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Old 08-29-2019, 02:36 PM   #1
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VDO Oil Pressure sender unit

My oil pressure gauge is giving erratic readings although oil levels and everything else seems fine. My gauge and sender is a VDO, for 80 max psi. A test for resistance of the sender unit gives erratic readings, rapidly fluctuating all over the place.

The sender has a mark: 0-5 bar. Defender.com shows several units that might fit and the look the same, but how to I determine which one is a suitable replacement? The mechanic in the boat next to me suggests going with a 2 wire terminals. The present gauge is for a single wire with the “ground” to the engine block.

Oh, and yes, I’m going to T-in a mechanical gauge for the engine room as well.

Advice?

Thanks,

Jim
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Old 08-29-2019, 03:13 PM   #2
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VDO senders are identified by the max psi rating of the gauge and whether it is a single or dual gauge setup. I found that my senders were mismatched for 30 years. Now with the correct ones I read 60 psi on both engines. Also be sure to check your wire connections for corrosion. There is no advantage to a two wire setup as the block is your ground. Do not use any Teflon tape on the threads
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Old 08-29-2019, 03:14 PM   #3
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A web search will tell you which one to order.
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Old 08-29-2019, 03:38 PM   #4
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A web search will tell you which one to order.
True. I wouldn’t necessarily go to 2 wire. Your going to have to ground one so you’re adding complexity imo.
If you T in a mechanical gage try not to use a pipe nipple. If you have to use hydraulic pipe it’s thicker.
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Old 08-29-2019, 07:51 PM   #5
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True. I wouldn’t necessarily go to 2 wire. Your going to have to ground one so you’re adding complexity imo.
If you T in a mechanical gage try not to use a pipe nipple. If you have to use hydraulic pipe it’s thicker.
Good advice. Brass will crack and break off from work hardening by vibration. Use steel from a hydraulics supplier.
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Old 08-29-2019, 10:38 PM   #6
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I’m still trying to figure out which sender I need. The mechanical gauge come later. But I will use a hydraulic line and the appropriate “T”. I will go to one of those industrial hose places. There’s a place here called Greenline hose and they have been very helpful in the past with putting together fittings.
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Old 08-30-2019, 06:45 AM   #7
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Advice?

VDO customer service was very helpful, quikcly responsive, etc., when I had questions about selecting new OP senders...

-Chris
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Old 08-30-2019, 10:48 AM   #8
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VDO Oil Pressure sender unit

Thanks all. I took the unit down the street to my friend the retired machinist and he look out his gauges and callipers to measure the nipple on the unit and has concluded that I need the unit with the 1/8 27 NPT nipple. This 80 psi dual station unit should work:

https://www.defender.com/product3.js...9100&id=154769

Jim
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Old 08-30-2019, 01:28 PM   #9
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I only recently learned that the temp sending unit must be matched to the gauge, not only that but both the upper and lower stations must have compatible gauges. Plus you must have a sending unit made to signal to two gauges.

Are you telling me that the same is true for oil pressure gauges on a two station boat ? This would explain why my upper station oil pressure gauge is unreliable.

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Old 08-30-2019, 04:02 PM   #10
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Yes all must match
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Old 08-30-2019, 08:02 PM   #11
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Agree. And unlike some other manufacturers VDO has a much wider variety of senders. One must match up the correct sender to the gauge or the readings will be off.

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Old 08-31-2019, 09:58 AM   #12
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I only recently learned that the temp sending unit must be matched to the gauge, not only that but both the upper and lower stations must have compatible gauges. Plus you must have a sending unit made to signal to two gauges.

Are you telling me that the same is true for oil pressure gauges on a two station boat ? This would explain why my upper station oil pressure gauge is unreliable.

pete

Yep. And VDO has single- and dual-station senders.

And then some gauges are "programmable" to a certain extent. In the CruzPro case, for example, it's a menu selection from among several choices.

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Old 08-31-2019, 02:49 PM   #13
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Yep. And VDO has single- and dual-station senders.

And then some gauges are "programmable" to a certain extent. In the CruzPro case, for example, it's a menu selection from among several choices.

