coleman evcon will not light off

[tmiller011]

coleman evcon 70 gas furnace- model #790e766..I realize this is a very old unit. It has been working great till this winter. It will not light off..
facts: ignitor lights up.(electric type)...ignitor box replaced last year...replaced ignitor ...replaced gas valve....getting 24+ volts at gas valve for about 3 seconds, then goes to zero.. ..transformer is working. ...induction fan works..checked all terminals and wires. i had a tech rep. out and he is puzzled. this problem was intermittent , but now seems like a permanant break...im pretty savy on troubleshooting , but im totally baffled ..any help would be appreciated..im cold! the sequence works ok, but will not lite off.

[Robert]

Hi,

Need complete model number for furnace, recheck and give missing digits/letters.

Need to trace back from gas valve to next component and determine where the 24vac is being lost.



Thanks,
Robert

[tmiller011]

Thanks robert for the quick reply. I rechecked the model #.. the number is correct. there are no more letters etc. i pulled it right off the label on the unit. I just think its really old.. If i check the terminals at the ignitor control box and i lose the 24 volts there, can i assume its the box, or is there something that would cause the box to stop sending the 24 volts.

[Robert]

Hi,

There is letters missing and yes it is an old furnace. I have the database to back beyond the furnace though.

All the E766 furnaces have four letters in front of the E and all begin with a 2.

For example: 2970E766


Will need this to know what is there and see what can cause this.

Each component should have voltage and through it.

When you find it entering and not leaving, you will have found the bad component.



Thanks,
Robert