I prefer vendors who stay in business by charging a reasonable amount for good work.

Mike C.

Shop B should charge 12 hours.

A discount to one customer is a penalty spread over the others.

Troubleshooting, by its very nature, is difficult to estimate, and it is always easy after the fact to say what they should have done. Hindsight is 20/20.

My goal is to tell the shop what to do to find the problem the quickest. This often means doing an experiment or two on my own first to narrow down the choices.

Diagnosis capability is severely lacking in most shops. The most common strategy is to replace the most expensive part and see if that fixes it, and repeat until it does fix it or the entire system is made of new parts. An involved owner can save huge sums of money by being able to direct these activities.

Mike C.

The trouble is the components being installed may be defective. Then the shop has to troubleshoot why it does not work and by eliminating the part just replaced it must be something else. Even though the part replaced first guess was the right one and it was a defective overhaul.

I just went through this with my Kingair Sperry Flight Director computer. Altitude hold failed.
Shop replaced FD computer with Duncan O/H unit first. It would capture just fine and then wander off when it seemed to be switching to the pressure transducers. Replaced air data computer with O/H unit. Same problems. so tried 4 different O/H air data computers. Wires traced etc. no changes.

I was 2 years without Altitude hold. Flew in cruise with the pitch wheel if I wanted the autopilot. Could not find a fix.

Finally rented a breakout box and had a different shop check it out. Found problems isolated to the FD computer.
I bought a different FD computer as removed off eBay and installed it.

Now it works great. But it was a long road to get to this point.
Warranty on the first FD computer box has long passed. But I do believe it never worked from the beginning. Shop guessed the right component first try but because it was defective the problem remained. Odds of getting another FD computer with the same problem seem low but there it was.

This type of thing happens very often. New or O/H component is defective. But how do you trouble shoot to determine this for sure before they will honor a warranty if it was being replaced to trouble shoot a problem to begin with? Especially since the shop is who typically recommends the vendor so they are basically responsible even though it is not their fault.

This is especially tricky if it is an engine or high dollar item.

Another example I have a O/H fuel control unit that overshoots or undershoots in torque. After noticing this I had the shop try doubling the springs on the cable. To see if it was cable slop or the FCU is defective. It is allowed to have two springs. No change on the flight home from this phase insp.
So now will have to see about the fuel control. Hopefully some warranty left.
Warranty does not cover remove and replacement, And cost to move the airplane... It's a bummer this happens so often.

My advice with legacy AP/FD issues is to bring it to Stevens in Smyrna, TN. Gary and Curtis see enough of these issues to know what is wrong without wasting time and effort trying to figure it out.