I have a EDM 700 engine monitor and I was wondering if anyone has changed the #5 spark plug cht sensor over to the Maule gauge and installed a new probe to be used for the engine monitor. From removing the spark plug the jpi spark plug sensor has a short life span and now doesn’t work so was needing to replace it.

My 830 came with this adaptor:

At additional cost an adapter probe P/N MM-113-3/8 is available that permits
the factory CHT probe and a JPI probe to fit the same bayonet location.

It allows you to install the stock probe inside the adaptor probe.

So am I understanding that you can use the same probe for both the maule gauge and the JPI monitor, spliced together somehow. The part number you gave is for a spark plug sensor.

The JPI CHT gasket probe is the one that allows both the stock Maule CHT and JPI sensor to be connected to the same cylinder. They aren't spliced together. You unscrew the stock Maule CHT probe and install the JPI gasket probe under the stock Maule CHT probe. This will change the spacing of the stock Maule CHT probe and it may read slightly lower (25 deg likely) than it used to.

Perfect. Thanks Andy

Andy I have another question. Should I switch the sensors around and use the maule sensor on the EDM 700. Would it be more accurate than the other one? I presume they use the same K thermocouple wiring.

You can't switch them around. They work differently. JPI uses K-type sensors that are grounded and use different thermocouple materials (Nickel-Chromium / Nickel-Alumel). They probably have different temperature ranges, too.

Thanks Andy But since I’m on a roll here, another question. My alternator light is staying on but my EDM says it’s charging so I must have a connection issue somewhere. The voltage gauge is showing it’s not charging but as I said the EDM says 13.7 volts.

The usual cause of the alternator light staying on is a broken connection between the inline fuse holder from the alternator control unit and the alternator control terminal post. Older Maules, like mine, used a plastic inline fuse holder with a fuse and a crimped ring lug on the alternator field coil terminal post. The fuse holder is usually near the alternator. Vibration over time would cause the fuse holder to flex the wire connection on the ring lug and eventually break it. Without current flow from the alternator control unit through the alternator field coil, the alternator will not produce any output voltage to charge the battery. I assume that you have already tried to recycle the alternator switch and the light still stays on. The EDM doesn't sense the battery charging, it only senses the supply voltage on the 12-volt bus. The battery could be supplying the power to the 12-volt bus and the EDM will not know if the alternator is working. The ammeter on the standard Maule instrument cluster does sense whether or not the battery is charging since it measures the voltage drop across a shunt resistor caused by current flow. If that ammeter isn't showing a charge, then the battery isn't charging.

You can fix the vibration that causes the fuse holder to break the wire at the lug by tie wrapping the plastic barrel of the fuse holder to a wire bundle so it doesn't flex. Put the tie wrap on the back side of the fuse holder so you can replace the fuse when needed without cutting the tie wrap. Take the fuse out of the holder and measure it with an ohmmeter to make sure it hasn't blown. If the fuse is blown, that may indicate a problem with the alternator control unit or the alternator.

Thanks Andy, my project this afternoon, let you know if I find it

andy wrote: ↑

Mon Jun 21, 2021 7:27 am

The EDM doesn't sense the battery charging, it only senses the supply voltage on the 12-volt bus. The battery could be supplying the power to the 12-volt bus and the EDM will not know if the alternator is working. The ammeter on the standard Maule instrument cluster does sense whether or not the battery is charging since it measures the voltage drop across a shunt resistor caused by current flow. If that ammeter isn't showing a charge, then the battery isn't charging.

How would a 12V battery be supplying 13.7V continuously? Voltmeter is a very reliable way to see if an alternator is working.

Well nothing is that simple for me. Everything looked good. So started it back up and the amp gauge showed charging right off and the EDM said 13.7 but my light is still on. Black tape!!!!! At least I know it’s charging. With the alt switch off battery shows 12.7. Should of told you that before. I had tried it while running.

You're right asa, a 12 volt battery shouldn't produce 13.7 volts. It might measure 12.6 volts when freshly charged but not 13.7. It's possible that the alternator off warning light switch is bad. I think there's a small circuit board inside the switch module that senses low voltage on the bus and turns on the light. On my airplane that's Maule drawing 7128B Rev. A part A-00258-1 (14 volts).

Another possibility is that the red wire from the alternator off warning light module in the drawing to the alternator is broken or loose where it connects to the alternator. It's a good idea to check all of the wire connections on the alternator to make sure nothing is loose or broken before you start replacing components.

I've also had the red light always on while the old alternator had been charging nicely for hours and hours. Actually, I've never seen that light off since I have the plane.
At some point the old alternator broke and I replaced it together with the voltage regulator with more modern equipment. I installed the AL12-70 alternator and R1224 14/28V Voltage Regulator - both from Plane Power. By doing this, I basically bypassed the little circuitboard/InOp-Switch behind the panel that controls the red light and used the built in lamp port of the R1224. Now the lamp is off and only turns on when it should do so.
I'm very happy with this solution now - and I removed the black tape from the red lamp

Thanks again Andy and Chris. Sure sounds like a relay that’s faulty. I’ll look for a loose wire under there first. Off fishing in a couple days so a least I’m not worrying about a dead battery when I’m there. When I get it fixed I’ll post the cure