Firstly, my thanks to all responders for your experiences and knowledge of diesel heaters but even more important, are my apologies for not getting back to you sooner. It's not been an easy course you've set for me.
Allow me to present a few more details the day the Espar died. Where's the problem ...... stat or burner ? Flip a coin, take your chances. You can only be wrong 50% of the time.
A week ago I was in the process of re-locating the stat as it had always had poor responce due to (most likely) being in a location with poor air movement and close the hull which is covered by a wooden veneer liner as shown below :
I was in the process of moving the stat to a more open area and well away from other sources of temperature error. I took my time and spent a week installing 8 ft. of additional cable using a junction box :
Nice and neat (lucky) as only one original colour from the Espar harness changed ie. white to yellow.
Just to be sure, I took a picture of the original wiring connections by Roton Industries in Vancouver which worked well for some 26 years :
The stat part number (PN 30100132) is the closest guess I could make with Espar's infinite selection of stats all separated by a vast array of part numbers and the last 2 digits are the only ones to change. Physically, all of the stat exteriors with PN 301001 _ _ look the same and they never tell you what the difference is between stats. All I can say is that mine is the simplest one they have to control a three level burner and the stat has no means of self analysis.
Once the new wiring was in place and the thermostat was relocated to its new location, the thermostat was wired according to the original photo. The single wire to the (+) battery post was once again connected and the heater turned on. The battery was fully charged at this time. The heater went through its startup as it always had before. I let the boat heat up for about an hour as it had cooled off while the thermostat was being installed (and it was freezing outside). After about an hour, I noticed the boat was getting warmer than need be and I turned the thermostat set point down. The burner stayed on high. Again I turned the set point down. The burner stayed on high. Finally I turned the set point down as low as it would go, but the burner output did not decrease from high by the sound of the fan and the ticking of the fuel pump. An IR gun showed the temperature of the thermostat body to be around 65°F and a wall mounted thermometer in the area showed the same. Just to be sure of what I was seeing, I directed a hair dryer at the thermostat until the body showed 80°F without any decrease in the output of the heater. At that point, I manually switched OFF the thermostat using the OFF/ON switch. After that shutdown, there was no sound or amperage draw when the ON switch was turned on at the stat.
The wiring to the stat and the purpose of each contact is shown below :
The figures on the left side are voltage readings from the battery connection (5=POWER) to the remaining terminals when the stat switch is turned OFF and below when the switch is ON. The voltages seen when the switch was in the OFF position are due to the battery charger being on and at the float stage. This terminal check was not done in relation to any of the above tests.
_______________________________________________
In the 26 years since I bought the boat, I have never allowed an outsider to do any work on the boat. From renewing anchor lights on the mast, to all engine maintenance, plumbing, electrical work, to bottom painting and all the grunting that goes with it (that and swearing, I really do quite well.) This was going to be a hard pill to swallow if I had to bring in someone to do it for me.
After posting my first post, early on I realized I was not going to get too far until I knew the exact model number of the burner unit. That, I was told, is on a sticker located towards one end of the heater. Unfortunately, I have always known that getting up close to this beast was going to be my Waterloo due to its location as shown below :
There are those that say this lazarette was the original "Black Hole of Calcutta" (BHOC). Stick your head in there and you too will believe. The interior is crowded with hoses, wiring, and rudder columns of every shape and dimension imaginable. This is what you're met with inside :
I was solo on this trip as all the members of my boat crew were off attending other social activities on this day. Seems that whenever I need help, these people are always tied up with every sort of social undertaking imaginable ............. births, deaths, weddings, divorces, funerals, Bar Mitzvahs, you name it, they're going to one. One couple was married twice in the same year and then once a year later, another one has been to a great aunt's funeral three times (same great aunt every time), and another attends their nephew's Bar Mitzvah every year (same nephew). I can't keep track.
Seeing as how I was not about to enter the BHOC by myself, I used my cell phone with the video set to 4K and 60 frames/second. Stuck it on the end of a selfie stick and jammed it into the BHOC. With my usual luck, I was not able to get close enough to the heater for a good shot. I moved very slowly at suspected label locations but was not able to get near it at the correct angle due to everything getting in the way. This one frame was as good as it got in the wet snow mixed with rain :
Just not able to get the camera high enough due to the obstructions. Also, not being able to
spreken ze deutsch (or English for that matter on a bad day) I can't see if there's a field for a model number elsewhere on the label. Lot's of junk to remove before I can even think about venturing down into no man's land.
