1973 Mercruiser in-line 6 water temp gauge issue

[Campfire]

Hi guys,

I recently replaced the water pocket, impeller assembly, lower unit, bellows, upper bellows, water shutter, manifold gasket, elbow gasket, etc. The engine is running much cooler now in the 140 degree range. The water temp reads accurately when the gauge is working. On any given boat ride of 5 minutes or more, the gauge drops to zero. Sometimes it comes back to life on a boat ride, sometimes not. I carry an IR gun with me and am able to verify appropriate temps when the water temp gauge is pegged.

Today, the gauge was reading both normal, zero, and full overheat even though the actual manifold was around 145 degrees.

I grounded out the temp sensor. When the gauge is working normally, I can ground the sensor to the block and it instantly pegs full hot. When it’s not functioning, grounding it to the block does nothing.

Do these symptoms clearly point to a specific thing?


thanks!

 

[Scott Danforth]

you have a bad connection somewhere in the signal wire (tan) between the sender and the gauge

 

[Campfire]

Thanks for the reply.

There’s only a single wire coming off the sensor and it is green. I’m guessing it was a patch if the color is wrong. Is it safe to assume that if I simply run a new wire from the gauge to the sensor that the issue will resolve? My idea is to take off the current wire from the sensor and directly wire the gauge to the sensor with a temporary wire.

How many wires are on the back of a lighted water temp gauge? The gauges appear to be original from ‘73.

 

[alldodge]

The gauge would have 3 wires.
Do check is there is a bad ground

White - 12V
Green - Sender
Black - Ground (not shown on wiring diagram)

The light is turned ON normally when ignition is turned ON

 

[Campfire]

Thanks for the diagram. I’ll get this sorted out.

You guys helped me get through a lot of stuff so far on this boat. I was getting gas today and I ran into the 4th generation of family running the marina. This boat (prior to me) has been serviced at that marina since before the 20 some year old mechanic’s dad was born. When I pulled up to launch last month, one of the oldtimer’s knew the boat. Didn’t think he’d ever see in again. She’s going to go for plenty more years now.

 

[Campfire]

Scott,

I realize you already know this but you were 100% correct that it was a signal wire going bad I bypassed the wiring harness and wired the sensor straight to the gauge. Gauge works perfectly now. Going to install the new wire permanently this afternoon. Thanks for the advice!

 

[Scott Danforth]

I keep forgetting about the non-standard wire colors used in the late 60's and early 70's

since the SAE standard, the signal wire is now tan.

you may find other near 50 year old wires having issues. the schematic AD posted will help.

 

[Campfire]

Luckily the wiring harness on a boat this simple isn’t that thick. I’m considering changing to electronic ignition when the distributor decides to move on.

I came home after sunset today and the backlighting for the water temp gauge isn’t working. Is there the world’s smallest light bulb on boat gauges? I may just install an instrument panel flood light if it’s not a very simple fix.

 

[alldodge]

Can probably find the bulb, but might be able to use a 12V led and place it in the socket. Maybe silicone or other to hold in place. Can even get led's in colors