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Discussion Starter #1
Today marked the first legit start of the '82 GL500. After cleaning the carbs, setting the mixture screws, and getting the jetting close enough (90's for the primary, all else stock), she fired right up, first try, so I'm psyched. I have the proper size primary jets (78's) on the way, but wanted to try the test start this weekend.

After several minutes of idling, the temp gauge on the tach didn't read any increase. It's a used ebay special, so the guage could be broken or I may not have the wiring correct yet. the tach itself worked fine.

I have not yet flushed the radiator or tried to overhaul any of the cooling system. As I start to diagnose the cooling system, the bigger question ... are the temp gauge, thermostat, water pump, and fan all hooked to the same circuit? Put another way, could the cooling system work even if the gauge doesn't function. Service manual has different test for both the sending unit and the thermostat, so my assumption is that they are independent.

One test would be to let it idle until the fan comes on, but if the gauge doesn't work I'll have no indication if she's overheating, so it's risky.

I found a wiring diagram, but it only showed a circuit between the sensor and the gauge. Nothing about the thermostat or fan.

I know that earlier CX's had the fan tied to directly to the engine, but the later ones and GL's have a traditional thermostat.

Any insight appreciated.

Steve
 

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All are independent, analog systems. The water pump is driven directly by the camshaft. So is the fan, unless someone did an electric fan conversion. 650s have an electric fan, stock.
The thermostat is mechanical.
The gauge is driven by varying voltage through the temp sensor. There's a 7v regulator behind the instruments that provides consistent source voltage as overall system voltage varies with engine speed. That's often the culprit. Murray sells a replacement.

Randall
 

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Discussion Starter #3
All are independent, analog systems. The water pump is driven directly by the camshaft. So is the fan, unless someone did an electric fan conversion. 650s have an electric fan, stock.
The thermostat is mechanical.
The gauge is driven by varying voltage through the temp sensor. There's a 7v regulator behind the instruments that provides consistent source voltage as overall system voltage varies with engine speed. That's often the culprit. Murray sells a replacement.

Randall
Thanks for the info. Looks like my understanding about thermostat turning the fan on was wrong. The rest of this bike is unmolested so i doubt anyone PO did a fan conversion.

I'll flush the coolant and probably pull off the radiator since there's still some critter remnants on the top of the engine.
 

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The pipe that runs back to front on the left side of the engine will remain cool to the touch for the first mile or so. Keep your finger on it and you can easily feel the thermostat opening.
 

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The pipe that runs back to front on the left side of the engine will remain cool to the touch for the first mile or so. Keep your finger on it and you can easily feel the thermostat opening.
Nice tip. I'll try that next idling session. The bike only has 4400 miles and still has what appears to be the original front tire, so it needs new rubber before even a neighborhood test ride.
 

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To clarify: The "thermostat" is a termo-mechanical valve (standard, easily replaceable automotive part) located inside a housing above the engine which controls the flow of coolant from the engine to the radiator and the "temperature gauge sender" is a thermo-electric device (resistance varies with temperature) screwed into the thermostat housing which controls the flow of electrical current through the gauge (models with electric fans also have a "thermo-switch" or "fan switch" screwed into the radiator which turns the fan on & off depending on temperature).

When the engine is cool and the thermostat is closed the water pump draws coolant out of the thermostat housing via the small bypass hose that runs directly from the thermostat housing to the pump so that the coolant recirculates through the water jackets around the cylinders until it is hot enough to open the thermostat and allow the coolant to flow into the radiator. And a few seconds after the thermostat opens coolant which is not as hot as that entering the rad but a lot hotter than the air will start passing through the water pipe on its way back from the rad to the pump (blue arrows in the pic below) on its way through the pump to the cooling jackets.

201384


An engine really needs load and varying throttle to warm up properly so it can take a long time to fully warm up just idling.

Current flows from the 7V regulator to the temperature gauge and from the gauge to the sender. The other side of the sender is grounded through the engine (return current path to the battery). If the gauge is not working the first thing to do is make sure that it is connected as it should be (is your tach from the same model or a different model and perhaps the wires to the electrical connector are in different places) and that all 3 of the 7V regulator's wires are connected securely and the wire for the sender is plugged onto it.
If the wiring is correct check for 7V at the yellow wire that goes from the 7V reg to the gauge. If not make sure the reg's black wire really does have 12V and its green is actually at ground potential (must be connected to a solid green wire - no stripes). If still no 7V the regulator may need replacing.
If the regulator is good disconnect the wire from the sender and touch it to ground. The gauge's needle should move to the right (full hot). If it does suspect the sender. If not suspect the gauge or the wiring.
 

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Discussion Starter #7
UPDATE - With today's test, the coolant return pipe (horizontal, left side of engine) did eventually get hot, indicating the thermostat had opened and coolant was running through the radiator. It took about 5+ minutes, so the thermostat was likely closed at the start as intended. I also felt air flow in front of the radiator that increased when I hit the throttle, so the fan is operating. I've heard of cracks and will inspect the fan closer once I dig into it..

The temp gauge still did not register, so i'll test those components as well.

I'm still a week away from a test ride, but the bike coming together nicely. With tank dents, frame rust, and engine surface blemishes, it will never be a beauty queen, but will make a nice bomb around/rainy day bike.

Thanks all.
 
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