Failed RMP Wire

[druidjaidan]

No sooner do I get the broken through bolt replaced and another random thing breaks. I got the broken through bolt replaced this weekend (see other thread for that), did a nice long run up and then flew near the airport for a bit before finally heading out on a short flight to a nearby field for lunch. Everything was going perfect, engine working great. Feeling pretty confident that the through bolt failure didn't have any latent issues.

Then I head back from lunch. A couple minutes after departing my EDM 900's tach display starts oscillating wildly. At first I'm a bit panicked, did something go wrong? However, nothing sounded or felt wrong. If my RPM was acting as crazy as the instrument it would have been obvious. So I landed and....the RPM started working.

Pulled the cowl an started looking for the RPM sensor. Found the cord and traced it forward...and realized it was just completely snapped off. It must have just been touching, and just the air through the cowl was making it move around and sometimes make contact.

I'm having trouble trying to figure out what actually will need to be replaced and I like to know that before I call in the mechanic. Since the part number 420807 on the wire is the pressurized mag part to me it looks like someone cut the end off a pressurized mag JPI sensor (I don't have pressurized mags afaik either so not sure why that part) and then crimped another wire onto the end? This doesn't really look like https://www.aircraftspruce.com/pages/in/probesandsenders_zjpi/jpi10-05459-61.php. 

Sorry, It's a little hard to tell what's going on in these photos, but I'm holding the broken off spiced wire. I had trouble finding where exactly it broke off of tbh. I'm assuming one of my mag will have the end of the sensor in it.

 

 

 

Edited 13 hours ago by druidjaidan

[EricJ]

Glad you got the motor stuff sorted out.   Do you still have a mechanical tach, too, or just the JPI?

[druidjaidan]

Just the JPI.

[PT20J]

Here's the page for the JPI sensors. https://www.jpinstruments.com/shop/rpm-sensor-for-bendix-mag/

I would confirm what mag you have and then call JPI to confirm the correct rpm sensor part number. If you don't have the correct part, I'd get the correct one. If your part is correct, I'd just splice the wiring either by re-pinning the Molex connector if there is enough cable or using butt splices if not.

[rklems]

That picture sure looks like a pressurized mag to me? Isn't that a hose going in to the mag right there? So the 420807 sensor mounts underneath the calibrated leak orifice plug on the magneto. Most of the pictures that you'll see online linked to the 420807 are the wrong picture... It's more like a washer with a lead. I'll attach a picture below. It would likely be a pain to pull the orifice and change the sensor with the mag in place, though depending on the clocking of the mag, it might be possible.

[druidjaidan]

I suspect I'm going to find something very rigged, considering the whole thing I'm holding is a set of butt splices. I looked in my logs and the mag is 10-79020-18, the unpressurized mag it looks like. I suspect the wiring is all the same it's just sensor end that's different. I suspect that for whatever reason when they where installing the EDM they ordered the wrong sensor, then cut off the end and wired it up to an existing sensor from whatever was in there before I guess. 

I need to get back to the plane for a better look to be sure, I didn't know what I was looking for yesterday when I had it apart. I suspect I'll find something loose from the sensor, might just end up ordering a new one to clean this up instead of adding yet another butt splice.

Edited 2 hours ago by druidjaidan

[druidjaidan]

I'm going to check again on the mag. I don't see them being replaced in the logs with a pressurized version, and my engine is a GB from 79 so I don't think it was changed over to pressurized mags. But that sure does look like a hose going into the mag now that you point it out

And that sensor looks a lot what my wire and I saw something on the mag that looked like I imagine that does under the shrink tube. Maybe it just pulled out? I'd expect that to be crimped in I guess.

[rklems]

16 minutes ago, druidjaidan said:

I suspect I'm going to find something very rigged, considering the whole thing I'm holding is a set of butt splices. I looked in my logs and the mag is 10-79020-18, the unpressurized mag it looks like.

I'd definitely take a look at the data tag on the mag. The 10-79020-18 is an S6LN-25, which is an S-20 magneto, not an S-1200 mag (which is what the 420807 sensor is for). So that would definitely be the wrong sensor if that's the mag you have. If you have a pressurized S-20 series magneto, the 420806 is the proper PN per the 930 installation manual "For the pressurized Bendix -20, 200, series & S6LSC-25P and non-pressurized Slick 600 use JPI P/N 420806." There isn't enough in the picture to positively identify the mag for me, but it being unpainted, feels like an older style mag.

[LANCECASPER]

I would have thought any unpressurized mags on early 231s would have been changed out by now. That engine needs pressurized mags.