Well, I posted about this nearly a month ago that I was experiencing a complete cut at 3800rpm. Rotaryressurection chimed in and pointed me in the right direction of things to look for.

After checking different things, we isolated the problem to the resistors and soon replaced them. After replacement, the car revved perfectly normal over 3800rpm. But after about 30 mins of driving, its cutting out at 3800rpm again, which is most likely due to the resistors failing again.

My question is what would cause the resistors to fail after 30-60 mins of running time? Is it a ground issue? Thanks in advance!

 

Recheck your connections before you assume a bad resistor. Also, what resistor did you use?

 

Do your resistors have a sufficient power rating? I believe they need to be at least 10W or they'll burn out from excessive heat.

 

Yea I believe they're rated at 10w. They lasted nearly a year before this started after the driveline swap. Now its going through them quickly.

 

Is the resistors electronically failing or is the leads breaking?
If they are electronically failing, try and step up to 20W units.


-Ted

 

connect them to the wires near the ECU instead of ontop of the hot engine. if they are not there already.

 

I just did the maths (V = IR, P = VI) and discovered 14V pushed through a 10ohm resistor actually dissipates 19.6W, so I'd take Ted's advice and get 20W (or bigger) resistors instead. Putting them somewhere where they can better dissipate that heat as Scott suggested is a good idea too.

 

Been there, done that. Thanks for reminding me. I had mine start failing fairly regularly till I relocated them to the ECU area.

Jarrett

 

not all the voltage is across the resistor though.

although if the injectors are 2 ohms?? 80% would be.

I've never tryed adding resistors but aren't you trying to go from low (2 ohms) to high (8ohms). but you are going from 2 to 12 with a 10ohm... shoudln't it be like a 6ohm?

 

Oh yeah, of course. Electrical brain fade. The voltage is proportionally distributed between components in series based on their resistances, because the current must remain constant through all components.

So assuming a 3ohm injector and a 10ohm resistor, the injector sees 3/13 x 14V = 3.2V and the resistor sees 10/13 x 14V = 10.8V. So the resistor dissipates 10.8V x 10.8V / 10ohm = 11.6W

So 20W resistors are still a good idea.

 

and that is peak power. RMS would be less which is more important

RMS would change with duty cycle.

 

That is also true. You're just full of info today aren't you.

Since it's an approximately square waveform, the RMS number would be the same percentage as duty cycle right? So at the safe max of 85% RMS power would be ~9.9W.

I'd still use 20W resistors...

 

Thanks again for the great information. I'll try out the 20W resistors further away from the manifolds.

So grounding isn't really an issue on this one then?

 

yep.

gota make use of all this new school stuff somewhere before I forget it all