Mercruiser 4.3 No oil pressure reading

[pcm]

Looking for some help with a Mercruiser 4.3 (approx 1989) which does'nt show any reading on the oil pressure guage. I suspect that it could be a duff sender unit or the guage itself - or even a break in the wiring somewhere. Does anyone have any tips on how these items might be easily checked?? I also need to know whereabouts the sender unit is mounted on the engine. Any advice greatly appreciated. The engine runs well but I'd really like to have a pressure reading!

 

[Bondo]

Re: Mercruiser 4.3 No oil pressure reading

Ayuh,.......

The Sending Unit is mounted either,........
Just ahead of the Oil Filter Housing on the lower Portside, just above the oil pan rail....
Or,....
Right beside the Distributor..........

Remove the wire from it,+ Ground Out the Wire,.....
The Gauge should Peg to Max........
If it Does,.... The Sending Unit is Junk......
If it Doesn't,.....
Use a jumper wire,+ Ground Out the Sender post on the Gauge,.....
It should Peg to Max,......
If it Does,.... The Wiring is Bad......
If it Doesn't,..... It's the Gauge.........

Of course,... The Gauge needs a 12V feed to it's Positive post before you start.......

 

[180shabah]

Re: Mercruiser 4.3 No oil pressure reading

Ayuh,.......


Remove the wire from it,+ Ground Out the Wire,.....
The Gauge should Peg to Max........
If it Does,.... The Sending Unit is Junk......

Or the sending unit is good and his oil pressure is actually zero. Although, a guage/sender/wire issue is more likely..

 

[Silvertip]

Re: Mercruiser 4.3 No oil pressure reading

And if the oil pressure is actually zero, there would be copious amounts of knocking and lifter clatter.

 

[achris]

Re: Mercruiser 4.3 No oil pressure reading

The harness across the back of those engines is very tight. The one on my old 4.3 rubbed through on the back of the block. The positive and the oil pressure wires were touching (luckily not positive to ground or I'd have been putting out a fire ), the pressure guage was giving all sorts of wierd readings. Once I found the damaged harness and taped it all off everything went back to normal, but not before I'd spent $100 on a new guage .

Chris......

 

[DHPMARINE]

Re: Mercruiser 4.3 No oil pressure reading

Being old school (and working on customers motors,not my own) I'd have to screw my oil pressure gauge into the engine to be sure there was pressure. That's just me.

Then I would probably pull the seding unit,clean it with carb cleaner,reinstall it and send the customer on his way.

DHP

 

[Don S]

Re: Mercruiser 4.3 No oil pressure reading

Then I would probably pull the seding unit,clean it with carb cleaner,reinstall it and send the customer on his way.

After checking with a manual type gauge, and finding the sender was bad, I would replace the sender with a NEW one, if it was in my shop. I don't want to do it the second time for free because the carb cleaner was only a temporary fix. UNLESS, the customer was aware of the quick fix and possibly may have to be done the second time with a new one at HIS expense.

 

[DHPMARINE]

Re: Mercruiser 4.3 No oil pressure reading

Good point Don.That would have been said to the customer.I don't think I've had 3 bad senders in almost 30 years.I don't even stock senders.Any I have cleaned never had another problem.Must have been sludge blocking the sender,is what I figure.

DHP

 

[pcm]

Re: Mercruiser 4.3 No oil pressure reading

Thanks to everyone out there for your advice. I'm on the River Thames close to Windsor in the UK and it's great to get such help from 'across the pond'.
I'm going out to the boat a little later and will follow up on what has been suggested. The boat is a much neglected Sea Ray which I'm trying to straighten out as a project. I'm sure I'll be back for more help later!!!

 

[pcm]

Re: Mercruiser 4.3 No oil pressure reading

.....have located the sender as described (just by the oil pan rim) but I'm mystified by another similar looking device which is located on the port side at the rear of the cylinder head. This has a blue wire attached. Any idea what this one is doing????????

 

[Bondo]

Re: Mercruiser 4.3 No oil pressure reading

Ayuh,........

Does your motor have an Electric Fuel Pump,..??
Does 1 of these units you've found have 2 wires, rather than 1,..??

 

[JustJason]

Re: Mercruiser 4.3 No oil pressure reading

1 is an oil sender, (1 wire). The other is an oil pressure switch.. (they can have 1 or 2 wires)

 

[180shabah]

Re: Mercruiser 4.3 No oil pressure reading

Lt. Blue wire should be for the oil pressure sender and the switch should have a purple wire.

 

[pcm]

Re: Mercruiser 4.3 No oil pressure reading

Thanks to everyone for your help. I've now found that the gauge was sticking. After a couple of sharp taps it showed a correct reading. In my checking, I found that the sender/switch (?) at the top of the engine has a blue wire connected to it and the one down near the oil pan has a single tan/blue wire. (only one wire to each of these devices). I have also established that the fuel pump is mechanical rather than electrical. I also see that there is a way of testing the water temp and oil pressure by turning on the ignition and waiting for 15 seconds for an alarm to sound - BUT THIS IS NOT WORKING! When I ground out the tan/blue wire mentioned above, then the alarm sounds. I'd be happy to ignore this but if anyone has any clues as to what may be the solution to this I'd be glad to hear from you.
As an afterthought, I'm still concerned over the fact that the two senders/switches look to be identical. Should they both look alike or is there a difference in appearance between a sender unit and a switch???????
The one in the lower position looks like it has been replaced recently and I'm beginning to wonder whether some inexperienced person may have fitted the wrong part - particularly since a wiring diagram which I found shows a tan and blue wire fixed to a switch rather than a sender.

 

[CharlieB]

Re: Mercruiser 4.3 No oil pressure reading

Yes, there is a difference.

A 'Sender' varies the resistance to ground in order to allow the guage needle movement.

A 'Switch' is either ON (NC normal closed contacts) or OFF (NO normal open contacts)

Someone has replaced your 'NC' switch with a sender.

Do you have a NAPA dealer nearby?

They should have the correct switch on the shelf OR they can order it, ussuallly next biz day.

 

[pcm]

Re: Mercruiser 4.3 No oil pressure reading

Thanks for that - yes the last owner obviously installed a sender instead of a switch!
So far so good.......new subject and sorry to show my ignorance, but can someone explain what is the function of the micro switch in the area of the gear shift mechanism?? It has a roller which sits in a detent between the two shift cables. I can see it on a wiring diagram but there is no explanation of what it does. Everything seems to be working correctly but it would be nice to know more about this. Thanks in anticipation for any advice....

 

[180shabah]

Re: Mercruiser 4.3 No oil pressure reading

That is the shift interupt switch. When you pull the shifter to neutral it briefly ground the ignition causing the engine to stumble. This takes the pressure off of the gears and allows them to disengage.

 

[pcm]

Re: Mercruiser 4.3 No oil pressure reading

That's a nice simple explanation - thanks for being so quick!

 

[180shabah]

Re: Mercruiser 4.3 No oil pressure reading

That's me - Quick and simple