Oil pressure gauge pegs to max

[The Dread Pirate Robert]

I want to write down what I have done to figure out why my gauges are not working so hopefully you can find something I have missed and point me in a better direction.

My engine oil pressure gauge and my transmission pressure gauge on both lower and upper helms of the starboard engine are pegging to max when the ignition is turned on. They go back to zero when the ignition is off.

I am working on the oil pressure gauge only for now.

1) I swapped the known working gauge of the port engine for the non working gauge of the starboard engine. The known working gauge on the starboard side pegged to max with ignition on.
2) I ran the port engine with the swapped non working gauge and it functioned properly.
That tells me my gauge is good.
3) I wired the sending unit from the port engine to the wire harness of the starboard engine and the starboard sending unit to the port wire harness. The gauges performed the same. Which means my sending units are ok.
4) I checked the continuity of the sending wire and there are no breaks.
5) I checked for any grounding of the sending wire and got no reading on my ohm meter.
6) I disconnected the sending wire from the starboard engine and connected the sending unit to the gauge with a new wire and the gauge still maxed out.
7) I connected an ohm meter to the sending unit and a ground but get no signal at all on the meter.

I hope this is not too confusing but I could really use the help in knowing if there are any other tests I can use to figure this out.

Thanks for your help.

 

[Lepke]

Get a mechanical gauge or a oil pressure tester. About $25 for either on Amazon.
Temporarily replace the sender on the engine with high oil pressure. The gauge or tester should screw into the port where the sender is now. Then start the engine and you'll know the real oil pressure.

 

[High Wire]

Did they used to work correctly and now do not?
I used to own a boat that when I bought it used, the oil pressure gauges pegged high with the ignition switches on but engine not running. Started the engines and they went to mid-scale (40 psi). Found out that when run hard then pulled back to idle oil pressure readings went UP (70 psi). Further testing showed the oil sending units were Teleflex but the gauges were VDO. VDO gauges needed senders that use a resistance scale exactly opposite than Teleflex.

 

[The Dread Pirate Robert]

Did they used to work correctly and now do not?
I used to own a boat that when I bought it used, the oil pressure gauges pegged high with the ignition switches on but engine not running. Started the engines and they went to mid-scale (40 psi). Found out that when run hard then pulled back to idle oil pressure readings went UP (70 psi). Further testing showed the oil sending units were Teleflex but the gauges were VDO. VDO gauges needed senders that use a resistance scale exactly opposite than Teleflex.

They used to work for years then one day they stopped. As Soon as the ignition is on they max out and when the engine is started and run they remain maxed out.

 

[The Dread Pirate Robert]

The issue is not too high pressure as tested when I switched wiring from starboard sender to port gauge and the gauge read proper. The gauges max when the ignition is turned on and remains maxed when the engine is stated and run.

 

[STB]

What brand of gauges do you have? Traditional gauges and some VDO gauges work differently.

For a traditional gauge, the resistance to ground goes up as the pressure goes up. So a gauge pegging high is grounded on the sender side. There is very little that can be done to peg it high other than to ground the sender side, because it is basically the current through the gauge that moves the needle. The more resistance on the sender side, the less current and the less needle movement. Even a high or low voltage won't do it, because the gauge is actually moved by magnetism and uses a coil configuration that balances out the difference in voltage due to difference in input voltage.

I guess there could be some type of interfering magnetic field. But, that much interference seems unlikely.

More likely to me is that everything checks out well while it is disassembled, but once assembled or pushed into the panel something is getting grounded that shouldn't be.

Disconnect the sender side of the gauge and really check to see if that wire is grounding itself. Move it. Wiggle it. Press it in.

If that doesn't turn up the problem, do the opposite. Connect the sender side of the gauge, disconnect the 12v side of the gauge, and FULLY reassemble and reinstall everything, including screws. Then, go down to the sender and disconnect the wire there. Check that wire to ground. That'll tell you if anything between that distal end of the wire and the gauge is grounding the wire. You can then go from there.

Can you confirm the gauge type?

 

[Comodave]

When I suspect bad wiring, and that is what this sounds like, I run temporary wires over the deck from the gauge to the sender and make sure the sender and gauge are both working. Then it is just a problem of finding where the wiring is bad. Check every connection.

 

[STB]

When I suspect bad wiring, and that is what this sounds like, I run temporary wires over the deck from the gauge to the sender and make sure the sender and gauge are both working. Then it is just a problem of finding where the wiring is bad. Check every connection.

I think he reported both swapping gauges and using a new wire in his original post.

Another thing he can try is to measure resistance to ground with an ohm meter at the top. That should be nearly the same as the measurement at the sender at the bottom.

Another thing he can try is disconnecting the wire on the sender side. If that gauge still pegs the gauge is obviously grounding elsewhere.

If that doesn't peg it, he can try adding a resistor between the gauge and ground in place of the sender, both at the gauge side and at the engine side.

I'd the gauges are really good and the senders are really good and the wireing is really good, at leaat as tested in isolation -- for reals -- it seemingly is most likely installation, e.g. wires, connectors, terminals, etc, getting moved or squished as the panel is assembled.

I'd check it front to back and back to front again and then

Some gauges and senders are set up for isolated ("floating") grounds. VDO makes a bunch like that. Thst introduces another couple of wires. Among thebreasonsnid likento.know what we've got.

 

[Bmarler]

I would check the positive voltage in the instrument cluster. It’s common to all gauges, and it seems like you’ve verified the negative side fairly thoroughly.

