Mercruiser 4.3 oil gauge mystery

[Lou C]

I would try pulling out the op gauge and put a voltmeter on the + & - terminals & see what it is getting, both with the ignition on & engine running. Also I’d pull apart that big cannon plug & make sure all the terminals are clean & free of corrosion. Do you have a dashboard wiring diagram for the Mercruiser?

 

[stresspoint]

I would try pulling out the op gauge and put a voltmeter on the + & - terminals & see what it is getting, both with the ignition on & engine running. Also I’d pull apart that big cannon plug & make sure all the terminals are clean & free of corrosion. Do you have a dashboard wiring diagram for the Mercruiser?

yes Lou , i do have the wiring diagrams , in the manual , i also have the wiring diagram for the boat as well.
i already put meter on that wire and its reading around 12.5 motor off and a tad more motor running @ idle. ground and sender wire also checked out fine.
as posted , what seems to be tricking all the tests is the gauge showed slight movement when i took the motor up to 2500 rpm on the muffs.

? does anyone know what reading i should be getting on the gauge if the oil filter is clogged / internally damaged and the oil using the bypass to circulate on a 4.3 .

i just looked up the shed and found a spare know good sender so i will swap that out and see what happens.

 

[stresspoint]

I think last time i bought a bosch gauge at the autoparts store it was like $25

Advance Auto Parts - Down for Maintenance

fittings on the block are 1/8” NPT…

hey Scott , iv been searching to find a cheap capillary gauge with lines and fittings here in Perth Western Australia and i cant seem to find one under100 bucks , looks like i will have to order one online and wait , that's not going to help now , but having it there in the future will be an asset .

 

[04fxdwgi25]

On the back of the tach is a selector switch for what type engine it is used on ie: 4-6-8 cyl. With a small screwdriver, move that switch back and forth from one end to the other and reset to the engine type you have. Then try it again.
Corrosion builds on the contacts of the switch wiper / pads and sends crap signal to tach.

When a tach starts acting weird / showing phantom engine speeds, that is 8 times out of ten what is causing it.

 

[stresspoint]

ok , so i could not wait till the weekend " you guys know how it is "

went out and started testing and investigating ,"""" result was a bad sender unit """".

earlier on i tested the spare know working sender unit against the one that was later proven to be faulty and they both tested identical, so after further investigation and testing of the system to find all was good ,i was about to give up , for the sake of it i swapped them out , bang , the gauge come to life .

as the thread says oil gauge mystery , well this one has me thinkin ? why are the senders testing identically and yet one works and the other does not.

thanks to all that replied and i hope the thread help someone in the future. .

 

[Scott06]

ok , so i could not wait till the weekend " you guys know how it is "

went out and started testing and investigating ,"""" result was a bad sender unit """".

earlier on i tested the spare know working sender unit against the one that was later proven to be faulty and they both tested identical, so after further investigation and testing of the system to find all was good ,i was about to give up , for the sake of it i swapped them out , bang , the gauge come to life .

as the thread says oil gauge mystery , well this one has me thinkin ? why are the senders testing identically and yet one works and the other does not.

thanks to all that replied and i hope the thread help someone in the future. .

How did you test them? Would think you need to apply different pressures and see if the ohms change , dont know the ohm range off hand but usually if you ground tge sensor wire gauge should peg one way or other

 

[dubs283]

i checked the gauge , wiring and sender , all checks out fine.

From post #1

result was a bad sender unit........earlier on i tested the spare know working sender unit against the one that was later proven to be faulty and they both tested identical..........why are the senders testing identically and yet one works and the other does not.

Depends on your method of testing. You make no mention of how you tested the sender(s) only that you tested them

Also, in regards to your first post mention of the system (guage, wiring and sender) testing okay leads others to focus on other areas that could cause the issue

 

[stresspoint]

i tested as per posts on the internet to test a switch type sender that show to connect ohm meter to - to the screw in thread and the + to the gauge wire post , according to my research and practice it works the same with a sender unit except its a variable reading , not just on / off. as with the switch tests,
i used compressed are about 1 inch from the tiny hole and the ohm reading changed on the meter , the range did vary depending on how much air pressure was applied and how close the air nozzle was, i avoided holding the air too close or using lots of air pressure so as not to damage the sender.

i replicated the test on the 2 senders and got close enough to the same readings ,
obviously this type of testing is inconclusive as i now know .

 

[stresspoint]

From post #1



Depends on your method of testing. You make no mention of how you tested the sender(s) only that you tested them

Also, in regards to your first post mention of the system (guage, wiring and sender) testing okay leads others to focus on other areas that could cause the issue

not just others here , it tricked me as well , so don't feel too bad .

 

[Lou C]

IIRC there was a procedure in my OMC shop manual for testing these if I can find it I’ll post it

 

[dubs283]

Wouldn't say I was tricked, rather misinformed.

Standard electrical guages read from 30-240 ohms resistance.

