I have a rinnai cosy glow 650 when i push the start button it glows up but soon as i let the button go it stops glowing

glow plugs on a diesel should be the same voltage as the battery voltage that vehicle has , the glow plug relay takes this power and allows it to go to glow plugs for first few minutes of starting , some of them cycle the power on/off several times a minute to control the temp of glow plug but the voltage is allways the power of the battery , this may only not be true on large trucks and tractors (im talkin big !) they use 36 volts to start but then as soon as started cut back to 12 or 24 volts

problem is proberly the thermocouple burnt out .FSD
(fail safe device) normally you push the button in and the pilot light or flame is ignited,this flame heats the thermocouple that generates a small voltage that holds the gas valve open.
if the thermocouple is burnt out or not set correctly in the flame or the pilot flame is to small , as you say when you release the button it goes out, looks like you need a new thermocouple.

Sounds like maybe air in fuel system, bleed fuel system and try.
are there any trouble codes now or before.
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Light Duty Truck Diesel Engine Diagnosis

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Check the big fuses in the power distribution box under the bonnet ,this is a UK reg vehicle kept outside no doubt and these fuse connections under the bonnet do get damp ,if still no joy for whatever reason then but a manual push button starter switch and wire them in direct manual overide ,power to switch then direct to the glow plugs ,so when the light dont come push button in for 10secs and no more and away you go,Clean the big fuses and spray with some 3in1 or a oil can with engine oil not that wd40 and lubricate the connections where the fuses push into.

What make , model , year and engine ?

The 4500 John Deere tractor sounds like the glow plugs are not working. Checked voltage at the glow plugs to make sure that there is 12 DC volts.(Might have to remove main wire from the glow plugs to check!) If no voltage at glow plugs there maybe a bad fuse,relay or wire connection in the glow plug circuit. If there is 12 DC volts at the glow plugs, it is possible that the glow plugs need to be replaced.

MOST TIME GLOW PLUG CONTROLLER ON THE REAR OF INTAKE MANIFOLD CAUSES GLOW PLUG PROBLEMS.YOU HAVE TO USE 12 VOLTS TEST LIGHT TO SEE IF POWER GOING TO GLOW PLUG CONTROLLER.IF NOT YOU HAVE BLOWN FUSE OR FUSE LINK.IF POWER GOING TO GLOW PLUG CONTROLLER.THE CONTROLLER IS FAULTY OR THE ENGINE TEMPERATURE SWITCH IS BAD.

Check the glow plug thermo sensor...found in the cylinder head.
Unlightly anything wrong with injector pump.
Good luck
John

Check the glow plug.
Take off the bottom front panel, push the start button and look for a red glow. This glow plug is about the size of your little finger and is hard to see when it's off but you will see a red glow if it's working. No glow, replace the glow plug.

If it glows for a few seconds then goes out then the solenoids on the gas valve should be checked with an OHM meter.