This month’s ShopTalk will discuss the confusing world of engine and fuel primary instrument replacement, along with the difficulties of obtaining repairs to these instruments and their associated senders/ probes. Ultimately, we will focus on the all‑in‑one instrument system, CHT, EGT, TIT, fuel level, manifold pressure, OAT, RPM, etc., as appropriate to the aircraft.

In the 1950s and 60s most of our fuel, oil pressure, oil temperature and CHT gauges were made by Stewart Warner. These gauges are coupled to specific senders and probes. In later years (70s and 80s) Stewart Warner got out of aircraft business and Rochester Gauges jumped in to manufacture new gauges with newer technology. Of course, their gauges and probes were not interchangeable with the Stewart Warner systems, so you had to replace the probe and the gauge as an assembly. A bit of a pain, but there was a choice when your old Stewart Warner system failed and could not be repaired or replaced. Now let’s think about this: There are more than 2,340 currently registered Mooneys over fifty years old; more than 3,540 older than forty years - over half the Mooney fleet of about 6,000 airplanes. How long do we really think these instruments will be available for replacement? Rochester stopped production over 20 years ago. Sigma-Tek made some of these instruments in the 90s but they are out of production for certified instruments also.

Here is the real problem: Because Mooney certified these instruments when your M20A-TN was produced you can’t just go to Sigma-Tek (or any other manufacturer) to purchase a new one even if it were available and being manufactured! You see, Mooney, not the instrument manufacturer, holds the approval to install that instrument in your airplane. The instrument has to come from Mooney with their return-to-service tag or stamp.

Another problem encountered when your primary instrument(s) fail is not only the inability to get a replacement but the lack of instrument shops that can legally repair that instrument and properly return it to service (FAA 8130 tag). Most manufacturers of these instruments will not allow the design data to be released to the repair shops. Without the FAA approved data, an instrument shop cannot legally repair your primary instrument.

Just because the item is out of production does not mean the manufacturer wants its part to be repaired and kept in working condition for another twenty years. Some of these companies are still in business making other items but many are no longer around. Try finding a B&D electric tachometer for your 252 or TLS. If you do find one, the price could be so high as to force you to consider other options for an FAA approved replacement.

I think the first thing an informed buyer should understand is the term, primary instrument. A primary instrument is the temperature or quantity instrument your airplane came with fresh out of the Mooney factory. When it received its type certificate from the FAA, all installed and optional equipment were the basis for that approval. This is unlike experimental aircraft that have no FAA type certificate, where anything may be used for temperature and quantity measurements. FAA certified aircraft are locked in to approved factory configurations unless there is Supplemental Type Certificate (STC) available. This rule is applicable for almost all configuration changes.

It used to be we would see plenty of the Stewart Warner instruments failing as they pass 40 or more years old but now we are seeing the Rochester and B&D instruments failing and this begs the question of do we change them all or just one at a time as they fail. When we in the maintenance field do an annual, all of your primary instruments must work correctly and be range marked correctly. When I say range marked correctly, you can’t have a CHT gauge with a redline of 460 degrees (TCM engine) installed in your stock M20J (Lycoming engine) that should have a 500 degree redline.

It’s the oil temperature, oil pressure and CHT systems for which replacement parts are difficult to find. This may mean you purchase an STC FAA approved replacement specific for your model and serial number aircraft. Today there are many manufacturers that make these FAA approved instruments; just ten years ago there were only a few.

Arguably, the latest and greatest idea for primary instrument replacement is the all‑in‑one engine instrument display which may also include fuel quantity gauges. There are a handful of manufacturers to choose from, but buyer beware as not all of these displays are primary approved. A good example is the JPI 830 from JP Instruments (JPI) (jpinstruments.com) . It looks just like the JPI 900 and is a great unit but not FAA approved for primary replacement. The JPI 900 and 930 are FAA approved as direct replacement for your primary gauges. Also, Electronics International (EI) (buy-ei.com) has an all‑in‑one FAA approved engine instruments that includes fuel quantity and Horizon Instruments (horizoninstruments.com) makes an approved tach. Both manufacturers have been in the primary instrument business for well over 20 years now.

