Temp Guage Problem

sandiego

Recruit
Joined
Jul 21, 2008
Messages
3
I have a 2000 Monterey with a 5.7 volvo. The temp guage works about everyother time. We had a mechanic look at it and said it was the guage. They bench tested the guage and said it worked but when they put it back into the boat it did not work. So they ran a new wire from the engine to the guage and told me that it works intermitantnly. After getting the boat back, the guage still works just occasionally and when it does work will only go to about 110-120. My questions are what should the normal operating temp be for this engine? Second, I looked at the engine to verify that they ran a new wire. I can find a new wire runing to a bolt or sensor that appears to be under the water housing on the front of the engine. But I also found another wire pluged into the top of the same housing that is not connected. Does anyone know what this second wire is and shouldn't it be conected to something? The last question is since they pulled the guage and ran a new wire, the guage does not illuminate when the dash guage lights are on - did they not conncect it correctly? Any advice before I call this mecaninc back will be very helpfull so I can at least know what I am talking about. Thanks!
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Re: Temp Guage Problem

Howdy,

A temp gage circuit is a rather simple thing.

There's a temp sender that is usually bolted or screwed into (and grounded to) the thermostat housing.

There is one wire from that sender to the gage(negative connection) The positive connection is connected to 12v + (when the key is on).

The sender is simply a variable resistor that changes value with temp. The electrical current displayed by the gage(Calibrated for temp), is controlled by that variable resistor (sender).


Assuming the gage or sender is NOT defective, the wire is not broken and you have 12v at the gage........ you have only 4 places to check for problems ,

The 2 posts on the gage, the 1 post on the sender, and the ground connection at the thermostat housing..... which of course should be grounded to the engine.


Sometimes the thermostat housing might not ground to the engine block well because the gasket is insulating it an there's some corrosion on the bolts.... Mercury has used little brass 'rivets' in the thermostat housing gasket to ensure the housing was positively grounded to the block.

You could check for this by using a short piece of wire with alligator clips on each end. Clip it to the brass hex part of the sender (NOT THE WIRE CONNECTION ) and clip the other end to a good clean bolt on the engine (or directly to the NEGATIVE post on the battery.)

If the gage starts working, you have a grounding problem in the circuit. If you have ground problems at the engine, might see other strange problems in the other gages and accessories.

The other place to look is the actual connections at the gage and sender. Some people are not very careful when crimping wires and don't get a good clean connection..... wiggling the wires will sometimes reveal a poor connection.

Your engine should probably indicate about 140-160 degrees on the gage. If the thermostat is stuck open it may not get that high. If the gage or sender is defective it may not show the correct temp. You'll want to check the temp with another gage/sender or infrared temp probe.



Cheers,


Rick
 
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