1989 Mercruiser 5.7 Water Temp Sending Unit

Karen S Little

New Member
Jan 9, 2021
28
Boat Info
1989 SR 300 Sundancer
Engines
twin Mercruiser 5.7
Greetings I have a 1989 SR 300 Weekender with twin 5.7s. Port engine has a water gauge and RPM gauge issue i am hoping someone will help with.

Water Temp - gauge is dead on zero. i checked the gauge for loose connections / grounds. anyone have a photo or exact description where on the engine is the actual sending unit so I can clean / heck grounds there?

RPM - gauge intermittently jumps around. the engine is not actually surging - just a gauge issue. again, Ive checked the gauge end for loose / good connections. can anyone help with photo / description where on the engine the wire actually connects for the rpm signal?

thanks
 
Certainly check the grounds, but the gauges may be bad or going bad. Tachs are notorious for eventually jumping around when they age. Since you have twins, try swapping them to the other engine and see if that helps.
 
Tach runs off the coil, I think it is the purple wire. Temp sensor on mine is near the thermostat.

IF the tach is the original SeaRay tach there is a adjusting screw on the backside. The temp sensor may have gone bad due ti it’s age.
 
Tach runs off the coil, I think it is the purple wire. Temp sensor on mine is near the thermostat.

IF the tach is the original SeaRay tach there is a adjusting screw on the backside. The temp sensor may have gone bad due ti it’s age.
thank you - if able, can you possibly send a photo of where on the engine exactly?
 
Here you go, Karen the first one is for the alarm (over heating) Second is the Temp Gauge. Any questions please ask and I will find the answer tomorrow while I’m at the boat
 

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Karen,
To check your gauge. Just unscrew the wire from the sensor. Touch it on anything grounded like the block. The guage should be at 0. ( all the way left ) Then when you touch something metal it will go to full hot ( all the way right ). If this happens the guage is good.

Next. The thermostate housing is where the sensor is. You should have 2. 1 goes to guage and 1 is a overheat alarm.
 
Here you go, Karen the first one is for the alarm (over heating) Second is the Temp Gauge. Any questions please ask and I will find the answer tomorrow while I’m at the boat
Thank you sir much appreciated. ill check tomorrow also
 
Karen,
To check your gauge. Just unscrew the wire from the sensor. Touch it on anything grounded like the block. The guage should be at 0. ( all the way left ) Then when you touch something metal it will go to full hot ( all the way right ). If this happens the guage is good.

Next. The thermostat housing is where the sensor is. You should have 2. 1 goes to guage and 1 is a overheat alarm.
Thank you Jeff Ill check tomorrow at the boat
 
Karen I will be at the boat tomorrow after work. If I can help I will PM my cell. I will also be with my engine guy till around 3est.
 
Karen,
Hope this helps.
First pic is temp sensor to guage
Second pic is overheat sensor to alarm
41mxZ7pm4FL._AC_SS350_.jpg
sd10482-nib-mercruiser-4cyl-v6-v8-sender-temp-audio-warning-thermostat-housing-806951.jpg
 
Just a follow up - replaced both engine sending units and over temp sending units. of course the ones the parts manuals have now is different from what was installed :) thanks for the help
 
Hey Jeff - sorry I should have mentioned the total result :)

I first changed just the sensor - it did fix the gauge reading but not the overheat. i changed the thermostat, Barnacle Bustered the heat exchanger, and changed the raw water pimp. end result was gauge works and the overheat is NO MORE - yaaaaaa :)
 

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