Aqua-hot flame sensor question

[John Compton]

I’m trying to diagnose a no ignition problem on my Aqua-Hot 450 DE. I’ve run all of the diagnostic tests in the service manual. However with the ohm test of the flame sensor I can’t seem to get steady readings with my digital meter. The ohms do fluctuate when covering and uncovering the sensor. I understand that the purpose of the flame sensor is to shut the fuel off if it doesn’t sense a flame within 25 seconds. My question is, can a bad flame sensor prevent the initial ignition of the flame?

[Bill Lampkin]

The flame sensor, as I understand it, is a CDS cell, or photocell. A photocell varies its resistance with the amount of light it 'sees'. My guess is that the flame sensor is sensing ambient light and is changing its internal resistance as it should. Have you used the little tool to align the electrodes? New fuel nozzle and fuel filter? If yes, see if Rodger Berke can help-he runs an excellent forum for AH units.

http://forum.rvhydronicheaterrepair.com/activity.php

[John Compton]

Yes, I’ve aligned and cleaned the electrodes.  I’ve done a full service on it and confirmed that I have fuel. I tested the coil and it sparks. I was told by an Aqua-Hot service center that the coils either work or they don’t. So I assume the spark it produced is sufficient.

[David T. Richelderfer]

The following is from the Service Manual Rev. B for the Aquahot in our coach, an AHE-100-02S, page 54.

http://www.aquahot.com/files/service_manual/AHE-100-02S%20Service%20Manual%20Rev.%20B%209-27-2011.pdf

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"Flame Sensor:
The flame sensor is a photo resistive device which supplies the diesel-burner’s controller with a DC
voltage signal when it detects a flame. When the flame sensor senses the light of the flame, it will
shut the ignition coil off, and send a signal to the diesel burner controller indicating that everything is
working properly. If the flame sensor does not sense the flame, it will shut the diesel burner down
in a default after 20 seconds.

"NOTE: In order to perform the following procedures it is
necessary to detach and reattach the diesel-burner
head. Be sure to review the detaching/attaching
instructions.


"Component Test: Ohms

"NOTE: Be sure to calibrate your ohmmeter prior to checking the flame sensor to proper resistance values.

"1. In order to perform the following procedures it is necessary to detach the diesel-burner. Be sure to
review the detaching/attaching instructions in section 7.  2. Once the diesel burner has been removed, Locate and detach the flame sensor’s green and blue wires from the C-plug harness wires.
3. Connect the flame sensor’s green and blue wires to an ohmmeter.
4. Check resistance by placing a shop rag over the flame sensor’s glass surface, to simulate a no flame
condition, and then remove the rag and expose to a light source, to simulate a flame condition.
5. If the flame sensor is operating properly, the ohmmeter should register high resistance, over 100K Ω
when the glass surface is covered, and less than 200 Ω when exposed to a light source.
6. If these resistance numbers cannot be obtained, verify the flame sensor’s glass surface is clean. If the
flame sensor is clean and the resistance numbers cannot be obtained, the flame sensor must be replaced.

"Flame Sensor Maintenance:
1. For light dirt, dust, and/or carbon deposits, simply wipe off the flame sensor with a soft cloth.
2. Should heavy carbon deposits be present, completely remove the flame sensor from the photo disc and
clean with brake cleaner."

[Bill Lampkin]

voltage output, not resistance; yup!

[John Compton]

David, that’s the procedure I used. I just couldn’t get a steady reading with my digital meter.