Oil pressure gauge

[mstailey]

Oil pressure gauge problem

I'm at my witts end. Flybridge starboard oil pressure gauge pegs when I plug in the sender wire and causes the main helm gauge to peg as well. When I disconnect the FB gauge the helm reads correct and the reverse, FB plugged in and main helm disconnected, FB reads correct. Here's what I've done so far:
Resistance on the wires from the sender, junction box to both gauges is nothing
Ordered and installed new sender (VDO) that is floating ground dual station
Ordered and installed new pressure gauge
Traced sender ground - by comparing with port wiring which works fine
Traced gauge ground both stations - low resistance with multi meter
Traced gauge power both stations - low resistance and shows 12.9 - 13.1 volts

From my understanding this appears to be incorrect resistance from the sender but sender is new and is dual station. What I've not done is swap port / starboard senders to see if it solves my starboard problem and makes the port gauge read wrong but that would be a hastle.

Any suggestions?

 

[Nonchalant1]

Yeah...it sounds like you've got a single station sender, but it is evidently not? Was it ever working correctly and then started this, or has it always been this way?

 

[mstailey]

Was working and I noticed about 3 weeks ago it had stopped.
Been trying to figure it out since then.

 

[TedZ]

Sounds like the teflon tape monster!!

Check the resistance from the brass body of the sender to the block right near the sender. The number should be small.

Speaking of "small", an ohm is a BIG amount of resistance in the scheme of things. When you say "low resistance" it would be helpful if you would say what it is and in what units. Something like 5 or 6 ohms is not small and will shift the gauge reading a lot!! They are working with something like 50 or 60 ohms in total as i recall.

A number like .005 ohms is small and shows continuity essentially.

Good luck.

Ted

 

[31FlyCruzr]

My vote is the new sender is a single station sender, unlikely that the readings would be right with only one gage hooked up unless the sender was the wrong one. If your sure the sender is ok, and you checked the teflon tape poor ground, check the voltage side while drawing a load with a test light. I clamp the multimeter onto the test light leads while checking. If you have a high resistance connection, very low current (no light) may get through and mask the problem, the test light amp draw shows this problem. When in doubt, check with and without the light, the readings should be the same.

 

[mstailey]

Ted / Bill,
The sender runs on a floating ground meaning I have 2 wires to the sender; 1 for the gauge 1 for ground that is a short piece of wire to the engine block so, I would think teflon tape has no effect one way or the other. Correct?

Ted, good point. The numbers I'm getting from post above are .006 - .009

OK, now that I've gotten a point in the right direction it's up to me to start the research - I do however agree with both of you that it appears to be a sender problem in that 2 gauges don't work. I'm going to fire up the port engine (good gauges) and get the ohms from the sender then test the "new" starboard sender.

Thanks for the point.

 

[TedZ]

Sorry, been traveling.

You are right, i missed the floating ground. So how good is the ground you have connected the sender to? That needs to be checked.

Ted

 

[31FlyCruzr]

I'm confused by the floating ground concept. Seems to me the gage opertation is grounded through the sender only. A lighted gage has a totally seperate ground wire for the light.

 

[TedZ]

Which is which? I have to look at a schematic. The fact is though you either need a good ground attached with a wire or you need a good ground to the sender depending on which you have.

Senders work by varying the connection to ground which in turn moves the needle in the gauge as the voltage varies. Yes, the ground to the gauge light is seperate. Obviously you don't want the lights to dim or get bright depending on oil pressure etc.!

Ted

 

[mstailey]

Ted,
The floating ground is a seperate wire on the sender directly to the engine block. At least that is how the port engine has it. Same with temp and trans pressure. The Hatt diagrams do not show this and Steve Mc. determined based on the gauges that I currently have, were not stock Hatters. They used Sun for this model boat and all the gauges are currently VDO Blueline. Note however the numbered wires per diagram do match for the actual gauge operation wire. In other words, diagram shows wire #25 for starboard and #15 for port which is where they are. After tracking down the service records and talking with the service manager he stated the owner did not like the chrome gauges so the Hatt dealer installed all VDO with the black square bezels as well as adding fuel gauges.

 

[TedZ]

That is exactly right and those numbers i.e. 15 & 25 are the same as on my boat.

So 15 (or 25) goes to the sender on one post and the ground to the engine block or whatever on the other. The ground should be perfect on the one lug and should be a resistance, whatever the pressure dictates. It either is right or it isn't at the sender. Now if the sender is fine and you still have that crazyness you have a bad gauge (doubtful) a chafed wire on ground (doubtful, but less so) or a mess up on the gage or on the flybridge terminal strip (likely). Seems to me with the goofy readings something is laying on ground. Insulated terminal no longer insulated that is touching the gauge case???

Just keep checking the sender as you make these wiring changes as you described. When the 15 or 25 goes to ground, you know that's the problem connection.

Good luck. You'll find it...be systematic. Write down the reading in different configs if necessary.

Ted

 

[Nonchalant1]

Steve Mc. determined based on the gauges that I currently have, were not stock Hatters. They used Sun for this model boat and all the gauges are currently VDO Blueline..... After tracking down the service records and talking with the service manager he stated the owner did not like the chrome gauges so the Hatt dealer installed all VDO with the black square bezels as well as adding fuel gauges.

Mike,

So when you got a new dual sender, is it specifically a VDO sender? Both the gauges and the sender will say VDO on them (obviously they need to be the same brand and type to work together).

Doug

 

[mstailey]

Hi Doug,
Good point -
Yes, sender(dual station) and gauge are both VDO that I purchased from SAMS.

 

[TedZ]

So how is it going finding the problem?

Ted

 

[mstailey]

Going to have it starting tonight. I'll post progress...

 

[mstailey]

TedZ,
Finally found this rascal.. Turned out to be the sending unit. It did have a floating ground but was not dual station. After looking up the specs on VDO's site (ohms) and getting the numbers I was able to confirm this. Also, they have a code number stamping on the sender itself which also confirmed this finding. I had a spare unit which was put into play.
This was a well learned lesson! Thanks to all for their suggestions.

 

[TedZ]

Just saw your post.

Good job.

Ted