Thorp Air Command - T18.net

While taxi testing my recently inspected T18 I noticed something strange that I hav never noticed before. Whenever I key the mic switch I notice a fairly noticeable negative deflection on my ammeter gauge. I realize my Icom 200 will draw a bit but does this seem normal or can someone recommend what to search for? Radio transmit fine though. Only other major problem is my Van's capacitive (sp) fuel sending unit doesn't give any reading and of course I didn't put an access cover for the sensor! duh!

What was the engine rpm at the time and what type of generation source do you have? A generator won't charge until the engine is around 1500 rpm, my alternator doesn't put out much charge at idle (500 rpm). Also, what other loads are on the system?

Don't know anything about capacitive fuel senders.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I think it was Rich who retroactively cut an access panel in their airplane to access the fuel sender. Whoever it was, I seem to recall they put a few photos up in this forum showing their work. I recall because I copied their work. You use a circle cutter to cut the skin out about the fuel sender, then install plate nuts on the under side of the skin so you can screw a cover plate back down. Pretty simple, but my biggest concern was determining the hole cutout center over the fuel sender while the tank was still installed. A few measurements solved that for me.

Do you think you could have a grounding issue with the radio? Electrical problems can be a real bear to figure out sometimes. Good luck.

Not sure on the PTT issue Bob but glad to hear you're in the taxi phase ... looking forward to seeing your bird! I had the same issue with the compatinance in N118BC ... convinced me to go back to using a float.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

The icom200 puts out 7W during transmit (much less while listening). I have one too. But I don’t recall ever watching the gage when I key the mike. Not sure if yours is sustained, but I wouldn’t be bothered if it just flicked the gage when you ptt.

Les Krumel
Albuquerque

How old it the battery ? Have you checked the output of the alternator ? I use the Westach capacitance fuel probe . No problems noticed with the probe except every few years I have to remove the "access cover" and tweek the E-F level screw slightly , then all is well with the world . If you can put in the access cover BEFORE mounting the fuel tank you are ahead of the game . If you're not that "Lucky" you can still put in a cover it just takes a bit longer .

RB

Thanks for the replies. Battery is new and checking all the grounds as well. Doing a little changes with the ammeter shunt but only was concerned because of the noticeable change while keying mic. Turning on strobes show a little normal flickering and the landing light shows a normal steady movement so I believe everything is ok. Doing all this at idle so will check when i get the rpm's up later. Fuel gauge checks out ok so now it's just a matter of troubleshooting the sender or replacing it if it has to come to that. I didn't put in a access cover but did install a removable floor which is what I'm working through now. May be able to tilt the tank back to get at the sender but if that's the option I may just bite the bullet and retrofit a cover. Sooo, my adavice to future builders is to incorporate the cover and the removable floor from the onset. After starting the project back in 1989 what's a few more days! lol.

Having electrical components "jump" when you key your mic button COULD be caused by faulty grounding, a condition called "ground looping". Here's what I mean...

The definition of ground looping is explained here https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29. If your components share a common ground, ground looping probably isn't the problem. If your affected components connect to separate grounding sources they may not have the same "grounding potential". When you key the mic to your ICOM creating a relatively large power demand (<3 amps but still valid) it could create a "ground loop" with other components.

I would start by visually and/or electrically tracing your PTT (shielded wire(s)) ground source, your ICOM main ground wire source, and your ammeter grounding source. If they don't share the same ground, you have some options. You can re-wire each ground to the same source and see if that fixes your problem. Before a re-wire, you could locate where each component ground connects to it's ground, disconnect them, securely attach them to one end of a jumper wire (alligator clipped wire, 18 AWG or so) and attach the other end to a solid ground. Power up and see if you've tamed your trons.

It could also be a simple as a loose ground, so taking an hour or two to check your connections could turn up something. I'm no expert but that's where I'd start the troubleshooting. Hope this helps...


Heat

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Paul "Heat" Baxter
N8JB

Problem with the PTT switch was indeed a ground issue. I had grounded the wire to the side of the radio tray originally but when I switched them to a common ground is corrected the large negative drop to a very minor/acceptable drop. Just have to sole my fuel sender unit issue and away we go!

You tron tamer you! Congrats!

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Paul "Heat" Baxter
N8JB

Fuel sensors are ground sensitive, too.

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Bob Highley
N711SH
SN 835
KLAL

Finally I get to contribute instead of just asking dumb questions! My fuel sender did nothing at all when I first got the plane. That's not true -- it always read Full. So I spent 140 buck$ + shipping and got this Westach sender from Spruce:
https://www.aircraftspruce.com/catalog/ ... elqty5.php
The 24 inch, obviously, not the 12: AP-Q24B3-D120-5V Capacitance fuel level sender 24" long, Spruce P/N 10-01262
You will need to cut a few inches off the bottom and may (or may not) need to "S" bend it a tad depending on exactly where your hole is. Follow all directions super carefully, especially when it comes to calibrating. "Short this to this and hold for 30 seconds while..." blah blah blah.
I can't BELIEVE how accurate it is. Naturally, you need a compatible gauge. Mine is the one in the photo attached, grainy as it is (zoomed way into a full panel photo). Probably a Westach, don't know the part number.

Between the gauge indication with the new sender, the EIS counting GPH, fuel burn calculations, and eyeballing the tank, everything agrees to within a few tenths of a gallon.
Good luck!

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Philip Mandel
N118BC
S/N 957
Beaverton OR

I initially calibrated my Westach capacitance sender before mounting it in the tank . I used a 5 gallon container filled with gas to get my full reading and MT was done...well MT . It has always been accurate (when full) to less than 1 gallon diff. between the gauge and the fuel truck .

RB