Heater won't light.

UPDATE:
It is fixed.
Since the sail switch and limit switch tested good, that left the igniter or control board. The control board was easier to do, so I ordered a new Dinosaur board.. I am very impressed with the quality of this part.
But it didn't fix it. But it did help me diagnosis it.
The green LED would not light up, but the fan would run. The Dinosaur instructions said that in this instance there would be something wrong in the sail/limit switch circuit.
So I retested the sail switch...It failed several times in a row. Then it passed several times, and then failed again. So I pried it apart and found that it was pretty much junk inside.
Got a new sail switch and all is well.
I was going to replace the original control board and sell the Dino board on Ebay to recoup some costs. But gave up on that as the wires were so short as to make reinstallation practically impossible. The Dino board has the connections on the other side, which allows just enough slack in the wires to make it doable.

Anyone wanna buy a used factory board cheap?:B

My 93 has two Hydroflame furnaces, both original, original control boards as well. I have replaced a burner assembly on one. I believe the weak link is the control boards. Start of every season I'll pull the boards, clean & re-install.

Our control board was the culprit. Similar symptoms as yours same furnace mod., I believe the cost was ~$200.

With the unit starting up, check for voltage across the gas solenoid, if you don't have voltage to open the valve, then you have either a terminal corrosion problem or the control board is bad. I had this problem and replaced the board for the fix. My unit is an Atwood, but the diagnostic would be the same.

The fan could be dirty and not producing enough air flow to close the switch on the sail switch.

Check the copper line to the furnace for a potential "oil trap" restricting gas flow.

The fan blower comes on so t-stat is working.....

Do you heard a 'clicking'......ignitor trying?

12V DC goes to module....when t-stat calls for heat power then goes to blower. When it comes up to speed sail switch closes and power goes thru sail switch and high temp limit switch and back to module. After a time delay (to purge) power from module goes to gas valve solenoids and at same to spark electrode (power to gas valves is only on for 6-8 secs) If no main flame and proof of main flame via signal sent back thru spark electrode then power shuts off

If no 'clicking' then power is not getting back to module (sail switch/high temp limit switch) OR power is not going to spark electrode OR spark electrode is grounded/out of alignment.
Could be high temp limit switch...located on chamber at the back.
Could be bad connection on module
Could be bad module

If you can access
Clean up module connector contact area----pull connector and use pencil eraser on contact area
Check for DC power out of sail-switch with blower running
Check for power at module on red (+) and yellow (-) with blower running

With furnace out
Check for continuity thru high temp limit switch
Check continuity of spark electrode high tension wire/cracked ceramic
Check spark electrode gap 1/8" and that it is positioned correctly

Random thoughts:
I would observe it while it tries to light. Do you see or here a spark? Do you smell gas? Could be a bad control board. Check for all connections to see if any are loose or dirty.

I hope so. Taking this thing out to work on it is a pain. And there is no way for me to bench test it. Gotta reinstall it to see if it's fixed.

Dang - That takes the easy stuff out of the equation. So now, I'm out of ideas. I'm sure somebody will be on here soon with some more helpful tips.