temp and oil gauge problem

[David Anderson]

I've got a goofy anomily with my temperature and oil pressure gauge on my eagle 10.  When I'm running down the road the oill pressure shows 55psi, water shows about 175-180.  When I turn on the lights, either runnining/clearance lights or headlights, (either position), the oil pressure drops to 50psi and the temp rises about 10 degrees which actually is accuarate per the engine room mechanical gauge.  I've lived with this since I bought the coach but sure am tired of it.

Today I tried some things.  I didn't start the engine but put a hair dryer at the t-stat probe and got it up to about 130 degrees.  I turned on the master and sure enough, when I turned on the lights the oil press needle would drop a bit and the temp needle would go up a bit.  I put the negative probe of my meter on the temp sending unit and read about 8.5 volts.  The voltage would go up the hotter it got.  The positive probe was connected to 12v hot. 

When I turned on the lights the voltage would drop .5 volts.  Same for the oil pressure probe.  The voltage dropped on both, but the needles ran opposite each other. 

Thinking some weird ground looping, I bonded the oil press and temp sending unit cases together.  No change, same thing.
I bonded them both to the battery ground, same thing.
I disconnected the gauge light circuit,  same thing
I bonded the grounds at the back of the gauges, same thing.
I bonded the 12v + at the back of the gauges, same thing.
I disconnected the entire lighting relay.  No voltage drop on the battery and no voltage change on the sending units, The needles didn't move when I did this, but hey, I can't drive without lights at night.

I can understand a voltage drop when loading the battery or alternator, but I cannot understand why the needlles would change opposite each other.  My tranny temp gauge stays on the money, no movement there. 
All gauges are VDO.

What am I missing?  This has me stumped.

[Len Silva]

I just happened to be looking at some VDO info for a problem I'm having and I can tell you the the temp and pressure gauges work opposite to each other.

A 0-80 pressure gauge has a 10-192 ohm sender with 0 psi being 10 ohms and 80 psi being 192 ohms.

The 0-250 degree temp is just the opposite using 287.4 - 14.25 ohms. 287.4 is 100 deg and 22.7 is 250 deg

I found this at http://www.sso-usa.com/performance/TechnicalSupport/pdf/Bullentins/PressureResistanceChart.pdf
and  http://www.sso-usa.com/performance/TechnicalSupport/pdf/Bullentins/TemperatureResistanceChart.pdf

Another item I found on the same site is that the ground leads for electrical pressure and temperature gauges should be 14 ga. wire.

I have no explanation for the trans temp not reacting the same.

Len

[JackConrad]

Sounds like a bad ground connection somewhere. Might try running a dedicated 14 gauge ground wire from the gauges directly to the battery ground terminal. If it still acts up, this will rule out a bad ground on the gauges. Hope this helps, Jack