Hi Jeff
Do you have a model number of the heater? I'm assuming it is a Hydroflame if not what make is it?
I can give you some "basic" info that applies to many of the Hydroflame units. I can't tell for sure this fits your model from 1988 but it could be right on or close.
I'm assuming the fan turns on and off when you turn the T stat up and down, Correct?
Now to the heat itself.
Need to know some things about your 12 VDC. Are you running off a battery only or are you plugged into shore power using the converter and a battery, or a converter and no battery? Do you have a volt meter you can test the battery voltage?
Reason for asking this is there is a saftey in the furnace called a sail switch. This is sort of like a paddle switch hanging inside the blower housing that the wind power of the fan pushes the paddle and then makes the circuit to allow the ignigtion to lite and gas valve to come on. If you have low voltage the fan does not run as fast as normal and then the sail switch does not make leaving the fan blowing at a lower speed but no heat.
If your converter is not working right or sometimes is and sometimes is not the 12 VDC can fluctuate up and down and so can the sail switch by the fan speed.
With a volt meter check the battery voltage with no shore power plugged in meaning the converter is not on and do not turn on anything in the camper to draw battery power. The propane detector may be on but that is OK. So what is the battery voltage? If the converter was on up until your shut it down and then test the voltage it may be up in the 13 volt area as a surface charge exists boosting the voltage. If you are on just battery for a long time, 8 hours etc, then if the voltage is down in the 12.2 area that means you could be 1/2 discharged. If it read 12.6 or 12.7 then your battery is just about full if not fully charged. Again this is with a battery at rest and has not been on a converter or charge in the last 8 hrs.
When you plug the converter in it should be in the 13.25 area if your converter has a float mode or in the 13.6 area if it is charging the battery. If you have much any thing in voltage above 12.5 volts you should have enough power to run the fan fast enough to make the sail switch. Now it will work at some lower voltage but again, if the converter is working and on you should be well above 12.5 and in the 13 volt plus range.
Next possibility that may line up with part of your comments with it is not cycling right. If the battery voltage is OK then do you have a heat duct or 2 that are blocked or the air intake blocked in the camper? In side the unit there is a temperature cut off switch (high limit) on the main heat chamber. When the heat chamber gets hot enough it opens up the temp cut off switch and shuts the gas off. If the heat ducts are blocked the unit can over heat and then not cycle very well as it is going into overheat too quick. This will also happen if the outside air inlet or exhaust duct is partially plugged.
Not knowing your model and if it has the diagnostics board of the newer ones, the newer ones have an LED on the main control board that can give some ideas to what is not working right.
You can see here on the newer models. If you take the grill cover off outside, you will see the motor compartment and a circuit board. This one is one of mine and it is what it looks like with the cover off. Yours I'm sure is not this new or may not even look at all like this. But if you have a circuit board I would look here behind the outdoor grill.
If it has a circuit board sensing system it may have a decal to count the number of blinks on the board LED to tell that fault code it is throwing.
I would start with the battery voltage 1st and make sure you have not blocked any inside or outside heat ducts. If the outside exhaust/intake is partially plugged the unit can overheat and trip the high limit switch as well.
Hope this helps and let us know some more info to better point you in the right direction of where to go hunting.
John
PS, do you hear the igniter clicking but not firing the flame? It is hard to hear over the blower but if you listen real close you can hear it when it is clicking.
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