-Chris
Rather than trying to find a sender to use with an old gauge and then finding out that the old gauge doesn't work well, perhaps just buy new gauges with matched senders out-of-the-box. Yes, more more money (yours) but the result would be gauges in which you would have confidence. I did that using VDO gauges at both the lower and upper helms. Very happy with that decision.
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Old 08-31-2019, 07:23 PM   #14
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Rather than trying to find a sender to use with an old gauge and then finding out that the old gauge doesn't work well, perhaps just buy new gauges with matched senders out-of-the-box. Yes, more more money (yours) but the result would be gauges in which you would have confidence. I did that using VDO gauges at both the lower and upper helms. Very happy with that decision.

Yes, I may yet regret my decision to replace only the sender. But the gauges aren’t that old (circa 9 years), and for sure they are VDO with dual stations at 80 psi. So hopefully it will all work.

I also have VDO temperature gauges both helms. I was hoping to “connect” another gauge to the temperature sender for display in the engine room, however from what I’m hearing that will not work.

Jim
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Old 09-02-2019, 04:30 AM   #15
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Yes, I may yet regret my decision to replace only the sender. But the gauges aren’t that old (circa 9 years), and for sure they are VDO with dual stations at 80 psi. So hopefully it will all work.

I also have VDO temperature gauges both helms. I was hoping to “connect” another gauge to the temperature sender for display in the engine room, however from what I’m hearing that will not work.

Jim
My concern was with getting a correct match of gauge to sender. Buy the wrong sender and gauge does not read correctly. I bought new to ensure that gauges were matched to senders from the factory. Of course, cost is often a factor to us all. As you said, hopefully, it will work. As for adding a third gauge, that will definitely not work. Doing so will cause all three to read inaccurately.

Here's another suggestion, for your temp gauges. When your engines are at normal operating temp, take a reading with an IR gun to determine actual temp, compare to gauges, and note the differences so that when you glance at a gauge you will know the actual engine temp. You will be surprised at the correction factors. They could be plus or minus. On my new VDO upper helm gauges, one is -4, the other is +5. I have re-checked several times getting the same result. The lower helm has similar but different correction factors.
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Old 09-02-2019, 06:54 AM   #16
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I was hoping to “connect” another gauge to the temperature sender for display in the engine room, however from what I’m hearing that will not work.

Jim[/QUOTE]

I had problems with the Teleflex oil pressure gauges slowly dropping as the engine ran/warmed up. I replaced the sender with a Sierra dual station unit.

At the same time I put a tee in at the sender and added a Bosch mechanical gauge in the engine compartment. All three now agree. YMMV

Rob
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Old 09-02-2019, 06:57 AM   #17
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I should note that the above oil pressure problems are not in the NP 39. We sold that boat last June in anticipation of our new NP 45.

We are currently enjoying a 1979 Campion 26 with a 305 and Alpha 1 outdrive.

Rob
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Old 09-02-2019, 09:27 AM   #18
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Thanks Cat and Dave. We will see how this works out and thanks for the mechanical gauge experience. I think I will hold off on the third temperature gauge and just use the IR gun. I don’t really want to tap another hole in the coolant tank.

Rob: the floors still look good and congratulations on the NPT 45!
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Old 09-02-2019, 08:36 PM   #19
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Rob: the floors still look good and congratulations on the NPT 45![/QUOTE]

Glad to hear it!

We have had a couple of delays but should get the boat in March of 2020. Since the weather here in the Northeast is miserable that time of year we will take delivery in Savanah and take our time coming north.

Rob
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Old 09-02-2019, 08:51 PM   #20
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I was also having a temp issue with this boat. My remedy was take off the intake from the seacock and send a gallon of CLR into the salt water side. After an hour of "soaking" I reconnected the intake and ran the engine. It looked like a bubble bath!

Next removed the heat exchanger caps and made sure all the passages were open. Still ran 178-180 (gas engine).

Next drained coolant and bought cleaner for coolant side. Flushed before adding cleaner. Got an amazing amount of rust and scale out of the petcocks on the bottom of the engine. Did this at least four times before adding cleaner. Followed cleaner directions and flushed 5-6 times more untill stopped getting scale and rust. Removed all hoses and back flushed. Got more scale and rust.

After every spot ran clear installed new thermostat. Ran engine, temp nearly 180.

Finally thought to use my IR gun. Temps are 142 at thermostat housing and 131 at the temp sender. Guess it is time for at least a new sender if not gauges as well.

At least the cooling system is clean.

Rob



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