not exactly something that will engage (sic) me every waking moment, but it still bugs me when I can't think of a sensible reason for it.

Thanks, John

[bevans6]

Some of the gauges on my bus don't return to zero.  The oil pressure gauge for sure, rests at 40 psi.  If they work, I don't think it's a problem.  If it is a problem, don't tell me, I don't feel like worrying about yet another thing right now.

Brian

[gus]

Brian is right, many gages stop where they were when last on and some don't.

The most likely reason is they are cheaper to manufacture that way, this is the American way of doing business.

If the batts are disengaged there is no way anything can be on.

[Cary and Don]

I don't know how your gauges are wired, but ours will only return to zero if after stopping the engine with the engine run switch, we flip the switch that supplied 12 volt to the gauges back "on" for an instant.  Otherwise, the gauges stay where they were on shut down. Our gauges don't get their 12 volt through the run switch.

Don and Cary
GMC 4107
Neoplan AN340