___________________________________________
@Windpilot No. My stat is old enough that it doesn't have any form of self analysis. The glow plug sounds like a good candidate for an "experimental replacement" seeing as it's relatively cheap. Thanks for that idea.
@Maine Sail No I have never cleaned the heater in 26 years. Early on I decided to adhere to that old idiot idiom that "If it ain't broke, don't fix it." This was only because of the lack of any idea how the heell to get to the burner. Once it craps out, I'll deal with it then. Well, it's crapped out now and it's time to pay the piper. You've seen how well the attempt to look at the model number worked out. Unless you see another number on the label that is the model number and the number I am not quite able to see only gives a general group number ie. D3LCC.
Put a clamp meter on heater B+ and let me know max current during start up..
I do not have a DC clamp meter and my other meters would not be able to handle the IIRC current of ~ 15 - 20A for the glow plug. Looking at my Link 10 battery monitor with no other DC loads on the 12 VDC wiring in addition to the battery charger being turned off, the Link 10 amperage readout started at 0.0A and did not show any change from 0.0A for the one minute the OFF/ON switch was ON at the the stat :
@jssailem I checked all of the initial checks such as enough fuel, no rags in the heating air intake, no rags in the combustion air intake etc, etc, etc. Nothing obvious to the eye.
@marcham
a) At this time I can't get in there. When Husqvarna has their annual chain saw sale, then we'll see who has the last laugh.
b) The heater has its own fuel line from the tank.
c) The fuel pump DOES NOT tick since it died. Otherwise a good healthy tick before. There is no leaked fuel under the burner.
d) No fan starts since the unit died.
e) Absolutely no movement in the heater.
f) All air flow tubes are clear.
g) is it possible it was switched off without allowing for the cool down cycle?
Yes, very possible. The unit would not shut down by turning the stat wheel to absolute minimum and it was getting warm in the cabin. I finally shut it down using the OFF button on the ON/OFF switch. I did not check the sound of the heater or the Link 10 ammeter to see if there was any cool down operation taking place in the heater. You can always see what is happening in the heater on the Link 10 ammeter.
@Kings Gambit The stat has always had poor control due to it being in an area of stagnant air and close to the hull which is covered with a layer of 1/8" teak veneer plywood.
It blows plenty of hot air, but it does not regulate cabin temperature as it once did. I do it manually by switching the unit on and off, etc., as needed.
In its last hoorah before mine died, the heater would not cut back via the stat so I had to shut it OFF at the OFF/ON switch on the stat. But unlike yours, it never started again with the switch in spite of the burnt offerings I placed around it.
There is a separate wire that attaches directly to the battery. If it corrodes through or falls off, the unit will not start at the thermoswitch. I've had to reattach mine a couple of times.
That's the wire I disconnected to de-energize the heater unit while doing the wire connections. It looks all new and pristine under a coating of SuperLube from several decades ago.
@Hello Below Thanks, I do have your installation manual for the D3LC and many more different D3LC installation manuals as well. Each one has another little nugget (but only one) in it so you need the entire bloody collection of manuals to get all the tips and tricks for the D3LC heater. I always thought these people were so bloody organized with their train schedules and brewing of beer. How did Espar go so, so wrong with their manuals ?
I do have an Espar 30100135 "Stat" that if it is of any use to you at all is yours for the taking.
I see your stat has a few last digits off from mine, yours ending in 35 and mine ending in 32. You've got to wonder what the difference is. This is a hidden nugget which I found in one of their manuals :
Thanks for the offer of your old stat. If it's compatible with my burner, I may jump a ferry to come over and see it. Got any beer

?
@east coaster Unfortunately, my heater is an after market installation (not by Espar) so no green light to give me a code as to what's screwed up. Sorry I can't show the quote since I have maxed out my allowance for 10 pictures.
Thanks for the address to the Espar library. I think I have what they are showing but there is little for the D3L which is my older model. I agree that the servicing looks a little challenging but that's what keeps me out of the beer parlors during the daytime. I'll try overhauling it and although it may wind up in the dust bin, that's the fun of boat maintenance in the winter. Yelling, screaming, and bashed knuckles.
Hopefully this added information may may help to bring out the gem of information I need to tell who's at fault here, the stat or the burner ? Thanks again to all contributors. Please keep the ideas coming.