Temp gauge problem, please help!

[Gunner54]

Water temp gauge works (reads resistance) until it hits 150, but as temp continues to increase, it goes to 0. Pick up speed and once engine temp drops the gauge starts working, but it will only read below 150 deg.

I have a 160 deg thermostat, so the temp reading of the gauge with the engine running is not accurate.

If I shut the engine off and turn the ignition to "ON" the temp gauge works and reads accurately above 150 as tested with an infra red temp tester.

I've replaced the gauge and the sender, but still have the same problem.
Sender is grounded metal on metal.
I've run a wire directly from the sender to the gauge but still, same problem.

What am I missing here??

 

[Watermann]

Check the ground wire for the gauge, run a good solid ground lead first to test it out.

 

[Bondo]

Check the ground wire for the gauge, run a good solid ground lead first to test it out.

Ayuh,.... If the t-stat was changed, 'n the gasket didn't have the brass crush rivet, that could be it,.....

 

[Gunner54]

Check the ground wire for the gauge, run a good solid ground lead first to test it out.

I tried running the gauge ground lead to the neg battery terminal with no change in results.

Ayuh,.... If the t-stat was changed, 'n the gasket didn't have the brass crush rivet, that could be it,.....

Yes I've tried a new thermostat and even tested it in hot water with a calibrated thermometer. It operates fine.

"Brass crush rivet" ... could you elaborate?

 

[Watermann]

How about some more info on what your working on.

What bondo is talking about is some of the gaskets look like the one below that have the crush rivets.

 

[achris]

What engine? Make, model, year, serial number. Thanks...

 

[Gunner54]

Thanks for the suggestions and comments.

It's on my '91 Glastron 195 SSV, originally a 350 / Alpha 1, now morphed into 383 with Alpha SS.

 

[achris]

So, the engine is/was a 1991 5.7L (not interested in the boat).... When you say 'morphed', do you mean the original 5.7 was stroked/bored, or a whole different engine was dropped in?

 

[Gunner54]

Yes the original 1991 5.7L Mercruiser engine was stoked to 383 using a 400 crankshaft. Bore is stock.

Still trying to get a temp gauge to work accurately.

 

[achris]

using thread tape on the sensor?

 

[Gunner54]

Nope.

 

[achris]

ok... Clutching at straws. The gauge and sender are both matched correctly? Senders and gauges are set for particular ranges. Most Merc gauges are 33-240 ohm, but some VDO and teleflex gauges are 0 to 100 ohm or 0 to 180 ohms... You don't have a mismatch do you?

 

[Gunner54]

ok... Clutching at straws. The gauge and sender are both matched correctly? Senders and gauges are set for particular ranges. Most Merc gauges are 33-240 ohm, but some VDO and teleflex gauges are 0 to 100 ohm or 0 to 180 ohms... You don't have a mismatch do you?

I appreciate the comments and thoughts, yes the gauge and sender matches.

The gauge will read above 150 deg on the warmed up engine accurately ONLY with the ignition on and engine NOT running. When the warm engine is started the gauge goes to 0 until the actual engine temp goes down.

I have 2 gauges that have been alternated, a Faria and the original Glaston both work in same way as described above.

I've tried:

1/ new wires,
2/ wired directly to the battery, eliminating the ignition switch
3/ taken the alternator belt off to eliminate the charge system.
4/ Checked the main wiring harness plug for loose connections
5/ Checked the main ground stud and put dielectric grease on the threads.
6/ Checked and cleaned the thermostat housing threads and applied dielectric grease.

Everything ends with the same "frustrating" result.

 

[NHGuy]

I wonder if your ignition system could be doing it. What are you getting your sparks from? If aftermarket, are there any installation remarks that could have been followed differently?

ps. Jealous of your crankshaft! Gotta do that at some point.

 

[NHGuy]

Leave it running and check ground continuity when the gauge zero's? Maybe some weird expansion? I'm following your thread, odd problem!

 

[gm280]

Gunner54, I have a crazy idea, can you take the thermostat AND the gauge and place the thermostat/sender in a pan of water while still connected to the gauge and heat it up to see if it continues to read. That would eliminate anything else but the sender and gauge and wire... If it reads straight through the entire range, then you have an engine continuity issue... Crazy but interesting... :thumb:

Just thought about another question. Is this temp sensor actually made for a gauge and not a on/off type?

 

[Gunner54]

I wonder if your ignition system could be doing it. What are you getting your sparks from? If aftermarket, are there any installation remarks that could have been followed differently?

ps. Jealous of your crankshaft! Gotta do that at some point.

Spark comes from the original coil Mercruiser coil.
I'll try your suggestion on ground continuity. Thanks!

Other than this temp gauge issue.... the engine works great with the updated crank and all the other bits thrown at it
here's a link to a short youtube of the boat.

 

[Gunner54]

Gunner54, I have a crazy idea, can you take the thermostat AND the gauge and place the thermostat/sender in a pan of water while still connected to the gauge and heat it up to see if it continues to read. That would eliminate anything else but the sender and gauge and wire... If it reads straight through the entire range, then you have an engine continuity issue... Crazy but interesting... :thumb:

Just thought about another question. Is this temp sensor actually made for a gauge and not a on/off type?

Thanks for the feedback.

Both senders are Mercuiser senders. And they both work (or not in my case) the same.

0-150 reads ok but soon as temp hits 150, the gauge blips back to 0. Until I drive a bit quicker and cool it down, then the gauge "blips" back to 150 or less....
While at 0, shut engine off and turn ignition back to ON, and the gauge (either of the 2 I have) will read above 150 to whatever the actual engine temp is.

 

[Gunner54]

UPDATE: threads in the thermostat housing are corroded to where the sender can not ground properly. No amount of dielectric grease would complete the ground...

TEMP FIX: Wrapped the sender body with copper wire and grounded the other end of the wire to the engine... Voila, it works!