Question about oil furnace CAD cell

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Old 01-18-16, 07:38 PM
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Question about oil furnace CAD cell

I'm still having problems.The furnace pump runs normally (starts right away nice and quiet), the blower comes on then 10-15 minutes later the pump quits it trips the reset button.

I've checked just about everything I can. Fuel flow (seems good) , inline filter (changed) , pump strainer (cleaned) , nozzle (changed) , Honeywell primary control (changed) , pump primed (good flow of fuel comes out).

The only thing I haven't changed is the CAD cell and that's because the one they ordered in for me didn't come with the pigtail assembly and the cell won't fit my existing pigtail socket.

What I'm wondering is could it actually be the CAD cell? I know they can "wear out" but don't they either work or they don't? Can they produce the necessary Ohms to keep the furnace running for a while and gradually , the Ohms drop until the furnace locks out? (I'm only guessing here).

I'd love to get an oil tech out here but there simply aren't any (it's all propane or natural gas now) and any parts I need to order have to be ordered in so I'm flying by the seat of my pants and learning as I go.
 
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Old 01-18-16, 10:22 PM
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Can they produce the necessary Ohms to keep the furnace running for a while and gradually , the Ohms drop until the furnace locks out?
Absolutely. A dirty cell lens can also cause a problem.

If you have an ohmmeter you can check the cell out of the equipment. Place the cell flat on a table (all light blocked) and it should show a very high resistance to open circuit. When you expose it to bright light it should be lower than 500 ohms or so.

It's actually more effective to check it in place. Disconnect the two cells wires from the relay(FF). Clip you meter onto the two leads. It should be extremely high resistance. Start the burner. I use a 500 ohm resistor but you can short out the FF terminals to keep the burner running. You should read 700 ohms or lower. Anything over around 1000 ohms is a problem.
 
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Old 01-19-16, 01:32 PM
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Pete's checkout is spot on. One thing I might add is to not leave the furnace with the jumper across F-F in place. Hook up the meter, fire the burner, install the jumper, then watch & wait. Sometimes the resistance will slowly creep up during a run. You just have to pull up a chair or sit on a bucket & watch the ohmmeter the whole time during the run. Let us know the results.
 
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Old 01-24-16, 10:53 AM
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Just an update..as I really don't know what I'm doing and it's all trial and error , I was nervous about letting the thing "run free" without any safety net (ie CAD cell). Better the cell lock it out than let it fill the bowel with oil eh??

Anyway , I had a new CAD cell (same part number but for some odd reason it doesn't fit the pigtail..loose and wiggles in it) and I checked the Ohm readings on both (the old one and the new one) out of the furnace. I covered with a hat to simulate darkness and took a reading then shone a pen cell flashlight down (but no directly into) the cells. Both readings on the two cells were very similar so I am assuming it's not the CAD cell.

Ohm meter set to 20K (excuse my ignorance but I'm not really sure how to read an Ohm meter) both CAD's
No light - returned about 2.0 - 2.6
With light - returned about .04 to .06

Either way , I'm ordering a complete assembly (cell and socket). Not that expensive and worth a shot.

For the life of me though , reasoning indicates fuel feed problem more than anything else.The day we drained the main line and bled out the pump , the following morning it ran 3/4 hour , purring like a kitten , before locking out. Next day , after a reset , it ran it's usual 10-15 minutes then locked out. The rep at Suntec seemed to think the same way..the furnace is burning the fuel pump reservoir (and some in the line) then ..no flame..lock out , but the fuel seems to be flowing through the line OK (drained about 2 gal in under 2 minutes into a jerry can).

I'm going to take the pump apart again and recheck the screen (also going to order a replace it) and the elbow fitting (copper fuel line into pump).

I know the tank is filthy and really need cleaning or replacing but the inline filter isn't plugged and it's too damn cold to disassemble the lines to blow them out right now (-21C),

Guess I'll just keep fighting it..thankfully we have a good wood heater and electric space heaters so we're managing.

Stumped but be damned if I'm gonna let some dimwitted hunk of junk outwit me..
 
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Old 01-24-16, 11:57 AM
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Ohm meter set to 20K (excuse my ignorance but I'm not really sure how to read an Ohm meter) both CAD's
No light - returned about 2.0 - 2.6
With light - returned about .04 to .06
Just so you know in the future. You try to set the meter just over what you expect to read or start on a high resistance like 20k and work your way down. (Same thing for volts)

Your next scale down was probably 2k and then 200 ohms. You'd use the 2k setting.

Since you used 20k..... that would be 20,000 ohms. So .04 is actually 400 ohms and .06 is actually 600 ohms. Which means your measurements where good.
 
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Old 01-24-16, 12:42 PM
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Thanks for Ohm reading info.
 
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