Ignition problem pt6a-34

[napalm747]

The other day I started an routing inspection on our pt6a-34. When I pulled the ignition cable off the LH plug in exciter box I found that the RH igniter was not sparking. So I plugged back in the ignition cable (after 10 minute wait for exciter to dissipate) and pulled the rh plug out from the exciter box and did the same. The LH igniter was sparking but seemed to do so erratically. I plugged them both in and checked again and the LH one fired (more consistantly I might add). So next I plugged the LH ignition cable into the RH plug on the exciter. No firing/sparking. I then plugged the RH ignition cable into the LH plug on the exciter box. No spark. This made me wonder what kind of condition my igniters were in so I pulled them both and found the RH igniter out of tolerance and actually worn a little into the cooling holes. This led me to believe that the RH igniter was bad. So I put the LH igniter (the one still sparking) in the RH position and vise-versa with the other ignitor. I then repeated the tests by removing the LH cable from the exciter box and seeing if the RH igniter would fire with the good igniter in that location. It did not. This told me either the ignition cable or exciter box was bad. I then did the same for the LH side with the bad igniter and no surprise it wouldn't fire. It appears to me that I not only have a bad igniter but also a bad exciter box. I am new to pt6a maintenance and just want to throw this out there to see if anyone with more experience has any advise for me before I buy another exciter box and new igniter. Can a worn/bad igniter cause an exciter box to fail?

[Beech_Boy]

I wouldn't have thought a bad ignitor would cause the box to fail, it's just one of those things I guess. Let us know how you get on.

[spanner 56]

I agree with Beech Boy.......faulty plug shouldn't cause any grief to exicter box.
Only 3 things can go wrong with that system....plug...chafed cable or exicter box itself. Have come accross alot of faulty cables in my time.

[vanjast]

It's decades since I've worked on a/c, but electrical/electronics doesn't change that much. A question

1) Is the exciter box electronic, or electro-mechanical ?

Ignition system are high voltage and require decent insulation. Heat and liquid or vapour are not good. As mentioned chaffing is also a problem.

Disconnecting the plugs (or pulling the cable off the plug) to check for spark can cause breakdowns as the generated spark voltage can be higher than it would be under designed operating conditions - This will send high voltage spikes back into the ignition system, causing damage.

Under these conditions, your best piece of test equipment .. is your nose.
High voltage/current carbon burns stink... sniff each ignition cable - the whole length of it (insulation breakdowns happen at a specific point) and sniff away at the ignition/exciter box... you'll smell burnt carbon/silicon here too.

Carry on sniffing the whole ignition path.. plugs included.

:)
Edt: Silly me, I forgot to mention the most important aspect - Safety (which you'll probably know anyway) = make sure the whole system is switched off or battery disconnected before you sniff away, otherwise you might have your brains 'carbonated'.

[stankou]

hi

As mentionned above, never heard about a bad plug causing damages on the ignition box.

Did you check the contact pin on the cable plug side? that pin can be worn out and cause erratic.
On what kind of airplane are you working on?

[Sevarg]

Going back to when I worked on 212's we had a fair bit of problems with the HE cables, the silicon would brake-up at the ends and allow the conductor to short to the screen. Easy to if the screen/plug is pushed back.
No where do you say you checked the supply to the RH box, don't forget basics, if you didn't check.