Slow cranking results found

[74-07]

Read up on all of the other threads. RV-8, 320 hours, IO360 200 hp., one blade and stop then, next try would crank. Used all the info here and once again, like many others, found starter contactor burned and pitted. Replacing both contactors. I do have one question. The airplane has EXP2 Bus system. Starter contactor has diode but master does not. Any reason they both shouldn't have a diode?

[vic syracuse]

Put a diode on both solenoids.

[74-07]

Thanks Vic. Got them ordered.

[JonJay]

How many hours?
Where was it pitted and burned?
The “contact” is a disk that should rotate freely. Near as I can tell, the intent is for vibration to allow that disk to randomly move allowing a “fresh” surface to contact the terminals.
I inspected mine at about 500 hours and the disk was fine. A few small arc traces and very minor discoloration in a couple spots but not even close to failure. No pitting.

The original Vans electrical diagrams do not call out a diode on the Master Contactor but I agree with Vic. So does Aeroelectric.

[snopercod]

Diodes across the coils are necessary, but you have to realize that they only protect the switches activating the solenoids. I installed the diodes to protect the starter switch and master switch, but I really wanted to protect the starter contacts. So I installed two large 18V DC MOVs across the starter solenoid contacts, from one contact to the center plunger, and from there to the other contact.

[74-07]

Jonjay,

The disc was pretty severly burned/pitted. I would post photos but I'm just photo challenged on this site. Also, interestingly, the disc apeared to have been moving but not 360 degrees as the pits were each in about a 100 degree arc. One contact was burned/pitted on one corner. I suspect the contact was rotated during aircraft assembly and was only touching the disc on that corner. 320 hours

[Ironflight]

Since the topic hasn’t been discussed here for several years, I’ll bring it up again. It is important that when you tighten the nuts to attach wires to the big studs on a contractor, you back up (hold) the inner nut while tightening the outer one. If you don’t you end up rotating the stud slightly in the case. The stud is just a square headed bolt and the bolt head is what the contact disk touches to complete the circuit. If this twists even slightly, the disk will only make contact on the corner - a much smaller area - and this will cause pitting, scorching, and decreased conductivity. If you keep the bolt/stud in the correct position, you will have much longer life, and better performance.

Paul

[JonJay]

Originally Posted by 74-07

Jonjay,

The disc was pretty severly burned/pitted. I would post photos but I'm just photo challenged on this site. Also, interestingly, the disc apeared to have been moving but not 360 degrees as the pits were each in about a 100 degree arc. One contact was burned/pitted on one corner. I suspect the contact was rotated during aircraft assembly and was only touching the disc on that corner. 320 hours

I bet you’re right about the contact rotating. That is all too common as you can’t get a normal wrench on the post to hold it while you torque down the nut. Many just crank it down without realizing the post turned, which rotates the internal contact. Then you have a sharp edge with little surface area making contact with the disk. Just a matter of time to failure.

(Paul was typing while I was!)

[Kyle Boatright]

Originally Posted by Ironflight

Since the topic hasn?t been discussed here for several years, I?ll bring it up again. It is important that when you tighten the nuts to attach wires to the big studs on a contractor, you back up (hold) the inner nut while tightening the outer one. If you don?t you end up rotating the stud slightly in the case. The stud is just a square headed bolt and the bolt head is what the contact disk touches to complete the circuit. If this twists even slightly, the disk will only make contact on the corner - a much smaller area - and this will cause pitting, scorching, and decreased conductivity. If you keep the bolt/stud in the correct position, you will have much longer life, and better performance.

Paul

One of the challenges I've found is that the inner nut is really hard to hold with conventional tools. The nut thickness is less than a typical nut and my wrenches are too fat to fit.

I'm sure I've solved this problem before, but don't remember what I did... (Maybe ignition wrenches??)

[JonJay]

Originally Posted by Kyle Boatright

One of the challenges I've found is that the inner nut is really hard to hold with conventional tools. The nut thickness is less than a typical nut and my wrenches are too fat to fit.

I'm sure I've solved this problem before, but don't remember what I did... (Maybe ignition wrenches??)

I ground down a few wrenches. 10mm for the control terminals and 14mm for the high amp. I think, Don?t hold me to that!
I keep all the cheap old wrenches I finding so if I need to weld, grind, bend, or whatever to make a special tool, I have the stock.