Carver 325 oil pressure gauge problem

[wingnut]

RE: 1996 Carver 325, 5.7L Crusaders, duel helm.

While underway, driving from the flybridge, 1600 RPMs (but I don' think that matters) the starboard oil pressure went full scale high. I checked the lower helm, starboard oil pressure at 25ish psi. My normal is 40 psi at idle, slightly higher with increase in RPMs. All indications on the port engine remained normal. Tapping the upper gauge caused the needle to jump all over, with no change at the lower helm. No low oil pressure light, and no check engine light illuminated. I believe this to be an indication problem, not a problem with the actual oil pressure.

My boat has VDO gauges. I worked with a VDO dealer, with the 30 year old gauge and part number in hand. My gauge is out of production, but they have the replacement which looks nearly identical and has the exact same spade type connectors on the back. So as to not cause a mismatch problem, I purchased 2 of the new VDO gauges. Installed both gauges, turned the key on and both gauges moved to full scale high. Thinking perhaps I also had an oil pressure sender problem, I purchased a new oil pressure sender from the marina parts counter. Although I have duel helms, there is only one wire to the sending unit. I did verify that the new sending unit is the unit for a duel helm application. With the new sending unit installed...same problem, both new gauges go full scale high when the key is turned on. With either the new, or old sending unit installed, the oil pressure indicated lower than normal at approx 25 psi. at both helms. Increasing RPMs the gauge would go down to about 20 psi, in stead of going up with increased RPMs. Thinking I may have crossed the wires when installing the gauges, I tried swapping the two leads on the back of the gauge (the third ground lead was obvious correct location) and the indications remained the same.

I'm stumped. From what I hear, duel helms can be tricky with some of this stuff. What am I missing. I value any input from the forum.
Regards,
Jeff

[bud37]

When you changed the sender did you use teflon tape on the threads of the sender.....it needs to have a good ground to the engine block, so no tape is better, it should seal just fine due to the pipe threads.

Oh and to add....was all this fine for a long time before this happened ?

[wingnut]

Great question. No, I did not use any thread tape or other sealant on the sender.
Before this happened, I had randomly had a few fluctuations at the upper helm, but not at the lower helm. And tapping the gauge would cause fluctuations as well. When I opened the helm to remove the old gauge it was broken open and discolored. Perhaps it had gotten wet at some point? I have owned the boat for 16 years, and the flybridge is enclosed and never exposed to rain, and the helm is covered when not in use.

Thanks for taking the time to respond.

[wingnut]

Problem solved. I worked with an engineer from VDO. They said I had the wrong resistance oil sending unit. They sent the correct unit, problem solved. FWIW, the new (but wrong part) had 118 ohms of resistance. The correct part from VDO had 4 ohms of resistance.