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Steve
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71 Posts
Discussion Starter #1
I am absolutley stumped. My 86 VN750 is still overheating. I have changed the coolant at all locations, ie heads as well as water pump, and overflow, changed the thermostat for a new one, changed the Radiator cap for a new one. The overflow tank is full when hot and medium level when cool.

The only thing I can think is the water pump is stuffed. When I take off the radiator cap and rev the bike the level does drop down about half an inch and rises and when it returns to idle. Is that normal ?

Its still overheats into the red in traffic. I am using 10/40 oil. And 50/50 coolant (green)

I have also ran the bike for 10 minutes with the radiator cap off it and rocked it from side to side to remove any air bubbles. What else can I try ?

The bike goes really strong so iassume not a head gasket. There is no oil in the coolant, and visa v.
 

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1986 VN750
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3,255 Posts
Is your radiator fan coming on?
 

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Steve
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71 Posts
Discussion Starter #3
Yes the fan comes on when the temp is about half way or a little higher. It only stays on for about 15 seconds at a time. Is that too short ?
 

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1986 VN750
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3,255 Posts
Yes the fan comes on when the temp is about half way or a little higher. It only stays on for about 15 seconds at a time. Is that too short ?
I've heard my fan run for much longer durations than that.

It's possible the fan is not running long enough, resulting in the heat you're describing.

As soon as you start moving again, does she cool right back off?
 

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Premium Member
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5,236 Posts
Is it possible that your sensor or gauge are simply not reading right ? If you think they are check your fan sensor in the bottom of the radiator ,ground the wire going to it and it should run constantly.if it still kicks off in a few seconds the fan relay may be dropping out.It is internal to the JB and can be by passed with an automotive relay externally.

I had to bypass mine on my750 with an external relay,after I bought Fan.To check it unplug it and run two wires straight from the battery and see if it runs constantly before bypassing the relay,I suspect either the sensor in the bottom of the radiator or the relay,but the temp sensor on the t-stat housing may be out of range causing a false reading on the gauge.
 

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Steve
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71 Posts
Discussion Starter #6
I think the temp guauge is correct. Its a replacement (due to tach not working). Both have shown overheating.

I will try the the fan option tommorrow by grounding the wire. Which wire should I grounds BTW, and what is a JB ? Excuse my ignorance.

When I start moving the temp backs off after about 30 to 40 secs, but not much. Even at 100 km the temps is still just over half.

When I bought the bike the guy had water in it . No coolant. I have flushed it twice. There was some rusted iron bits in the overflow tank. Could these be stuck in the system somewhere casuing a blockage ? Its just a thought. I am just at a loss right now.
 

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Premium Member
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5,236 Posts
I think the temp guauge is correct. Its a replacement (due to tach not working). Both have shown overheating.

I will try the the fan option tommorrow by grounding the wire. Which wire should I grounds BTW, and what is a JB ? Excuse my ignorance.

When I start moving the temp backs off after about 30 to 40 secs, but not much. Even at 100 km the temps is still just over half.

When I bought the bike the guy had water in it . No coolant. I have flushed it twice. There was some rusted iron bits in the overflow tank. Could these be stuck in the system somewhere casuing a blockage ? Its just a thought. I am just at a loss right now.
about halfway on the gauge is about right ,running down the road.At the bottom of the radiator there is a sensor that makes up a ground to complete the circuit for the relay circuit,unplug that wire and ground it and turn the key on the fan should run constantly.If it does you have found the problem .If it doesn't and still kicks off it is the relay in the JB(Junction Box) or the fan motor,which is less likely.If the fan runs constantly with the wire I mentioned the fan sensor switch is bad (the one you unplugged the wire from).
 

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Banned
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10,354 Posts
Check your overflow cap seal too. Mine was a lil too warm once, and when I removed the overflow cap, idle went up and it cooled down. Turns out the system was a lil overfilled, but the overflow tank wasnt allowing the extra to overflow ! Make sure your coolant level is right in the overflow as well.
 

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Steve
Joined
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71 Posts
Discussion Starter #9
Thnaks Denny. I did the test with wire and the radiator fan runs constantly. The fan sensor switch needs replacing. Can I ask how this is done ?
 

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romeobravo172
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842 Posts
Fan switch unscrews from rad, where you pulled the connector off of! Got one, PM me
 

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Premium Member
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5,236 Posts
Thnaks Denny. I did the test with wire and the radiator fan runs constantly. The fan sensor switch needs replacing. Can I ask how this is done ?
the wire terminal you took the wire off on the bottom of the radiator is on a sensor that screws out of the radiator.First thing you need to do is get a sensor.There was someone on here parting out a bike,you might buy one off him or you can check Ron Ayers cycle parts or Cheap cycle parts for new.if you need to run the bike untill you get one, mount a toggle switch to that wire and run the other wire to ground and put it in easy reach and turn it on as needed.

After you get the sensor/switch,drain the coolant and unscrew the bad one and put the new one in it's place and wire it back up like it came.

Edit:^^^^^ there you go one already popped up!
 

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Steve
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71 Posts
Discussion Starter #12
PM Sent. Thanks for the great advice. I have a toggle switch handy, so that sounds like a cool (pardon the pun) in the interim.
 
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