Mercruiser Oil Pressure Alarm switch troubleshooting

[mhopeng]

I have twin Mercruiser GM EFI 5.0L V-8 2 bbl engines (1999-2001) with Thunderbolt V ignition. Engine serial 0L639284 / 0L639281. When I start the port engine, the audio alarm buzzer sounds for a few seconds until the engine generates oil pressure, as expected. But when I start the starboard engine, the alarm does not sound. I can't tell if the alarm circuit is working or not.
I verified the buzzer by grounding one side of it while the ignition switch is on and it works. I also shorted the transmission fluid sensor and the buzzer comes on. So I feel that the buzzer and ignition switch are ok.
The service manual #24 and various forum posts describe the three audio alarm switches as being simple normally open switches with a single wire (tan/blue) that connects the buzzer to ground if the alarm conditions occurs. This is true for the transmission fluid switch. However, the Oil pressure switch has two wires, purple and purple/yellow. This is true on both engines, and purple/yellow is a connection to ground on both. Also, the switch is open (no conductivity between the two wires) when the engine is not running (and no oil pressure), which is the opposite of what I would expect for a "simple connect-to-ground = alarm" circuit.
So, can anyone advise me on the next step in troubleshooting? Where can I find a circuit diagram for this oil pressure switch circuit? Thank you for your help!

 

[tank1949]

I assume that you have electric fuel pumps and have the special oil pressure switch wired correctly? Swap switch on motors and see if problem moves to different motor.

 

[alldodge]

Oil pressure switch has two wires, purple and purple/yellow. This is true on both engines, and purple/yellow is a connection to ground on both.

Both have purple, that's new
As you can see in Manual 24 page 4E-10 it should be a Yellow and a Black. Check to see if your purple goes to ground. Guessing but it may have a bad connection

 

[QBhoy]

Hi. I don’t think it’s normal for the alarm to sound for a few seconds until it builds oil pressure. Not at all, I’m sure.
Is it definitely an alarm for this reason ? Is it perhaps more likely that it’s an alarm for low voltage for a few seconds as a result of a failing alternator or slipping belt ? Just when you say it doesn’t happen if the other engine is started first ? Unless I’ve read you wrongly.
But it’s definitely not normal for an alarm to stay active until oil pressure is built up. That’s almost instantaneous.

 

[mhopeng]

Hi. I don’t think it’s normal for the alarm to sound for a few seconds until it builds oil pressure. Not at all, I’m sure.
Is it definitely an alarm for this reason ? Is it perhaps more likely that it’s an alarm for low voltage for a few seconds as a result of a failing alternator or slipping belt ? Just when you say it doesn’t happen if the other engine is started first ? Unless I’ve read you wrongly.
But it’s definitely not normal for an alarm to stay active until oil pressure is built up. That’s almost instantaneous.

Well, that is my understanding of how the alarm circuit is supposed to work, but I try to keep an open mind... The logic is that there is no oil pressure when the engine is not running, so if you have an alarm switch for "no oil pressure", then it will be activated from the moment you turn the key and supply 12V to the circuit until the cylinders have moved a few times and oil has been pushed onto the alarm switch (say 3-5 seconds, basically until the engine starts). This confirms that the alarm circuit is working. I believe this is a common feature of older engines with minimal computer control.
In my case, this alarm behavior does happen with the port engine, but does not happen with starboard. The order of starting them does not matter.
It would be great if the alarm is not supposed to be activated at ignition, then I don't have a problem The wiring diagrams that I have definitely don't match my switch, so maybe I am wrong about the expected alarm behavior. In which case I should be troubleshooting the port engine <forehead slap>

 

[mhopeng]

Both have purple, that's new
As you can see in Manual 24 page 4E-10 it should be a Yellow and a Black. Check to see if your purple goes to ground. Guessing but it may have a bad connection

Huh, now this is interesting... Page 4E-2 is my engine, but the diagram is about the same. I see now that there are TWO "oil switches" in that circuit diagram. One of them is in the audio warning circuit (Group C, item 1), and one of them is in the "ignition" circuit (Group B, item 5). The switch in Group B has purple and purple/yellow wires (just like I see), and the purple wire is connected to the alternator and the distributor, for what reason I don't know. Thats the one I have been working on, and this means I have been effing around with the wrong %$#@! switch; it has nothing to do with the audio alarm circuit. <head explodes>
So, new question, where is the oil switch for the audio alarm circuit?!? The oil sender and the oil switch #5 are on a tee together, low on the port side of both engines. Anybody know where the audio alarm oil switch is?

 

[dubs283]

I have twin Mercruiser GM EFI 5.0L V-8 2 bbl engines (1999-2001) with Thunderbolt V ignition.

Is the engine efi or 2bbl carb?

It can't be both.....

 

[alldodge]

The alarm switches are designed as designed for carb motors. Key ON alarm, start alarm OFF

Page 4E-2 is for a MCM which means sterndrive (note the shift interruptor), MIE is inboard. It list only 5.7 but this would also include 5.0

Both are Tee'd together, the pressure sender is the larger of the 2

 

[mhopeng]

Is the engine efi or 2bbl carb?

It can't be both.....

Good point, has a carburetor, not EFI.

 

[dubs283]

Anybody know where the audio alarm oil switch is?

On a carbureted engine most obvious place would be aft of intake manifold, near the distributor

 

[tank1949]

On a carbureted engine most obvious place would be aft of intake manifold, near the distributor

Pray it is not near oil filter!!!!!!!!!!!!!!!!!!!!!!!!

 

[mhopeng]

OK, problem solved. The engine has *two* oil pressure switches and one oil pressure sender (for the gauge). The sender and one of the switches are together on a tee connector, low on the port side. I'm not sure what that switch is for. But the switch for the audio alarm circuit is separate, it is located on top of the engine, near the distributor (thank you @dubs283). That one looks completely different and it has the correct wire color per the manual. Sure enough, I checked it and it is bad; replacement ordered. Easy once you know what you are doing
I'm attaching some pictures here just for completeness. Hopefully this will help someone else. Thank you to everyone who read and responded!

 

[mhopeng]

On a carbureted engine most obvious place would be aft of intake manifold, near the distributor

YES, that is correct. Once I went looking for it, there it was. Thank you!

 

[nola mike]

OK, problem solved. The engine has *two* oil pressure switches and one oil pressure sender (for the gauge). The sender and one of the switches are together on a tee connector, low on the port side. I'm not sure what that switch is for. But the switch for the audio alarm circuit is separate, it is located on top of the engine, near the distributor (thank you @dubs283). That one looks completely different and it has the correct wire color per the manual. Sure enough, I checked it and it is bad; replacement ordered. Easy once you know what you are doing
I'm attaching some pictures here just for completeness. Hopefully this will help someone else. Thank you to everyone who read and responded!

If you're asking about #5, it's the oil pressure switch that runs the fuel pump