{"id":55,"date":"2026-08-05T18:25:44","date_gmt":"2026-08-05T18:25:44","guid":{"rendered":"https:\/\/flyrc.info\/?p=55"},"modified":"2026-08-05T18:28:04","modified_gmt":"2026-08-05T18:28:04","slug":"pet-precision-extension-tester-rc-tools-2","status":"publish","type":"post","link":"https:\/\/flyrc.info\/?p=55","title":{"rendered":"PET – Precision Extension Tester (RC Tools #2)"},"content":{"rendered":"\n

After writing up my impressions on the Servo Signal Display, I realized I had a few other goodies and gadgets around the shop that I could spend some time on as well. So next up, lets talk about the Precision Extension Tester.<\/p>\n\n\n\n

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The Precision Extension Tester (PET) is a tool used to measure the resistance of each lead in a servo extension wire. <\/p>\n\n\n\n

Extensions are one of those things we don’t think much about most of the time. You install servos and buy an extension lead with enough length to get the end to the receiver and your good. At least in theory. In fact, those little wires can be a source of issues, and you probably will never know they are an issue unless something fails in a way and at a time where you can actually be sure what caused it.<\/p>\n\n\n\n

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I had a 50cc aircraft that definitely went in due to a servo lead failure (and stupidity on my part for not taking corrective action for what I knew was a part about to fail… a whole other story). For most folks, when a servo lead has problems… short of a full break or disconnect… they probably will never figure out what the issue was. This little tool can help spot some trouble before it becomes an in-flight problem. So here is the basic idea.<\/p>\n\n\n\n

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The PET will measure the resistance of each wire in a servo extension and display those on the LED screen. If one of the wires is not connected at all… it will show “Open”. Obviously, this indicates a loss of continuity in that lead, and you will want to either dispose of that servo extension or repair it. Otherwise, you will see a measurement for each wire. A quick note here. When you power up the device with the test leads connected to each other the device will run a calibration. Essentially it is measuring the resistance present in the connectors and wire of its own test leads. It can then exclude that from the measurement taken when you insert your servo lead you are testing. If the wires aren’t connected it will fail the calibration but will still measure any extension you subsequently insert. Calibration is NOT a must for the device to work.<\/p>\n\n\n\n

A properly functioning servo lead will have a very small amount of resistance. Typically in the 10s to low hundreds of milliohms. That’s end to end including the crimped pins and wire length. And that is the resistance measured in each of the three wires. If you see a lot of variances between the 3 measurements that is a problem. If it’s the signal lead (orange or white in most cases), the servo may have trouble understanding the positioning signal and fail to move properly. I would expect jitter would be a likely result. If the problem is on the positive or negative lead the servo may have trouble pulling enough current when it is needed under load and\/or could see a voltage drop that will cause it to lose torque\/speed etc… In any case, nothing good is going to happen.<\/p>\n\n\n\n

The actual value is typically of less importance (to my mind) than the comparison between the three. If you see one wire showing twice as much resistance on one lead than the other two, it stands to reason that lead is simply not making as good a connection compared to the other two and might need some attention but keep in mind these are very small units of measure so don’t be too quick to condemn all your extensions. In my own stash of servo extension wires I have seen some that (especially very short extensions) show very large differences lead to lead when measured after calibration and looked at as a percentage. I had one lead that showed 8mohm on one lead and 25 on another… that’s 300% difference! The reality however is that when you get down to such small measurements, it may not be quite what it seems. First of all, the calibration is measuring a slightly moving target. Every time you connect those leads you can get slight variances. Did you get the lead totally seated? Is there just a bit of corrosion on the leads? etc… This variance is then built into all subsequent measurements. For my money, I like to see the calibration happen when I power up just to see the meter is functioning but then I power back down, unplug the leads and go uncalibrated from that point on. Your mileage may vary.<\/p>\n\n\n\n

So, for the example given… 25 mohm is still a good connection. A truly bad connection if likely to show up as hundreds of milliohms not teens. So, you do need to tinker a bit and measure a number of leads to get your brain calibrated to what to expect and what is bad. Unfortunately, there is no red light\/green light indicator to say what is good or bad but after you measure a couple handfuls you will start to get a feel for when to worry. I also like to pull lightly and twist the leads a bit to see if things change drastically. I have found a few loose crimps this way that I can quickly solve by cutting off that end and crimping a new end in place. Crimps, when done right are claimed to be airtight and as good or better than soldered connections but that “when done right” phrase is where the issue lies. No person or machine is perfect. I would also encourage a few connect\/disconnect cycles as I have also seen that things often improve after a few insertion cycles. I would presume this is due to corrosion or dust on the connectors garnered from being stored in a drawer in the shop for weeks\/months\/years. I think I’m going to invest in a bit of contact cleaner and try to get in the habit of using it when I install an extension in a new airplane.<\/p>\n\n\n\n

I’ve also started using the PET to test any new or repaired extension I produce myself as well as testing all new extensions. New is just an alternate way to say “untested” so don’t assume new leads are perfect. Even the high-end manufacturers likely don’t do 100% quality control checks on items as inexpensive as a servo extension. Unfortunately, those inexpensive servo extensions can cost you a very expensive airplane!<\/p>\n\n\n\n

So far, my experience with the PET is a positive one. I found and fixed a few extensions laying around the shop and maybe that made an airplane fly better or longer somewhere down the road. It was not expensive and it gives me another tool toward ensuring my precious aircraft fly reliably and predictably. If you do much building\/repairing in the RC airplane world I think you’d find this tool to be useful. It’s not an everyday, carry it to the field with me type of tool, but I do like having in the toolbox in the shop when I need it. If you don’t have one, at least find a friend who does and get on his good side so when you need it….<\/p>\n\n\n\n

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After writing up my impressions on the Servo Signal Display, I realized I had a few other goodies and gadgets around the shop that I could spend some time on as well. So next up, lets talk about the Precision Extension Tester. The Precision Extension Tester (PET) is a tool used to measure the resistance of each lead in a servo extension wire. Extensions are one of those things we don’t think much about most of the time. You install servos and buy an extension lead with enough length to get the end to the receiver and your good. At […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-55","post","type-post","status-publish","format-standard","hentry","category-tools"],"_links":{"self":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/55","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=55"}],"version-history":[{"count":3,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/55\/revisions"}],"predecessor-version":[{"id":60,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/55\/revisions\/60"}],"wp:attachment":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=55"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=55"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=55"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}