When using 3 VB921s for WS...

Jon k · Post by **Jon k** » Sun Jul 09, 2006 12:23 pm

When using 3 VB921s for wasted spark, I fused the +12v to the GM coil I am using. I have a 10Amp fuse inline with the 12v so as to add a little protection, but I've been starting the car maybe 10 times in the last few days I blew 2 10AMP fuses. It's always when cranking. It will hickup like its oging to start and then it won't - I get out and check the fuse and its blown. I swapped in a 20 AMP since I don't have any 15 AMPS and so far so good... but isn't the load of a VB921 theoretically 7.5A at max load? And thus, shouldn't 10 AMP be enough?

Jon k · Post by **Jon k** » Sun Jul 09, 2006 12:31 pm

Mike_Robert wrote:On a stalled start, all of the ign lines *may*, though not always, go high and turn on the 921s until the stall detect routine kicks in and shuts off the FP, etc. 7.5 * 3, there ya go. 10 amp won't like it!

-Mike

Ok sweet so I could prob put a 25 Amp in there to keep it happy? And in the event of a short or something, a 25 Amp fuse will definitely blow before significant damage to coils or MS happens yes?

PSIG · Post by **PSIG** » Sun Jul 09, 2006 1:21 pm

I prefer to fuse each coil with a 7.5A Slo-Blo fuse for max protection. The coils can draw that much each but normally only milliseconds at a time for a lower average current. Besides, this way they can be used as a diagnostic based on which coil(s) popped, and the engine may continue to limp on the remaining cylinders to the next expressway exit...

FWIW,
David

Post by **jsmcortina** » Sun Jul 09, 2006 2:11 pm

An easy way to blow a fuse is to drop the clutch (by accident) at idle when running MS1/Extra wheel decoder. The stall code isn't quick enough to stop all coils from charging. A better implementation would have a "dead man's handle" so that it would enter stall mode faster.

James

Jon k · Post by **Jon k** » Sun Jul 09, 2006 2:30 pm

jsmcortina wrote:An easy way to blow a fuse is to drop the clutch (by accident) at idle when running MS1/Extra wheel decoder. The stall code isn't quick enough to stop all coils from charging. A better implementation would have a "dead man's handle" so that it would enter stall mode faster.

James

I am thinking that the coils are blowing the fuse when my engine is not tunring off completely... the rpms drop to like 300 - 400 as you saw in my datalog due to my "run on" condition. This is probably what's blowing the coils, not cranking - that's my thought anyway - and then when I go to crank her up again the next time its already blown... sound about right?

I'd fuse each coil individually but they are GM 3 tower coils, there is only one common +12v and 3 individual grounds (3 coils, wasted spark).

PSIG · Post by **PSIG** » Sun Jul 09, 2006 3:39 pm

James, is this a big enough concern to run auto or manual reset circuit breakers on typical wheel decoder installs? Sounds like life could get pretty miserable under the right - or wrong - circumstances.

hobieboy · Post by **hobieboy** » Mon Jul 10, 2006 6:49 am

umm... so am I asking for trouble?

I'm running 6 VB921's (twin plug) and they are fused to a single 25A circuit.
Even if I increase that to 30A it is still far less than 6x7.5A.

Post by **jsmcortina** » Mon Jul 10, 2006 7:06 am

I've only managed it twice since last August and I was running a 10A fuse per coilpack (V8)
I would not be unduly alarmed, just take some spare fuses.

James