Wiring Diagram for RobertShaw 780-745

[2BKing]

I recently replaced the ignition module in my 1991 Lennox furnace with the RobertShaw model#780-745 from the older SP735L. This is supposed to be a direct replacement. I switched all the old wires up to the similar terminals on the new module.

I can't get the furnace to ignite. I can hear the electric pilot clicking but the furnace won't come on.

I was wondering if there is a diagram or manual to hook the ignition modual up properly. I've done an internet search and come up empty handed. I can't figure out what I've done wrong and it's really cold in Toronto Canada right now. Any help would be greatly appreciated.

[16x80]

2BKing

Welcome to the Home Repair Forums

It maybe that you have it hooked up right and that you have another problem also. In a unit like that there are safety switches that could be bad not allowing your gas valve to open! You can check to see if you get power to the gas valve with a volt meter. When you call for heat at the stat you then put your meter leads on the two wires to the valve when it starts clicking you should get 24v at the valve! If not follow the wires back to the board you will find that one of them run thru some limits, Check for continuity on those limit's (with the wires off or you'll get bleed thru)You need to check the wiring diagram on the furnace to see if the switch is normally open or normally closed for proper a reading!

Check it out and also you can check www.robertshaw.com for contact info if you still believe you have mis-wired the board!

When all else fails call the pro's

Good Luck

16x80

[2BKing]

Thanks for your help 16x80.

I was unable to find any diagrams or manuals for the Robertshaw Module or for my Lennox furnace. They seem to be very secretly gaurded

However, I figured out what I did wrong and now I'm nice and warm.[^]

Again, thank you

[need-a-fix]

2BKing,
since figured out what your problem was, can you post it here? I have similar problem.
Thanks

[hopeful]

I've still got the Robertshaw SP735L. I can still turn the heat on at the thermostat by turning the heat off then back on. But the heat sometimes stops before reaching the set temperature, and then when it tries to restart, I get the ignitor clicks, but no flame.