Intermittent engine alarm

[nennedk]

First my engine is a Mercruiser 5.0L carbed from 2001. SN 0M359543
The engine alarm came on/off starting from around cruising speed (3000 rpm). I then changed the water temp sensor (the one with yellow wire), and it seem to be working for a couple of days. The alarm only sounded when I started the engine until oil pressure was up. All good.
But now it has begun again, after a longer cruise and worse than ever. Still not constant. If I change the rev. a bit it will stop, but soon after start again.
I have checked temp, oil pressure and gear lube reservoir. Gauges reads: water temp. steady at around 65c, oil pressure between 35 and 55, and gear lube looks ok.I have also checked temp. by temp. gun. Knees not hotter than I can keep my hand on them, Starboard a little hotter than port.I believe that's normal.
Oil switch was changed a couple of years ago (see my earlier post)
Any suggestions to where I begin my search and how, will be appreciated.

 

[kenny nunez]

Do you have a water pressure gauge, if not it will help to find out if the pressure is where it should be. Has the water pump ever burned up or lost a blade? How old is the impeller nd pump housing?

 

[alldodge]

Having a TB5 i also has a knock sensor, which can sound the alarm
The sensor is on the starboard side below the exhaust and beside the starter. Disconnect the sensor and see if it stops. If it does then there may be a knock or sensor is bad

Also check the Tan/Blue wire on lube and water temp. Disconnect the wire and see if it stops

 

[nennedk]

Do you have a water pressure gauge, if not it will help to find out if the pressure is where it should be. Has the water pump ever burned up or lost a blade? How old is the impeller nd pump housing?

No water pressure gauge, its not a MPI engine. I have changed impeller every year since I got the boat in 2016, but this year I haven't changed it because they always are like new when I pulled them out. So now I'm going for 2 years interval. Pump house looked fine and no lost blades.

 

[nennedk]

Having a TB5 i also has a knock sensor, which can sound the alarm
The sensor is on the starboard side below the exhaust and beside the starter. Disconnect the sensor and see if it stops. If it does then there may be a knock or sensor is bad

Also check the Tan/Blue wire on lube and water temp. Disconnect the wire and see if it stops

I don't think there is a knock sensor. See attached.

I will try disconnecting lube and water temp.

 

[tank1949]

Having a TB5 i also has a knock sensor, which can sound the alarm
The sensor is on the starboard side below the exhaust and beside the starter. Disconnect the sensor and see if it stops. If it does then there may be a knock or sensor is bad

Also check the Tan/Blue wire on lube and water temp. Disconnect the wire and see if it stops

That's news to me. All this time, I thought knock sensor just signaled "brain" to retard timing if a threshold was met. Does another tan/blue wire come out of knock control module to tie into alarm wiring? When I examined all of my TB5's wiring for breaks, corrosion, etc., I didn't notice a tan/blue wire coming out of control module. Former owner may have removed it? He also didn't have tan/blue wire coming back to any alarm at helm or on motor. I had to add 2 separate audio alarms and wire feeds. They work now. Thx!

 

[alldodge]

Does another tan/blue wire come out of knock control module to tie into alarm wiring?

Yes, in the above wiring diagram you can see the Tan/Blue. In this diagram there is no knock module so the alarm will just retard timing

The ones with the knock module will also have the sensor, which show on 5.7 motor

A worn spot on the wire will also sound the alarm

 

[tank1949]

Yes, in the above wiring diagram you can see the Tan/Blue. In this diagram there is no knock module so the alarm will just retard timing

The ones with the knock module will also have the sensor, which show on 5.7 motor

A worn spot on the wire will also sound the alarm

I wonder if nennedk has 2 different alarms: bilge and engine? I am restoring an 1986 SR Amberjack (TB5 w/Knock sensor) . I could only find a highwater bilge alarm. Tan/blue wires didn't go back to helm. I couldn't find engine alarm on block either. Is the engine audio alarm supposed to be on motor, part of TB control module or at helm?????? I suspect that my boats former owner removed engine audio alarm, which was not too smart.

