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Discussion Starter #1 (Edited)
2002 750 ace
Overheating red light problem.
Bike was sitting for a good period of time when I bought it.

1. cleaned and flushed coolant system... used vinegar and distilled water.
2. replace the thermostat and clean the housing.
3. Replace the relay at the thermostat housing.
4. The fan wasn’t turning on while riding, when I jumped it it would.. so I replaced that relay located on lower part of the radiator.
5. Also I haven’t seen any indication of overheating besides just the red light

riding the bike to my gym and back which is only about 2 miles the red light comes on. Also the fan is not turning on . Any help or suggestions would be greatly appreciated ?
 

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I would use coke to clean the system.
Then check the temp sensor for connectivity issues.
If the light comes on but the fan is not fired up then it should be the sensor.

The cleaning process includes replacing the coolant with coke, run the engine for 10 to 15min to release all rust and dirt.
empty the system, flush it with tap water, empty it again and fill up with normal coolant.
 

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Wonder what the sugar would do to the cooling system? The system has been cleaned and flushed, and the fan works, so the coolant is most likely not circulating. The water pump may not be working, but more likely the system is air locked. Keep leaning the bike left and right, pump the rubber cooling hoses as much as you can... WORK it until you know it’s been ‘burped’.
 

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Discussion Starter #4
So just burped the motorcycle… The light is still coming on and the fan is not… About to buy a water pump and hopefully that fixes it all
 

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1983 Honda vt750 Shadow
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Test the pump flow first. It is too much trouble to change if it isn't the problem.
Take the rad cap off and fill it to the top. Start the engine and see if coolant flows out when you rev it.
With the thermostat in it won't be as active, so if in doubt, take the thermostat out and fill the housing to the top, and try it again. It should push some fluid when revved.
If it doesn't flow then the impeller fins are rusted off which is a rare case but can be the problem.

You can also do a flow test of the rad by taking both hoses off and pouring water in the top and it should run out the bottom almost as fast if the tubes are open.
 
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2003 Ace Deluxe
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not sure about coke in the cooling system, not sure if the citric acid clean is to used for motorcycles.

it's worth the time to see if the water pump is working with the t stat out.

replacing the water pump is a real pain, i did it a few years ago without removing the motor from the bike.
 

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The fan only comes on when the temp in the bottom of the radiator is REALLY hot, which usually only happens with excessive idling on hot days, or some other problem. How long does it take before the light turns on? If it’s not long, the issue is more serious. Also, when you short the fan (I’ve seen any number rigged with a physical switch), how much does the temperature drop?
 

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Discussion Starter #8
The fan only comes on when the temp in the bottom of the radiator is REALLY hot, which usually only happens with excessive idling on hot days, or some other problem. How long does it take before the light turns on? If it’s not long, the issue is more serious. Also, when you short the fan (I’ve seen any number rigged with a physical switch), how much does the temperature drop?
If I’m letting it just sit at my house 10-15 minutes the light will come on

If I’m riding it around the same . But it can vary if I don’t have any lightBut it can vary if I don’t have any stop lights it’s longer .
 

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Discussion Starter #9
Also an update I found a hairline crack in the overflow reservoir… The reservoir was empty I don’t know if they’re supposed to be a little bit in it for pressure reasons… But I believe it might be causing air to be sucked in
 

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What is the overflow sensor you mention?
Look at the seals on the radiator cap = both rubber seals have to be in good shape.
And lift the small metal disc gently with a pocket screwdriver and see if it pops back down snug.
It has to work for the overflow and reservoir tank to be able to operate and keep the radiator full.
And look at the rubber tube for cracks.
 

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The reservoir is not strictly required. It's a way to recover 'wasted' coolant if temperatures get too high and it gets blown out as steam/liquid. For your testing, you could probably plug the hose, but I don't ultimately believe that's the issue. Ten to fifteen minutes of idling is plenty of time for it to get warm, especially with no fan. Were you ever able to directly power the fan to see if the overheat light shuts off after a minute or two of the fan running?
 

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1995 VT1100C2
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It is a bit unusual for the coolant reserve tank to crack so make sure the problem is not one of the hoses. Coolant is required in the reserve tank and capacity is 400ml (.42 US qt). The reserve tank will allow coolant to move between the radiator and reserve so keeps the rad "full" and operational. Plus Honda recommends filling via the reserve tank when topping up since it is much simpler.

