DD 8.2 alarm/buzzer

[cosmo]

Detroit 8.2T-250 hp. Twins in my boat. Recently I am getting an alarm/buzzer from port engine. Doesn´t happen at startup. Once she´s under load and at temp, alarm/buzzer starts in at both lower and upper helm.

What I can say is that gauges at lower and upper helms match in terms of oil pressure and temps (all good readings). I have good water flow out the exhaust. The buzzer is at both helm stations. No visible signs of leaks anywhere in the engine room.

Is there a ´bolt on´ alarm/buzzer on the engine itself that can go haywire? Why would it be silent at idle but once underway, it starts in?

In reading posts on TF, it crossed my mind of a possible high water alarm? The alarm buzzer is the same sound that is made when key is turned to 'on' position and goes away once engine is started.


Thanks!

 

[SeaDogAK]

Could be a bad sending unit for temperature or oil pressure. When it goes off, you’ll need to start unhooking wires from the various sending units.

 

[cosmo]

Thanks SeaDog.

 

[Greg QS]

I agree a bad sending unit. Alarms are there for a reason don't disconnect / bypass it. Find out what is going on and repair it.

 

[OldDan1943]

Does the alarm sound only when you make a significant throttle change?

 

[cosmo]

The last time it sounded, I had throttled up a little from idle to maybe 1000rpm if I remember. She idled for probably 15 mins while we pumped and rinsed our holding tank. No issues. Left the pump out, throttled up and the buzzer started. Gauges indicated no issue. Back to the dock we went.

 

[OldDan1943]

It does not sound when you lower the RPM to shift gears?

 

[cosmo]

No. Once it’s on, it’s on. Prior to departing our slip and tying to pump out, gear shifting has no effect. Just when we are underway.

 

[SeaDogAK]

I was not suggesting disconnecting wires to get rid of the noise. I meant unhook wires to diagnose which sending unit is bad. The fact that it only starts after you’re running for a while suggested oil pressure or temperature to me.

I had a Tollycraft 37 with DD 8.2’s until recently and had the oil pressure alarm fail and it acted a lot like that. If you’re not sure which sending unit it is you could disconnect wires from the alarm until you figure out which one it is, and then trace the wire back to the engine.

Mine had a panel with lights which would show which alarm is going off, which was helpful.

 

[Barking Sands]

Usually those alarms are simply grounded out through the particular switch. Usually just an oil pressure switch or a coolant temp switch. As others have said...recreate the condition...then go to the oil pressure switch and pull the wire. If the buzzer goes away then that is where the buzzer is getting its ground. If it does not go away then plug it back in and go to the water temp switch and pull the wire to repeat the process. It really should only take a few minutes to figure out where you are getting the ground.

If you had a corresponding gauge condition pointing to either low oil pressure or high temp I would suggest something else. But since you state the gauges are good its reasonable to suspect a bad switch.

If you dont want to do it that way you can bench check the switches with a multimeter set to ohms. Many times switches that are intermittently bad will show an ohm reading through the switch on the kilo or mega ohm range when in fact they should be completely open. Any reading through a normal pressure or temp switch is cause for rejection. Of course you would have to apply pressure to the oil pressure switch to open the contacts.

My guess is its the oil pressure switch since it actually has moving parts that have only so many cycles. Many times they will rupture internally allowing the oil into the opposite side of the cavity. When that happens both sides of the switch have the same pressure, say 50 psi, and the switch now relaxes which will ground out the switch and set off the alarm. When you shut off the engine both sides of the switch will eventually get back to 0 psi. This allows the switch to work normally again on the subsequent start up until the pressure again equalizes on both side.

Some oil pressure switches and senders have weep holes to indicate the switch is leaking internally. Oil completely filling an internal cavity will also show very high resistance reading with an ohm meter when it should be completely open. In an old school system with incandescent light and old school buzzer you might also get a dimly lit indication when this happens.

 

[cosmo]

Thank you both! On mechanical trial, zero issues. The next day, the stbd oil gauge fluttered, would peg both directions and collapsed. Long story short, engine was fine, oil pressure sender failed. Install new one and gauges are fine. The buzzer is the issue. You both might be pointing me in the right direction. For the sender to go bad and the buzzer, perhaps a gremlin or coincidence?

 

[SeaDogAK]

On mine there were two senders, one for the gauge and one for the alarm. They’re cheap, if it’s old I’d just change out the sending unit and see if that fixes it.

 

[Barking Sands]

Thank you both! On mechanical trial, zero issues. The next day, the stbd oil gauge fluttered, would peg both directions and collapsed. Long story short, engine was fine, oil pressure sender failed. Install new one and gauges are fine. The buzzer is the issue. You both might be pointing me in the right direction. For the sender to go bad and the buzzer, perhaps a gremlin or coincidence?

I personally would take your old sender home and bench check it if you had the resources. In a situation like this where you might question what is happening because of multi-failure on the same system I like to double check my work. Putting in a new component and having that system work again is usually sufficient. But when its electro-mechanical in nature the confidence level may be only 80 or 90% that the new component was the actual cause of the fix. I cant tell you how many times I have fixed something just the way you did and have it work for a time only to fail in the same way shortly after. Many times the act of changing a suspected bad electric-mechanical component that was actually good...for a new component wiggled a wire just enough to make better electrical contact to get things working temporarily.

I would also bench check it because of your description as it failed. A swing full scale high and then collapse could have been a real high pressure event simultaneously killing both the pressure sender and the pressure switch blowing out the inner bellows or seals. A bench check of the old sender might reveal that it works fine, has been blown out internally, or is erratic electrically or open electrically.

I would also be keeping a very close eye on the oil pressure gauge with new sender to look for anything erratic or inconsistent.

And of course..despite all this its probably just a coincidence. Both sender and switch are probably just old and failed near the same time.

Here is a short video I made to check pressure switches and senders.