 

[Bmarler]

It was mentioned there was no signal to ground on the sender. Is it the same for both? Did you try changing the ohm scale on the meter? Sometimes you can be out of range, or the auto range has trouble.

 

[The Dread Pirate Robert]

I have run new temporary wires from the sender to the gauge and it still goes to max.
When I disconnect the ground from the gauge it stays at zero when the ignition is turned on.
With the power line disconnected from the gauge it stays at zero.
They are all 1987 VDO gauges.

 

[The Dread Pirate Robert]

My panel hinges forward so the back of the gauges are accessible without removal.

 

[Mischief Managed]

Some engines, like a CAT 3208, have a dedicated ground wire for the starter and a separate ground wire for the engine block. Perhaps the ground to your block is no good and the gauges are now trying to be part of the ground circuit (other than the starter ground) for the affected engine.

 

[The Dread Pirate Robert]

New info as I do have Cat 3208s. Thanks. However wouldn’t all the gauges act the same way and max out or would it be just the engine oil and trans oil pressure gauges?
I will check this tomorrow.

 

[Max Simmons]

My Starboard engine temp was maxing out shortly after warm up. It was a problem with the ground on the sending unit.

 

[The Dread Pirate Robert]

My sending units only have two wires. A numbered black which is connected to the gauge on the dash and a green which is connected to the low pressure alarm switch. If they are grounded it must be by the threaded fitting that attached then to the engine block.

 

[Barking Sands]

If you dont figure it out I may be available Thursday and Saturday. I am also in NPR and would be glad to come help.

 

[The Dread Pirate Robert]

Thanks. I am going to try one more thing tomorrow and will get with you thru TF

 

[SteveK]

My sending units only have two wires. A numbered black which is connected to the gauge on the dash and a green which is connected to the low pressure alarm switch. If they are grounded it must be by the threaded fitting that attached then to the engine block.

earlier you said when you disconnect the negative from gauge, it goes to zero. (same as when the sender is not grounded).
It is my understanding that the gauge receives positive power when key is turned to run and the other wire goes to the sender which grounds out with pressure moving gauage from zero.
If you were to apply positive to the sender through a bulb now it should light up suggesting it has failed, if my thought is correct.

I think you need new sender. You can test this by swapping senders.

 

[The Dread Pirate Robert]

Using a lighted circuit tester, I connected one end to the ignition wire and the probe end to the wire coming from the sender. The light did not light.
However when I touched the probe to a ground it lit.
When I connect my stb sender to the port engine wire harness, run my stb engine, and turn on the ignition to the port engine, my port gauge reads as it should.

 

[SteveK]

Have we agreed the port sender needs to be changed? Or more thought needed?

Have you taken the sender wire from port to the starboard gauge to se if it pegs.

 

[The Dread Pirate Robert]

More thought needed since the stb sender works using the port wire harness and the port sender makes no difference to the stb gauge. It still pegs.

 

[The Dread Pirate Robert]

Are you available tomorrow Thursday?
I have run a fee more tests today and still no joy!!
I am in Gulf Harbors.

 

[Mischief Managed]

Correct me if I'm wrong, but all your tests have proven that both port and starboardd sending units are good, both gauges are good, and the wiring from both sending units to both gauges is good. That leaves the starboard engine or an electrical fault upstream of the starboard sending unit as the problem for the engine oil pressure. Since the starboard transmission oil pressure gauge an the same does the same thing and the gauges peg with the engine not even running, that means that there's an electrical problem upstream of the sending units. The problem also has to be electrically common to both sending units and as far as I know, the only thing common between them is the ground (or "common") connection.



I recommend that you clean a convenient spot on the starboard engine to bare metal and attach one end of a jumper cable to that spot and the other end to the engine starting battery - post. Turn the key on and see if if the gauges peg or not. If the engine gauge does not peg and transmission gauge does, clean a convenient spot on the starboard transmission to bare metal and attach one end of a jumper cable to it and the other end to the - post on the starting battery and retest. If both gauges now work normally, the problem is a missing ground on the engine and transmission.



I too have 3208s and I recently found evidence that my port engine starter negative battery cable must have been poorly connected to the battery at some point in time because my port engine AWG6 bonding wire was absolutely cooked, meaning the starter was pushing all the starting current through that wire to the battery - post instead of the 1/0 cable that it was supposed to push current through.

 

[CharlieO.]

Do you have any wires that "daisy change" from one gauge to the other?

 

[PM]

That indicates a dead short. trace it and find it.

pete

 

[The Dread Pirate Robert]

Yes all of the power wires from the ignition and all of the ground wires are linked

 

[Nick F]

Its frustrating watching this thread continue. Without bragging (I hope), I could (maybe) diagnose the issue quickly if I were on your boat.

However, to chase it remotely is tricky.

You seem to be in doubt as to which senders are working correctly and which are defective. This is easily checked - a sender simply changes its resistance with pressure. Below is a table for the most common sender:

Check the resistance with engine stopped (zero ressure) on the engine which seems normal. It will probably be 240 ohms. Now check the other engine - resistance should be the same. If this is is good, check the resistances with the engines running.

Note: to check the resistance you must disconnect the harness wire from the sender.

 

[Bmarler]

I ask again, did you check to see what the positive side voltage is at the gauges? If you don’t have good positive voltage to the gauges it will skew the reading.

 

[The Dread Pirate Robert]

Thank you all for the suggestions and help. After many tests and repeat test and even though I swapped senders to wire harnesses with presumably positive results, I swapped senders to motors and now the stb side works and the port side does not.
Original stb side sender is faulty.
Now to find one.