The temperature light is controlled by a thermosensor switch separate from the fan switch so it is possible you have nothing really wrong (except the light) especially since you are not experiencing overheating. If you can burp and no overheating I dont believe this is a waterpump problem. I think your temp light is in the speedo casing (US and Canadian models are a bit different). But there is also the possibility in a short of the thermosensor to light wiring.

Unlike the thermostat, the thermosensor functions via electrical resistance at a particular water temperature so when temp is high the resistance is low at under 20 ohm and clicks the light on. However it is possible the resistance is low do to failure of the sensor or wiring problem.
 

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Discussion Starter #13
The reservoir is not strictly required. It's a way to recover 'wasted' coolant if temperatures get too high and it gets blown out as steam/liquid. For your testing, you could probably plug the hose, but I don't ultimately believe that's the issue. Ten to fifteen minutes of idling is plenty of time for it to get warm, especially with no fan. Were you ever able to directly power the fan to see if the overheat light shuts off after a minute or two of the fan running?
I have not but I can jump it to see what happens as soon as the light turns on
 

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Discussion Starter #14
It is a bit unusual for the coolant reserve tank to crack so make sure the problem is not one of the hoses. Coolant is required in the reserve tank and capacity is 400ml (.42 US qt). The reserve tank will allow coolant to move between the radiator and reserve so keeps the rad "full" and operational. Plus Honda recommends filling via the reserve tank when topping up since it is much simpler.

The temperature light is controlled by a thermosensor switch separate from the fan switch so it is possible you have nothing really wrong (except the light) especially since you are not experiencing overheating. If you can burp and no overheating I dont believe this is a waterpump problem. I think your temp light is in the speedo casing (US and Canadian models are a bit different). But there is also the possibility in a short of the thermosensor to light wiring.

Unlike the thermostat, the thermosensor functions via electrical resistance at a particular water temperature so when temp is high the resistance is low at under 20 ohm and clicks the light on. However it is possible the resistance is low do to failure of the sensor or wiring problem.
I do believe I replaced that sensor it’s connected to the thermostat correct… I have thought maybe it is a bad ground somewhere
 

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Discussion Starter #15
Well fixed… Ended up being the overflow reservoir had a hairline crack that was air. Ended up taking the hoses off the thermostat housing and pouring water through... no restrictions. Took the lower hose off the water pump and reved it , water shot out so the water pump was good. So I filled up a gallon of water and stuck my overall flow hose in it… Fixed everything… Pretty stoked right now thanks guys
 

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So was the crack at the top, middle or bottom of the reserve tank? Any idea why it cracked? Did you just silicone it?
 

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We‘ll take that for the win! LOL! But I’m still puzzled. The cap is supposed to keep the top sealed until enough pressure builds up to vent. Which shouldn’t happen if your cooling system is working normally... Well, removing the hoses definitely blew out anything else plugging up the system for sure! ;)
 

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...I’m still puzzled. The cap is supposed to keep the top sealed until enough pressure builds up to vent....
Thermal expansion and pressure of coolant at about 100C (fan comes on) and about 120C (light comes on) is approximately 6% so with 2000 ml coolant capacity you need minimally 120ml reserve coolant. However drawdown (as engine cools) capacity is calculated at 12% so no chance of air so minimally 240 ml coolant from reserve. Honda, being somewhat conservative and having Min/Max line levels, has a Max of 400 ml reserve coolant.

Depending on where the reserve is leaking from (top/middle/bottom) it is possible the OP is drawing air into the rad system when drawdown due to cooling.
 

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Discussion Starter #20
Thermal expansion and pressure of coolant at about 100C (fan comes on) and about 120C (light comes on) is approximately 6% so with 2000 ml coolant capacity you need minimally 120ml reserve coolant. However drawdown (as engine cools) capacity is calculated at 12% so no chance of air so minimally 240 ml coolant from reserve. Honda, being somewhat conservative and having Min/Max line levels, has a Max of 400 ml reserve coolant.

Depending on where the reserve is leaking from (top/middle/bottom) it is possible the OP is drawing air into the rad system when drawdown due to cooling.
It was cracked about midway. I do believe it was causing air in the system. Whenever I shot it with my thermo gun the rad was 90 but the block was overheating, why the fan wasn’t turning on. I though I might have had a blockage but when I flushed it nothing of the sort. As soon as I put the overflow line in the gallon of water and re filled and burped the system, the block was staying cool and the rad was hot, Fan started working. I guess that hairline crack was just causing that much air .
 
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