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Author Topic: How do you test a Oil Pressure Sending Unit?  (Read 3131 times)
Dave Siegel
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« on: September 19, 2006, 09:02:10 AM »

OK, OK  I realize I am putting myself up for ridicule on such a simple subject, but this is a need to know situation.

I had 12 volt VDO gauges in our coach and they worked fine, but I decided I wanted a better looking dash so I changed them out to Cyberdyne Digital gauges. The Cyberdynes are cool looking but one day the oil pressure gauge pegged at 80 pounds (not working) and last weekend when coming home from our rally in North Ft. Myers the water temperature gauge started acting irratically.

I want to go back to the VDO gauges and I want to test the sending units.

My thought on the oil pressure gauge is to attach a 9 voly battery to the wire that goes to the sender (at the motor) and then check the voltage at the wire that goes to the gauge. I would think if there is a voltage loss I would have to look for a problem. If I have 9 volts I have a good wire, I know I have a good VDO gauge so then I have a bad sending unit.

Is that how I would do it?

Dave
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Dave & Jan Siegel    1948 GMC  "Silversides"
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ChuckMC8
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« Reply #1 on: September 19, 2006, 09:45:34 AM »

Dave, In my MC8, I have a pair of mechanical gauges at the engine...oil pressure and water temperature.The dash gauges are electric, and  I beleive that the mechanicals gauges are more accurate. I can compare the dash gauge and engine gauge in case of odd readings (like abnormaly high water temp or low oil pressure)
   If you dont have the gauges at the engine, I reccomend installing a qualityset, such as Stewart Warner or comparable. Not much $$$ to help with a major PIA on the road- Not exactly what you were asking about,but will tell you what the engine oil pressure really is....... HTH anyway...chuck
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Eagle
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« Reply #2 on: September 19, 2006, 10:06:12 AM »

You also need to match sending units with gauges.
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Len Silva
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« Reply #3 on: September 19, 2006, 10:28:38 AM »

Dave,

Lots of good VDO info here:

http://www.egauges.com/Inst_PDF.asp

and here:

http://www.sso-usa.com/marine/TechnicalSupport/index.html
« Last Edit: September 19, 2006, 10:30:21 AM by Len Silva » Logged


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JackConrad
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« Reply #4 on: September 19, 2006, 11:44:23 AM »

Dave,
  Try running a dedicated ground wire from each gauge to the batteries. Poor grounds can cause all kinds of "goofy stuff".  Your oil pressure gauge pegging may have been a poor ground and erratic operation is usually a poor connection, possibly a poor ground connection.  Jack
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rayshound
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« Reply #5 on: October 09, 2006, 04:25:58 PM »

Good info on using matched senders to gauges. To do a function check on most electric guages & senders and wiring is to unhook the wire off of the sender the gauge should drive all the way upscale. Now ground the sender wire it should drive the gauge down scale. This will work on oil pressure, water temp & gas gauge. The DDEC series a whole different story.  Ray
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Hartley
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« Reply #6 on: October 10, 2006, 11:57:24 AM »

I have the Cyberdyne gauges also. I found that really good grounding on the engine to frame and gauge cluster
to frame is required or they will flake out and give you all kinds of wierd readings.

A floating ground will make them read way high and out of range or not work at all.
A ground loop between gauges with a bad dash ground will make them read low since the (-) Ground is floating
slightly positive.

I had one on my water temp reading 285 degrees and my oil pressure reading LO or 6 psi up to 25 PSI..

Fixed the grounds and now with the engine at 150 I get 150 on the water temp, Both the stock one and the electronic one.

My oil pressure idles at 18 psi and running at speed at 48 psi ( warmed up ). This is what the mechanical guage says in the engine compartment. ( no didn't ride back there, Just revved the engine!) Cool

As far as liking the CyberDynes... I can live with them but make a lot of light at night...I have parts of digits going on and off
at times. But every time I decide to change that one it self corrects.. so much for that...
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« Reply #7 on: October 10, 2006, 12:28:02 PM »

Dave,
I thought I'd actually answer your question...

To test the sender, (or the gauge) first find out from VDO what the resistance range of the gauge and sender are.  typeically you'll find numbers between 50 and 200 ohms.
Once you know that, to test the sender and all of it's wiring, simply disconnect the sender wire from your gauge and hook an ohm meter from that wire to ground.
Measure the resistance with the engine stopped, and with the engine running- the value should change radically between those two situations, and somewhat match up with the resistances you got from the manufacturer.  If the resistance is always zero (or very few ohms) your wire is probably shorted... if it's infinite or higher than a few hundred ohms it's probably broken or intermittent, and if it doesnt change between running and stopped (it should follow your engine speed just like the gauge will when it's working) then your sender is bad.  You can disconnect the sender and do the same test right at the sender to verify if your wiring is kaput...

To test the gauge, get a potentiometer with roughly the resistance that the manufacturer told you, hook it between ground and the gauge input, and vary it.  The gauge should follow along... again to test the wiring and the gauge, you can put the potentiometer back where the sender is (disconnect the sender) and it should make the gauge wiggle happily from back there...

If you can't figure out the resistance value from VDO, it's probably good enough to start with a 500 ohm potentiometer- that may not match up entirely well but it will give you a good idea if things are working or not, and which ones aren't

If this is clear as mud, let me know and I'll try to reword it... to me it's clear but this stuff is what I do... explaining it is another story!!! Smiley

Cheers
Gary
« Last Edit: October 10, 2006, 12:31:10 PM by boogiethecat » Logged

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Dave Siegel
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« Reply #8 on: October 10, 2006, 01:08:56 PM »

gary Thanks,
You are right it was as clear as mud. But you did try very well, it's my limited brain matter that's the weak link.

What I did was , bought new VDO sending units and re-installed the dash gauges (replacing the Cyberdynes). I made sure I had a good ground from the gauges to the frame. When I started the bus they worked fine and gave me reading that are very much like the mechanical gauges that are in the motor compartment. I'm happy with that.

I have yet to hook up my rear back up camera, and when I do the monitor has a provision to monitor several sources and I was thinking about installing another camera facing toward the mechanical gauges in the motor compartment. Then every once in a while just for a double check I would view that screen and double check the both pairs of gauges against on another.

Thanks again to everyone that tried.

Dave Siegel
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Dave & Jan Siegel    1948 GMC  "Silversides"
               Naples, Florida
   Dave is Host to the "Help Assist Pages"
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RTS/Daytona
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« Reply #9 on: October 10, 2006, 02:36:05 PM »

Dave

Almost ALL types of VDO senders are 10-180 OHMS (That's at VDO standard) - only exception is usually fuel gauges

to check a sender - Disconect the sender wire from the gauge (important) and use an ohm meter and check the resistance
example 0-80 psi VDO Gauge - 40 psi is approx. (1/2 scale) should = (10-180) / divide by 2 ==> or  95 ohms

to check a Vdo gauge - disconect the sender and replace with a know resistor   example
0-80 psi VDO gauge - (10 ohm = 0 psi) --- (95 ohms = 40 psi (half scale))  ---  (180 ohms = 80 psi (full scale))

I like VDO method cause shorts will peg the meter lower than 0 (lowest reading)
and opens will peg the meter over full scale

Pete

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