Audio warning system on 2003 Mercruiser 5.0

[Alphawater]

I have a 2003 Bayliner I/O with Mercruiser 5.0 with thunderbolt V ignition control module and electronic fuel pump and Alpha One stern drive - also has partial closed water cooling system. The audio system alarm currently goes off (continuous horn) when I hit 3000rpm or more while under way - stops when I reduce RPMs:

• While horn is going off the oil pressure gauge reads steady at 60PSI and the temperature gauge reads steady at 175.
• My Mercruiser manual states that the audio warning system will sound with a continuous horn if the engine oil pressure is too low or the engine temperature is too hot. The audio system will sound with an intermittent horn if the sterndrive oil level is too low.

Prior to the current situation, the following has taken place:

• I replaced the oil pressure sensor and switch last summer due to fuel flow restriction while trying to start engine - successfully done. I don't recall the audio warning system going off after this was completed.
• I then had local marine mechanic go through the stern drive last summer - replaced gimbal bearing and bellows. He also removed the lower unit and installed a new water pump assembly and then refilled the gear oil.
• I have not used the boat since the repairs until a few weeks ago - the alarm system (continuous horn) would go off intermittently at different RPM levels and then stop after say 5 seconds or so - very random except at about 3000RPMs when I could expect it to go off.
• Local marine mechanic recommended I replace the sterndrive oil level bottle and included sensor - which I did within the last few days
• Now the alarm system goes off at 3000RPM and greater with continuous horn on a constant basis while under way. The local marine mechanic does not have a recommendation for how to proceed next.

Help is appreciated. Thanks.

 

[alldodge]

Unplug the lube bottle, oil pressure switch and temp switch then see which one is turning it on. The switches just turn ON the alarm

You replaced the oil switch but maybe you actually have low oil pressure or something else. If the oil switch does turn the alarm OFF, then connect a mechanical gauge to switch port. Need to verify if you boat gauge is off or its the switch

 

[Fun Times]

Also try carefully disconnecting the 2 wire harnesses going to the grayish color ignition control module mounted to the port side exhaust manifold system, carefully make sure the wires and pins are clean and pushed in tight so they’re making contact, leave them unplugged for a minute or two, reconnect and see if the alarm stops at 3000 under way…. 50/50 chance it may help.

 

[dubs283]

Could be knock module/system, if equipped

 

[QBhoy]

Hi. Anything slightly above 175 is overheating I’m afraid. Throw into the mix, in slightest inaccuracy of the gauge on the dash….

 

[nola mike]

so this has come up a few times lately.... the tks/tbv system looks different than the TB iv in terms of the alarm system. all of the alarms go through the icm in tbv, and the op switch for the warning is NC with engine running.
unless I'm reading things wrong. otherwise all of the recent advice to disconnect the op switch for testing is wrong.

 

[alldodge]

TKS was not installed with a TB4 ignition

There is a difference in connection between V6/V8 and 4 cyl

 

[nola mike]

TKS was not installed with a TB4 ignition

There is a difference in connection between V6/V8 and 4 cyl

that's what I'm saying. switch polarity is reversed between TB iv and v

 

[Alphawater]

Thanks to all for input - current status update. Per guidance from Alldodge, I disconnected all three items. Was able to identify that the engine coolant temperature switch was causing the beeping. Am planning on replacing the sensor switch when replacement part arrives. Will advise if problem still has not been resolved. Thanks again.

 

[Alphawater]

Update: Yesterday (in driveway) I changed out the ECT sensor and functions stayed same as before with dash Faria temperature gauge starting at 0 degrees and climbed to 175 degrees at idle after warming up. Today (in driveway) I changed out the ECT sensor switch (used Quicksilver replacement part) - at cold startup the temperature gauge immediately read about 110 degrees and then climbed to about 200 degrees at idle after warming up. I then used an infrared thermometer while still at idle and the highest reading I could get was about 152 degrees on the outside of the closed cooling system heat exchanger. No alarms sounded during the whole time. I double checked the parts used with a local marine repair store and I used the correct ECT sensor switch which provides data to the Faria temperature gauge. I also checked with the closed cooling system manufacturer master technician and he felt the cooling system was functioning properly at idle as described . Both ECT sensor parts that were replaced and the Faria gauge are original equipment. Trying to determine what to do next. Additional input appreciated.

 

[alldodge]

It's either the gauge, sender or connections.
Old sender 0 to 175
New sender 110 to 200

My first thought is old sender was fine and new is not the correct one. What was the part number?

The gauge sender has no connection to the alarm and has one wire connection. The alarm switch has a two wire connection. Could look it up with your serial number?

 

[Alphawater]

The part number used is 806490T - I verified this number with the local mercruiser dealer and he verified it was the correct one by cross referencing with the engine serial number. Since then I reinstalled the original sensor switch (single wire) and retested in the driveway. The temperature gauge went from 0 to 175 after warmup at idle. At the same time, the infrared thermometer read a maximum of 155 degrees. I plan on taking the boat back out on the water soon and use the infrared thermometer while in use and see if I get any more audio warning system sounds. One item I haven't replaced yet is the water thermostat (I believe it is the original part of 20 years old) - am wondering if it could possibly be contributing to the short phases of the audio warning system.
Will advise regarding results. Thanks again for guidance info.

 

[alldodge]

That's the correct part number for the gauge but still say, the gauge has nothing to do with the alarm

Your thermostat housing showing 155* then it's not over heating, might just be the 2 wire temp switch

 

[Fun Times]

Agreed with alldodge, you’re testing /diagnosing the wrong temperature sensor…. It should be the one located on the port side of the thermostat housing with 2 wires going to it… Yellow and Black…, Item number 6, https://www.mercruiserparts.com/bam/subassembly/31839/8182/170

Yours mentioned above is item number 3 which is for the gauge only… Not the warning system.

 

[Alphawater]

Thanks for the responses - understand what you are advising here. I already replaced the ECT sensor (part #805218) on October 4th and driveway tested it with no problems. I mentioned this in my post on October 5th but can see how it got overlooked with how I worded it and my focus on the temperature gauge reading. I have yet to put the boat in the water and test this part while under way at higher RPMs. Hope to do so in the next few days - will post results when completed. Thanks again.

 

[alldodge]

Update: Yesterday (in driveway) I changed out the ECT sensor and functions stayed same as before with dash Faria temperature gauge starting at 0 degrees and climbed to 175 degrees at idle after warming up. Today (in driveway) I changed out the ECT sensor switch (used Quicksilver replacement part) - at cold startup the temperature gauge immediately read about 110 degrees and then climbed to about 200

This is where our comments come from.
No clear difference as to which one is controlling the gauge