External BEC needed for RX8 high voltage/Mech brakes?

[Aceldama]

Originally Posted by BrianG

You might want to check/replace the servo. I had a Hitec steering servo that would operate fine on the bench, but in the vehicle, it would sort of jam sometimes. Inside the servo case, there are several gears, and they ride on "axles". Well, it turns out that one of the axle holes had elongated and was causing a bad mesh internally, but only under a decent load. Since it was a MG servo, I just replaced the servo case and it worked fine thereafter.

Not saying this is exactly the case here, I'm just saying it could be the servo itself for whatever reason. If the servo is jamming and/or stalling, the high current flow could be dragging down the BEC voltage to the point where it is causing odd behavior in the ESC brains.

I'll try to get a hold of a temporary replacement servo just to give it a test. I wonder if I could get away with a standard plastic gear servo from the LHS.

Edit:

I guess I could also try running a receiver battery to see what happens.

[RC-Monster Mike]

Running a receiver battery won't tell you if the servo is drawing excess current - the problem may seem to go away, as the load will no longer be on the ESC BEC, but the root cause may still exist. My guess is that the servo is drawing excess current, as mentioned a couple times in this thread - dropping in a good servo will fix the issue. Also mentioned in the thread - you get what you pay for with servos for the most part. You don't need a super powerful servo, either - there is no throttle return spring to fight. :)

[Aceldama]

Originally Posted by RC-Monster Mike

Running a receiver battery won't tell you if the servo is drawing excess current - the problem may seem to go away, as the load will no longer be on the ESC BEC, but the root cause may still exist. My guess is that the servo is drawing excess current, as mentioned a couple times in this thread - dropping in a good servo will fix the issue. Also mentioned in the thread - you get what you pay for with servos for the most part. You don't need a super powerful servo, either - there is no throttle return spring to fight. :)

Last night I ran one last test with the existing brake servo. I disconnected the linkage and it still drew enough to shut down the BEC like I'd been describing.

So I went ahead and swapped it out with a Traxxas standard servo I had laying around the shop. From a limited, midnight, shakedown run it seemed fixed but I'll try it today and really wind it out. I'll report back in a few hours. The servo was just drawing too much juice it seems.

[BrianG]

Maybe the steering servo is causing the problem? An unloaded servo shouldn't pull hardly anything unless it is faulty.

[wallot]

Some servos are extremely power hungry like Hitec or JR. Example:
Protec UBEC with warning LED that goes on when pack voltage goes bellow 6.2V
Fully charged 1500mah LiPo rx pack who knows what rating
Nitro buggy on a car stand
Servos at full brake and full steering (EPA set right )
With either 2 Hitec (one 56xx one 59xx) or 2 JR servos (9100T and S) the warning LED goes on so servos must draw quite a lot of Amps to drop from a fully charged to under 3.1V per cell.

Does not happen with Futaba Brushless servos

And since RX8 has "just" 5A bec it may be overloaded by power hungry Hitec servos causing issues.

So either run RX pack or get less power hungry servos like Futaba BLS line.

[Aceldama]

It seems to be working fine with the Traxxas standard servo in the brake slot. I guess my servo combo was drawing too much juice. Either the brake servo is defective or the combination draws too much.

Thanks for the suggestions guys. Wallot, I might upgrade to those Futaba servos over time, they sound really nice.

[BrianG]

I've had good luck with ACE servos. They are built tough, have good specs, and are relatively cheap. Some people say they wear out quick, but I haven't noticed that and I have them as steering servos in three vehicles.