Currently, in our shop, we are installing a JPI 900 in a 1970 Mooney M20E (see Figure). Because the JPI 900 comes with all of its own transducers for oil pressure/ temperature, CHT and EGT, rpm and manifold pressure it’s just a matter of locating transducers and routing harnesses. The major problem with any of these replacement systems is interfacing the old original analog fuel quantity senders with the new digital display systems.

The original senders in most small aircraft were a resistance float-actuated device, essentially a rheostat. The Mooney senders have a resistance of 0-30 ohms (30 being the full/up position). Over years of operation the wiper or resistance coil may have worn out or corroded in some areas creating flat spots that may impede the float’s motion or present a high resistance. Aircraft that have long sat idle are very susceptible to this problem. For a fuel gauge to correctly interpret the correct fuel level the sender must read in a linear manner.

Here is the lesson learned at KNR: If the plane is over 40 years old or if there are erratic fuel level readings, pull all the senders, clean them, have them overhauled or, worst case, get new or serviceable replacements (some of these senders are also getting hard to find).

To calibrate a new fuel quantity system the aircraft must be completely defueled. All fuel is drained through the tank sump drains with the airplane in the parked position. Add in unusable fuel with the aircraft in flight-level attitude. Calibrate (to empty), record the fuel sender reading and continue adding fuel (say five gallons at a time) keeping close track of added amount. Chart these individual readings as you continue to add fuel. This chart will provide checkpoints at each new fuel level all the way up to topped‑off tanks. The new digital fuel‑level instrument can interpret these know quantities and provide a fairly accurate fuel level reading in level flight. This part of the job will take all day, even if your fuel senders are serviceable.

When you order a primary replacement all‑in‑one unit they are pre‑programmed to your specific airplane. For example, if you have a 252 converted to a Rocket, the manufacturer has to program the unit for the Rocket engine parameters not 252 parameters. Therefore you must be precise when ordering the unit. This also applies to auxiliary fuel tanks; the manufacturer must know the total usable fuel for each tank. The fuel calibration must still be specific.

Be sure you have enough instrument panel open space for an all‑in‑one unit as it can take up most of a radio rack, as in the case of the JPI 930. Also, it must be installed within a specific field of vision from the pilot’s view. Additionally, relocating a bunch of radios will add to the installation price.

So what should one expect to pay to have one of these all‑in‑one primary replacement units installed? We have found it is typical to blow through four or five full days by the time you mount the instrument, remove or relocate items that are in the way, route all the harness, install all the transducers (EGT/ CHT/ TIT, oil temp and pressure probes, tachometer signal pick up, manifold pressure sensor), calibrate the fuel system and possibly interface to the GPS system. You do the math.

These all‑in‑one systems are way cool but complex to install. Often there is inadequate instrument panel space available and you have to consider spending a whole lot more money replacing old obsolete radios to create that space, a complete instrument panel/avionics upgrade.

If all‑in‑one is too cost prohibitive, several manufacturers make individual STC primary instruments. Today there are a lot of innovative engine instrument options when original instruments fail and you can’t find anyone to repair them. However, not all are FAA approved for primary replacement for your FAA type certificated aircraft: make, model and sometimes serial number - buyer beware! Some (but not all) manufacturers with STC instruments are: EI, Aerospace Logic (aerospacelogic.com), Alcor (alcorinc.com), Insight (insightavionics.com), Horizon tachometers (horizoninstruments.com).

If you have questions about this or any other ShopTalk article, please e-mail me at shoptalk＠knr-inc.com or call me at my aircraft repair shop, 307-789-6866. All ShopTalk articles can be read here at www.knr‑inc.com. Until next time, enjoy flying your Mooney.