Mercruiser 5.7 Temperature sensor issues

[alschorn]

Hi everyone.

New member & first-time poster here.

I have a 1999 Bayliner Cierra 2655, Mercruiser 5.7 Thunderbolt ignition & Alpha 3 stern leg. I’ve picked up the (seemingly not uncommon) issue of an intermittent alarm from the temperature sender, usually only when running at over 3,000 rpm. Alarm stop as soon as I throttle back.

The engine doesn’t seem to be running hot at all - dashboard temp gauge is rock solid & I keep checking the engine temperature with an IR temp gauge & it stays consistent as well. When I unplug the wire from the temp sender, the alarm goes away.

So far I have changed the thermostat & temp sender, but the problem hasn’t gone away. I’m thinking the next options are to change the wire from the temp sender to the Ignition Control Module, or to change ICM itself.

I’ve seen a few discussions on similar issues dating back several years, but so far I haven’t come across anything that seems to provide a definitive diagnosis or solution. Some posts have suggested that the alarm is going off because the ICM isn’t receiving a signal from the temp sender.

Any advice or insights will be very much appreciated.

Regards
Al

 

[alldodge]

Motor serial number please?

 

[alschorn]

Engine serial number 0L030924.

 

[alldodge]

Thanks, TB5 and may have knock sensor

The ICM doesn't get info from any sensor other then the knock sensor if installed. The alarm has 12V on one side and the Tan/Blue wire is attached to oil pressure switch. gear lube bottle, temp switch and knock module. Temp goes thru knock module

Edit: if temp switch has yellow wire then it goes to knock, if its Tan/Blue then it grounds like the others

This is a V6 but wiring is the same for yours


Are the temps your getting from the top of the thermostat housing or on the side right where the temp gauge or switch is located? Measure on the side and compare to the gauge. If measuring on top the housing you can be getting cooler water

 

[alldodge]

Might be something wrong with the drawing I posted
If the temp switch has a yellow wire, then OK, otherwise it goes to ground like the other 2

 

[nola mike]

Might be something wrong with the drawing I posted
If the temp switch has a yellow wire, then OK, otherwise it goes to ground like the other 2

So it looks like the icm is connected to the buzzer and sensors by the tan/Blu and yellow wires. Unclear if the if the icm uses the tan/blu as input or output, both? I also don't see the ks on there, I thought it was a separate control box

 

[alldodge]

5.7 with knock

 

[alschorn]

Hi gents.
Thanks very much for your inputs - I’ll check out the various suggestions this weekend.
Apologies for the delayed response - it seems I wasn’t getting notifications of your posts.