John Deere model 855/856 (late '80's) review/cold start

Testing out a circa 1986 John Deere 856 for possible purchase. Less than 3,000 hours and seems to have been well maintained.
I believe it to be a Yanmar 3 cyl. diesel with the intake manifold heater instead of glow plugs for cold starting. I cannot find a button or key position to activate a solenoid to send power to the heater. It has the little lighted figure thingy on the dash to let you know when the heater is activated, but it remains dark. There are some fuses on the 'firewall' but none of these state it has anything to do with the heater. I don't have an operator's manual for this unit. Thanks for any pointers.
 
I have an 850, it sounds like it is similar. To activate the heater, rotate the key counter-clockwise and hold for several seconds. It also has a compression release that you pull to aid starting, but I understand that is the sign of a very early model.
 
We have an 01 4600 and on it you hold the key in, and it starts very well. If it's below 0 F I plug it in, and it always
starts immediately. After it starts it might start to smoke a little and if you push the key in for a few seconds it stops
smoking.
 
see if this gets you started, the main idea is the controller decides if it's cold enough for the heater to come on and for how long,
if you'll send me your email, I'll send you a PDF of the complete technical manual

Joe

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I wonder if the ignition switch was changed out and ergo the heater circuit was eliminated? I went out and tried the counterclockwise thing again. There is no 'return to off' feature in any position except start. The ignition key pins are at the bottom of the switch, which is rare due to the possibility of dirt getting into the pins. Thanks for the replies.
 

Thanks to everyone for the excellent information. On the weekend I will try to narrow the problem down with the information I now have. Awesome!
 
I have a 650 2 cylinder. To activate the cold start heater you simply turn the ignition key to the left for about
30 seconds or so. This is called a thermo-start. It starts a little fire of sorts with the small amount of diesel
in the cup on the element and it preheats the incoming air. Mine has always craned good when I have had to use this
system.
 
855 has a controller for the cold start. It looks similar
to a relay mounted on the firewall under the hood.
 
I have an 87 JD-850 with preheat feature. I just turn the key left for 15-30 seconds and start normally. Rarely use the feature unless it is really cold, which in Texas is below freezing. Starts easy every time with using the preheat feature. I do not have the decompression feature on my tractor. The tractor is almost 30 yrs old and the only maintenance issues is replacing the battery at 17 yrs and replace a fused link to the starter at 18 yrs. If you don't abuse the two stage clutch and changed all the filters and fluids on schedule it will be bullet proof. Just wish mine had some remotes. JD no longer stocks the conversion kits and I haven't been able to find a kit for mine. Good luck.
 
On my 4300 you turn the key to the on position then "push" the key inward, toward the dash till the indicator glows red.....have no idea if that will work on yours....
 
On my 855 you turn the key on let it sit the glow plug light on the dash will light for 24 seconds then you start as normal.
 

Thanks for the information. Turns out I was able to troubleshoot the wiring with the diagrams sent. Normal operation of the the heater appears to be to place key in the 'on' position.
The relay is time/temperature sensitive and will hold the manifold heater on for a period of time if cold enough out. After the indicator light goes out, move key to the 'start' position and the rest is history.
The relay wiring harness checks out ok but the relay itself shows an overheated transistor. A new relay is about $60 Canadian and should fix the problem without modification. Again, Thanks to everyone for their input.
 

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