 

[alldodge]

The alarm should be under the helm.

 

[nennedk]

So I went to the boat today, and can say there is no knock sensor. But then I disconnected the tan/blue temp.sensor wire. The temp. gauge went out but still intermittent alarm. I then disconnected the tan/blue gear lube wire. Still intermittent alarm. Finally I disconnected the yellow and black wire from the other temp. sensor. C3 at diagram. Succes no more intermittent alarm.
But as I wrote earlier, that part has just been changed.
Have I been unlucky to install a faulty one, or could it be something else? I noticed this in service manual for reinstalling thermostat housing:
"IMPORTANT: Gasket has continuity rivets. Do not coat with Quicksilver Perfect Seal, or Audio Warning Temperature Switch may not work properly."
Maybe that's where the problem is?
Whats is the best way to test the temp. sensor? An ohmmeter and then heat some water?
Thanks
Ps. top of starboard elbow 58c, top og port elbow 40c. Starboard manifold around 65c. Temp. gauge steady at 150F (65c). Cylinder head around 70c. All measured while cruising around 3000rpm. Don't that sounds right?

 

[alldodge]

Place in pan of water on stove and connect meter to leads, with thermometer

While the rivets are good to be there, not so much an issue with the 2-wire switch such as yours. Previous switches used the ground of the case.

 

[nennedk]

I don't know if I got any wiser. I have suspected either resistance or not. How should I test a switch?


C Ohm

22 302

36 203

44 131

51 109

54 091

57 084

59 076

60 073

63 063

67 057

70 051

73 045

75 041

80 036

 

[alldodge]

Looks like the temp sender is off



The switches are based by part number

 

[nickmo]

I had a phantom alarm a couple seasons ago. It was intermittent and only stayed on for a couple seconds. On the port riser there is a module. I disconnected the plug, sprayed some electric cleaner on both sides (they were not corroded or dirty) reattached and never had a problem since.

 

[nennedk]

My switch has part no 805218T. I don't see that number at the chart.

805218T is also used at a 5.0L MPI. I have noticed that some parts at my engine is using "newer" parts than what's original was used at the carbureted versions.
Do you have have the diagram for 805218T?

 

[nennedk]

I had a phantom alarm a couple seasons ago. It was intermittent and only stayed on for a couple seconds. On the port riser there is a module. I disconnected the plug, sprayed some electric cleaner on both sides (they were not corroded or dirty) reattached and never had a problem since.

I will try that thanks.

 

[nennedk]

Okay, just did a new temp. measure with more patience, now including the old one which I still have. They both settle at around 490ohm at 70C.
I think I must try to look at the wires to that sensor and ECU connectors. Thanks so far. Still if anyone know what the Ohm table for the part no. 805218T should be I would like to know.

 

[alldodge]

Don't have a diagram and agree your parts list indicates the 805218
This is the same sensor which is used on the MPI

Might be worth a call to Merc to ask the question, or maybe muc mmight know

Resistance values for sensor

 

[nennedk]

I spayed some electric cleaner at the plugs on ECU and the temp sensor (with yellow and black wire). It helped a little, but the problem is still there. It is better at flat water and really annoying when sailing in waves (with hard punches).

Disconnecting water temp sensor (yellow and black) is the only thing that truly works. I believe that narrows the explnations, to wire problems or ECU. As I see it, the yellow wire goes directly to distributor. Could it be bad connecting in there?

Maybe I could make a new strong ground connection for black wire, just to rule out.

Is there any way to test the ECU?

 

[alldodge]

Don't know of a way to test

Thought of a way to bypass it and still have the alarm if it overheats. Cut the Yel/Blk wire and connect it to the Tan/Blue. If motor gets hot the alarm will sound, but the motor will not retard timing