Mercruiser Temperature switch

[jak2108]

I have recently bought a 1990 mercruiser 4.3L. I started it up today with muffs on the drive to bring the engine up to temperature to check all is good before we take her out next week. The problem I have is the temperature gauge did not move up at all when the engine had been running for 15-20mins. When I looked I could see there was a cable not connected to what I think is the temperature sender switch. The cable is white or cream and blue. Am I right to say this is the ground connection?? How can I check the operation of the temperature switch and also the gauge.

Also, after changing the water impeller at the weekend I checked the speedo pick up hole and it was totally clogged. I cleaned out the hole and blew compressed air through the pipe and all was clear but when I re-connected the pipe to the back of the speedo clock and blew the air through the clock did not move at all!! Any ideas??


By the way the engine sounds cool when shes warmed up and purrs like a beast!!
Thanks for your help in advance.

 

[Bt Doctur]

Re: Mercruiser Temperature switch

wiring manuals are in the adults section. You should have 2 senders . one is a switch that senses a overheat and trips the alarm. can be tested by shorting it to ground with the ign "on" motor NOT running.The other is the temp sender. Variable resistor type, wiring to the gauge can be tested by grounding it , gauge should go to full scale.

 

[CheeseCutter]

Re: Mercruiser Temperature switch

As Bt Doctur said, see section 4-E in manual #7 in the Adults only section. That's the manual for your engine. Manual 18, while not for your engine, has much better info on how the sender works and how to test the sender. The specs of the sender may not be the same, but it gives you a better idea of how it works.

The tan/blue wire is for the SWITCH which sets off the audio alarm in an overheat. The switch does not affect the guage. The wire is not a ground. The switch and senders are grounded by their connection to the housing they are mounted in. The temp SENDER should be near by with a tan-only wire, which is what feeds the signal to the guage. With the ignition on, pull the tan wire and patch it to a ground. The temperature guage should peg to max if the guage is good.

On my '93, the sensor is on the cold side of the thermostat, so if it gets stuck closed, the sensor won't show the overheating. You could test the tstat in a pan of hot water to see if it's opening and at what temperature.

I had a similar situation in my '93 4.3LX. Turns out the problem was that the sensor wasn't grounded to anything because the tstat housing was not grounded to the engine. There is a gasket with conductive rivets which ensure the housing grounds to the engine, which in turn grounds the sensor. The prior owner of my boat had replaced that gasket with a standard gasket, which broke the ground of that circuit. I put in the conductive gasket, and all was fine.

Hope that gets you started troubleshooting that.