Voltage Drop under cranking on Coil wire

tightwad · Post by **tightwad** » Wed May 28, 2014 2:09 am

I just installed my MegaJolt with the Hard Rev Limiter board connected. I get a very weak spark. Checking the voltage shows just 8.4V under cranking, with a confirmed 10.7 at the 12V in under cranking. If I power the coil directly the engine fires. I traced the voltage drop all the way to the Rev Limiter Coil pin, so it isn't a wiring issue along the way.

Another issue I am seeing is a Key-On 9V PIP signal, instead of the expected 12V. I doubt the issues are related but wanted to put all the oddities out there that I could.

Can I use a relay to trigger the coil using the 8.4V signal (not optimal but getting desperate at this point)?

Any ideas? I really need the rev limiter...

Post by **brentp** » Wed May 28, 2014 10:12 pm

Hello,

Base-line: Did you get the Hard rev limiter with the board installed by us, or did you connect / install it yourself? If the latter, could you post pictures of how you have it wired?

You are correct, the PIP voltage you are seeing is likely un-releated.

Thank you,

tightwad · Post by **tightwad** » Thu May 29, 2014 2:52 am

I am a moron. I went to take a picture and discovered I had the ground and trigger lines swapped, not sure how I did that or how I missed it. Oddly enough now I get bizzare voltage readings under cranking but the motor fires. One step closer!

tightwad · Post by **tightwad** » Mon Jun 02, 2014 1:59 pm

tightwad wrote:I am a moron. I went to take a picture and discovered I had the ground and trigger lines swapped, not sure how I did that or how I missed it. Oddly enough now I get bizzare voltage readings under cranking but the motor fires. One step closer!

I spoke too soon. Once again I am seeing around 8.4 volts under cranking. I had to bypass the Rev Limiter in order to proceed with testing. I did notice that the RPM reading I get when the motor is not running isn't zero, instead its a very large number that naturally exceeds my set Rev limit, which might be the problem?

Post by **brentp** » Mon Jun 02, 2014 3:54 pm

Unfortunately, having the GND and trigger lines swapped on the module and trying to run the engine might have caused damage to the MOSFET transistor on the rev limiter board.

You may return the rev limiter board to us for evaluation. contact sales@autosportlabs.com to follow up.

tightwad · Post by **tightwad** » Mon Jun 02, 2014 3:58 pm

Thanks for the heads up! Can I test the transistor? I can replace it if that is all the problem is, and it would save me time for sure...my Rally is in 2 weeks so I am running short...

Post by **brentp** » Mon Jun 02, 2014 4:01 pm

So you are, in effect, testing the transistor by what you are measuring on the circuit. When the megajolt unit is powered on and the rev limiter board is wired correctly, you should measure very close to battery voltage on the COIL output pin of the rev limiter.

tightwad · Post by **tightwad** » Mon Jun 02, 2014 4:18 pm

Can you provide a part number for a replacement? I can see about sourcing one locally and swapping it, unless you don't feel this is doable on the bench.

Post by **brentp** » Mon Jun 02, 2014 5:56 pm

Sure thing! It's a fairchild semiconductor 512-FDS6681Z . Not sure if you'll find it locally, but you should be able to find it on Digikey or Mouser.

toga94m · Post by **toga94m** » Tue Jun 03, 2014 12:23 am

brentp wrote:Sure thing! It's a fairchild semiconductor 512-FDS6681Z . Not sure if you'll find it locally, but you should be able to find it on Digikey or Mouser.

The 512- prefix is actually Mouser's manufacturer code. You can find the part at lots of places - I use Findchips.com daily as the Google of electronic components.
http://www.findchips.com/search/FDS6681Z
Mouser / Digikey / Newark / Allied / Online Components will all sell to anyone with a credit card. Some of the distributors only want to talk purchase orders to large companies.

tightwad · Post by **tightwad** » Tue Jun 03, 2014 6:24 am

I ordered 2 of them (because somehow ordering just one didn't make sense...) Shipping was 4x as much as 1 part. Hopefully this is the problem and I will be well on my way to actually tuning AND using the Hard Rev. It is needed because when the watercraft jumps out of the water the overrev can destroy the motor...and they stopped making them 25 years ago or so.