Temp Gauge show Higher Temps w/ Engine Running

[OneLapper]

Last weekend I ran the bus for 600 miles and it's obvious that the new engine runs hotter than the old engine (4106 w/ 8V71).  If I use a IR pyrometer on the thermostat housing it reads 182 degrees.  With the engine off, but the switch in Day, the dash temp gauge reads 185ish. If I immediately start the engine and after the Gen Light goes out, the temp gauge jumps to 200 to 205.  

At first I thought the wiring had some resistance which caused the gauge to indicate hotter than the engine is actually running, but readout is correct with the engine not running.  The sending unit and the gauge are about three years old.  I never thought anything about it with the old engine because I never saw temps over 190, and normally the gauge indicated 185.  The old engine has 170 degree thermostats.

What would cause the gauge to read 15 to 20 degree too hot with the engine running?  I'm betting the engine's generator is causing the error, but how would I correct for it?

I'm going to install the 170 degree thermostats from the old engine this week, btw.

Mark

[zubzub]

on the '04 there is a resistor on the supply wire to each gauge.  If the resistor was faulty  then higher voltages (like when the gen was charging)  would  lead to inaccurate readings.  BTW some kind of voltage control for the gauges is common in all the rigs I have worked on over the years so I'm pretty sure you should check that out.  Somewhere in your book there should be specs for this.  There should also be specs for the resistance of the temp sending unit, but by your description it really sounds like the resistor/voltage control.

[brando4905]

I have similar problems with my temp gauge too. When I turn the headlights on the temp reads 5 to 10 degrees higher.

[OneLapper]

I have similar problems with my temp gauge too. When I turn the headlights on the temp reads 5 to 10 degrees higher.

Yeah, I didn't even bother to mention that my gauge also jumps up when I turn the headlights on, too.

The gauge is reading correctly at 12.2 vdc or thereabouts.  The problem is when the generator is producing DC power, which is usually around 13.5 vdc. When the lights are on, the voltage kicks up even more, making the gauge further off.

I'll figure this out and post the fix!  Might take me a little bit, though....

[David Anderson]

Mark,

I had a similar problem and added a short ground wire from the negative post on the gauge to the bus frame.  The problem went away.  That is a cheap, easy start for you to diagnose your problem
 
Also, I changed my 170 t-stats to 180's and my heaters worked much better.  You may not like those 170's.

David

[busnut104]

What I did is put a mechanical gauge in the bed room, If the dash gauge is running a little hot I can have someone go back and ck the mechanical one. 

[Len Silva]

I think all these problems are due to bad grounds.

It's a bit scary because a temperature gauge will tend to read high if there is a problem, while the oil pressure gauge will tend to read low.

[OneLapper]

Mark,

I had a similar problem and added a short ground wire from the negative post on the gauge to the bus frame.  The problem went away.  That is a cheap, easy start for you to diagnose your problem
 
Also, I changed my 170 t-stats to 180's and my heaters worked much better.  You may not like those 170's.

David

I'll try the ground wiring first.  Sounds like a logical place to start.

Once I figure out what my OTR temps are I'll be able to determine if I should switch thermostats.  I have been running the 170 stats for several years and coach heat is excellent, never a complaint, even in Mont Tremblant in January.  Of course, it took me 6 hours to start the darn thing, but that's a different story...

I also thought about installing a simple voltage regulator to the temp and oil gauges.  A regulator that will keep the voltage to 12.2 which is where the gauges read correctly.

I'll see if the ground wire works first.  Much easier!

Thanks!

[white-eagle]

is Len Right?  (sorry Len, i don't mean to be offensive, just confirming what you said) and hoping you or somoene else can verify.

bad grounds can cause a myriad of issues with guages?

[Lin]

Bad grounds can cause all sorts of weird looking problems.  Look what happened to Ted Stevens.