Accuracy of gauges?

[5B Steve]

I fired my bus today to circulate my winter Diesel Fuel Additive, after about 20-30 minutes of  fast idling (I know that's to long)

my temperature gauge on the dash  was  210 plus.  Shut it down, ran to the back to see the temp, it was around 160.  Has

anyone else experience this?


Steve 5B........

[robertglines1]

not un common. I think the rear is mechanical gauge. and front is electric.so a wire going to ground or a bad ground would give you a false reading

[gus]

I'm having those exact same problems with my two gages right now on my 4107.

The rear mechanical failed completely today and the electric dash gage suddenly started showing 165* with the lights on when it has been showing 130* for the past four days of driving??

I agree that the electrical gages suffer from old wiring and corroded connections so they are not the most reliable. What we need is wireless gages!

[JimW7]

My 7 does the same thing. I was reading at 170, didn't know how old thermostats were, so I replaced them with 180's. Then gauge went up to 210. I thought I did something wrong, but mechanical guage in back reads 180. Mechanic says that's normal, about 2/3's of their fleet read wrong, even the newer buses. Too much resistance in over 40' of wire. So now I just use the gauge as a reference point. Still doesn't feel right when gauge reads too high.

[Sean]

Guys, electric gauges are incredibly simple devices and there is no reason you should not be able to get them to work properly.  At the very worst the extra distance to the engine should mean the gauge reads a degree or two low (not high).

For most gauges, there are really only five things to check:

1.  The sender.  This can be tested with a normal ohmmeter.  If the gauge is way off, this can be done right on-engine -- check the resistance when it's cold and again when it is at normal operating temperature.  For more subtle problems, like the gauge is off by a few degrees in only part of its range, the sender needs to come out and you'll need a pot of water on the stove and an accurate thermometer to test the sender through its full range of values.  If the sender is bad, new ones are about $20.

2.  The gauge unit.  This can be tested with a 500Ω potentiometer (variable resistor), about $3.  If the gauge itself is bad, replacement temperature gauges are about $30-$50, and if you have a special style to match your dash, there are instrument repair shops all over the place that will repair them for perhaps twice that.  I just went through this myself (see http://www.busconversions.com/bbs/index.php?topic=18328.0); I ended up opening it up and repairing it myself.

3.  Power supply.  Power is supplied to the gauge itself, and the circuit is completed through the sender.  The gauge has been calibrated  for a certain input voltage (either 13 or 26, depending on your coach voltage) and bad voltage at the gauge can give you a bad reading.  For example, if you have 10' of wire from the fuse panel to the gauges, and someone has tapped a light, CB, or whatever off that same feed, then when that device is powered up the voltage available to the gauge will drop, affecting the reading.

4.  Grounds.  The gauge circuit through the sender is completed via the chassis ground.  If the sender has a bad ground that will affect the reading.  Occasionally you will see a sender that someone has installed using thread tape or pipe dope -- senders should never be taped or doped and should always be fitted dry.  Depending on the gauge make and model, the gauge itself may also be grounded.  I had a problem with my gauges where all the readings changed when I turned the parking lights on -- turned out to be because Neoplan had used the same ground wire for the gauges and the dashboard lighting, so when the lights were on, the gauges "saw" a different ground potential.

5.  The single wire from the sender to the gauge.  This might be the hardest to replace, but it is easy to test.  Get a 50' length of #18 or larger wire (the thicker the better) and hook the two together temporarily, then warm the coach up in place.  The other thing you can do here is measure the resistance of your 50' wire, then use it to complete a full circuit between your ohmmeter and the existing sender wire.  Measure that resistance, then subtract the resistance of your 50' test wire to find the resistance of the sender wire.  It should be less than 1Ω (50' of 18-gauge, typical for coach senders, should be just 0.38Ω).  Also make certain that an ohmmeter reading between the disconnected sender wire and chassis ground reads dead open (infinite resistance).

And Jim, you might want to get a different mechanic to look at your gauges if the one you have now thinks 40' of wire presents too much resistance.  Some of these guys can rebuild a cylinder head blindfolded but are, shall we say, electrically challenged.

No way would I drive my bus without working temperature and oil pressure gauges -- too risky.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[5B Steve]

   Sean,

    Thanks for that  valuable post!

     Steve 5B..........  Merry Christmas.

[Sean]

Sorry, forgot to mention:

Senders and gauges are matched to one another.  So a VDO sender, for example, will not work with a Stewart Warner gauge, and vice-versa.  Even within brands sometimes there are differences between models.  Some manufacturers, such as Datcon, make different gauges to work with a variety of other OEM senders and vice-versa.

Having the wrong sender for your gauge (or wrong gauge for the sender, depending on your point of view) will likely get you incorrect readings.  In some cases the gauge will even read backwards.

This is why I did not list what the readings should be back up in #1 in my last post -- the correct values will depend on your specific gauge.  VDO temperature gauges, for instance, often use a 700Ω-22Ω sender, whereas Stewart-Warner typically uses 240Ω-33.5Ω.  Without knowing who made the gauges on your 5 I can't tell you what the resistance curve should be.  Although I would expect that if you have the factory gauges, the resistance should be specified in the maintenance manual, as testing the sender would be one of the procedures therein.

Anytime someone replaces a gauge without changing the sender or vice-versa there is the potential that they will no longer be correctly matched, and this might be the issue with Jim's gauge.  Here again I think many mechanics are just not aware of this, and if you drove in with a converted coach where all the gauges had been changed to, say, Teleflex (which is what, I think, the Custom Instrument Panel guys use), and the mechanic decided to pop a new sender in after working on, perhaps, your thermostats, he might just go to his parts room and pull an OEM sender off the shelf, which will probably not be the correct match for the Teleflex gauge.

Again, FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

Sean, in your experience is resistance high or low when sensor is cold/off?  22 ohms cold to 500 ohms full scale hot?

Brian

[luvrbus]

Steve probably nothing wrong with you dash gauge if the mechanical and the electric are in the same housing location it could be a problem but most of the time they are not I see that on buses where the thermostats don't open and you get that kind of reading because of the difference in location of the 2 gauges


good luck

[Len Silva]

Here is a resistance chart for VDO Gauges
http://www.nsifleet.com/pdf/TemperatureResistanceChart.pdf

and one for pressure gauges
http://www.nsifleet.com/pdf/PressureResistanceChart.pdf

[Sean]

Sean, in your experience is resistance high or low when sensor is cold/off?  22 ohms cold to 500 ohms full scale hot?

Depends on brand and model but I would say most frequently for temperature gauges it is high resistance cold and decreasing resistance with increasing temperature, as shown in the charts that Len linked.

For many fuel and pressure gauges the direction of resistance change is different for European and US gauges which is why I said that a mismatched gauge/sender combo can actually read backwards.

-Sean
http://OurOdyssey.BlogSpot.com

[Busted Knuckle]

Some of these guys can rebuild a cylinder head blindfolded but are, shall we say, electrically challenged.
-Sean
http://OurOdyssey.BlogSpot.com

Hey Sean I somewhat resemble that! (but the truth is the truth!)
  BK