OP method of testing although not necessarily accurate would in most opinions be considered reasonably sound seeing as how both senders appeared to correlate output readings based on similar input. Not exact but close

Personally I would have rigged a pressure tester to fit the sender, apply appropriate pressure and read ohms across the sender(s). That being in my opinion a fair witness method, ymmv

 

[Lou C]

That sounds like the best you could do; in the manual it said to check gauge first if it’s good then replace sending unit if the reading isn’t normal; doesn’t even have a procedure to test the OP gauge.

 

[stresspoint]

Wouldn't say I was tricked, rather misinformed.

Standard electrical guages read from 30-240 ohms resistance.

OP method of testing although not necessarily accurate would in most opinions be considered reasonably sound seeing as how both senders appeared to correlate output readings based on similar input. Not exact but close

Personally I would have rigged a pressure tester to fit the sender, apply appropriate pressure and read ohms across the sender(s). That being in my opinion a fair witness method, ymmv.

the first set of tests i did were rushed through, all seemed to be reading good enough for the gauge to work.
the thing that threw me out was the fact that @ 2500 on the muffs i was getting a gauge movement , not accurate but a movement , in retrospect that would imply the system was working IE: sender wiring and gauge and it therefor could have possibly been an engine issue , but it ran fine for a good hour after i noticed "no pressure on the gauge " hence .starting the thread with the title containing the word "mystery".
today i did extensive testing and the results were the same. till i swapped out the sender , with that said , i am guessing even rigging a device as you mention to the sender it would have read that it was working , as that would have been basically the same result as using the compressed air,
your fancy set up would be fine if i wanted to calibrate the gauge ,but i just needed to know the sender was working IE: ohms changing as it got air pressure in the hole.
so really i did not give misleading info i gave what i had at the point of starting the thread .

what does ymmv mean ? .

? does anyone have any idea what would cause the sender unit to spit the dummy while the boat was sitting at the dock , now that is a mystery to me .

 

[dubs283]

what does ymmv mean ?

Your mileage may vary

As in, your experience, method or opinion may differ from mine

 

[Scott06]

i tested as per posts on the internet to test a switch type sender that show to connect ohm meter to - to the screw in thread and the + to the gauge wire post , according to my research and practice it works the same with a sender unit except its a variable reading , not just on / off. as with the switch tests,
i used compressed are about 1 inch from the tiny hole and the ohm reading changed on the meter , the range did vary depending on how much air pressure was applied and how close the air nozzle was, i avoided holding the air too close or using lots of air pressure so as not to damage the sender.

i replicated the test on the 2 senders and got close enough to the same readings ,
obviously this type of testing is inconclusive as i now know .

A switch and a sender are two different things.

A switch like the oil pressure switch used for the alarm circuit will be open when the pressure is above the set point ( 4 or 7 psi). When it is below the setpoint like when starting or ignition is keyed on but engine is not running, the switch closes thus making an electrical connection to ground and the buzzer goes off... on ohms switch would read 0 or close to it when below setpoint, infinite ohms (open) when above it.

The sender will vary the resistance (Ohms) across it as pressure varies. normally like 30-260 ohms or something like this. Gauge and sender are matched in ohm range. To test you need hook up compressed air to the sender and measure ohms across it (body to signal wire plus) while varying the pressure to it . Normally lower ohms will peg the gauge high so easy to take the sender wire off at engine to test and see if gauge reads high...or vice versa...

 

[04fxdwgi25]

Here is a quick guide on gauges from Telflex. Would translate to mercruiser gauges, as they are the same setup

 

[tank1949]

How did you test them? Would think you need to apply different pressures and see if the ohms change , dont know the ohm range off hand but usually if you ground tge sensor wire gauge should peg one way or other

Indeed, and there may be an ohm compatibility issue between sender and gauge. Been there!

 

[stresspoint]

Indeed, and there may be an ohm compatibility issue between sender and gauge. Been there!

i think that is where the problem possibly could have came from. something gone wrong inside the sender like diaphragm stuck or gummed with crap.

guessing the oil pressure sender was OEM to the boat when new , so that would make that sender 30 years old,
also it had been through a lot of engine carnage since i owned the boat and in the past , one motor that water intruded several times from a hard to detect split block that only showed when the engine was under load.
a spun a main bearing from the water intrusion.
a another engine that split a cylinder bore from hydro lock and the PO of the boat cooked the original motor.

the engine block that is in there now , a used high hour motor that i have had some extensive performance machine work done , parts from all over were use on reassembly.

that poor little sender was probably in need of replacement , but who would think,

i have taken the advice and i have ordered in a capillary gauge set up so as i can directly monitor the oil should there be a need in the future.

i keep a keen eye on the oil and the temp gauges (not so much the tach or GPS speed ).

the motor in there now is one tuff little beast that has had lots done to make it perform with reliability , i would hate to loose it because of a faulty oil warning switch or cut off sensor not working ,so they will also be replaced before next outing.

 

[04fxdwgi25]

Maybe someone used a teflon tape or sealer on the threads of the sender that gave it a crap ground connection

 

[Lou C]

Maybe someone used a teflon tape or sealer on the threads of the sender that gave it a crap ground connection

possible because you need a good ground (return) path for it to read.