Blown Fuses

708803 · Post by **708803** » Mon Jan 20, 2014 9:52 am

Fuses keep blowing. It is always the 5 amp injector(single injector TB) fuse and the 2 amp fuse that goes to pin 28. It does not happen every time I run the car but it has happened 3 times in the last few months. It can be at idle or when accelerating. I can't determine which goes first.
Any thoughts?

kjones6039 · Post by **kjones6039** » Mon Jan 20, 2014 1:16 pm

Just curios here......

What version MS are we talking?

Is that injector high or low impedance?

Ken

708803 · Post by **708803** » Tue Jan 21, 2014 8:28 am

MSII and the TBI is from a 90's Geo which I believe is High- 10 to 16 ohms (Saturated Circuit Drivers/Injectors). What causes this is sporadic because it does not happen every time I run the motor.

kjones6039 · Post by **kjones6039** » Tue Jan 21, 2014 11:41 am

Does the MS get warm/hot during normal operation?

Ken

708803 · Post by **708803** » Wed Jan 22, 2014 7:04 am

I will check the MS box temp.next time I run the car. Right now I am working on the header so the car is down. Also on further investigation and checking my assembly notes the injector appears to be low impedance. Maybe I need a resistor change.
Thanks for the assistance. I will probably have the car running this weekend.

trakkies · Post by **trakkies** » Wed Jan 22, 2014 7:29 am

MS2 can drive low impedance injectors directly by using PWM.

kjones6039 · Post by **kjones6039** » Wed Jan 22, 2014 7:44 am

708803 wrote:Also on further investigation and checking my assembly notes the injector appears to be low impedance. Maybe I need a resistor change.

This is something you need to be sure of and could certainly be responsible for your blown fuse issue! Did you actually measure the injector?

It sounds like you are already using resistors...... is that correct?

I personally, would not use resistors. As Dave points out above, low-z injectors can be easily driven using pwm.

Ken

708803 · Post by **708803** » Thu Jan 23, 2014 7:33 am

I checked the injector and the resistance is 1.1 ohms at the connector on the side of the TB. I also checked my assembly notes and I installed the PWM flyback components(#69 in assembly list).

kjones6039 · Post by **kjones6039** » Thu Jan 23, 2014 7:43 am

708803 wrote:I checked the injector and the resistance is 1.1 ohms at the connector on the side of the TB. I also checked my assembly notes and I installed the PWM flyback components(#69 in assembly list).

Yep...... Definitely qualifies as low-z.... Post your msq so we can have a look at your settings.

Ken

708803 · Post by **708803** » Sat Jan 25, 2014 6:59 am

Here is the .msq. This set up allows the motor to start and run. There is only 5 miles of driving with this file and during that test no fuses blew. Prior to that test the motor only ran in the garage and the (2) fuses blew about every other time the engine ran.

kjones6039 · Post by **kjones6039** » Sat Jan 25, 2014 8:19 am

As I suspected, the PWM Current Limit for your injector is set at 70%. I suspect that is certainly a big factor in your issue. My experience is that, 30% should be adequate for most lo-z injectors. FWIW, several years ago I toasted a set of hi-$ Holley injectors by running them at 70%..........

Ken

trakkies · Post by **trakkies** » Sat Jan 25, 2014 8:52 am

Notice it's also 1 squirt per cycle?

kjones6039 · Post by **kjones6039** » Sat Jan 25, 2014 8:55 am

CORRECTION: Oops..... Your setting is actually 75%, not 70% (even worse IMO)

Ken

kjones6039 · Post by **kjones6039** » Sat Jan 25, 2014 9:37 am

trakkies wrote:Notice it's also 1 squirt per cycle?

I agree! 2 squirts would, in theory at least, cut the continuous current in ~half.

In any case, 75% PWM is way to high, IMO.

Ken

708803 · Post by **708803** » Sat Jan 25, 2014 6:05 pm

Thanks for the info. I will make these changes. I don't remember why I set the PWM at 75% but I did set the squirt to 1 because both pistons travel together with only one firing per revolution. Can you give further explanation on both these changes? I would like to understand the changes.

trakkies · Post by **trakkies** » Sun Jan 26, 2014 1:55 am

http://www.megamanual.com/ms2/tune.htm#pwm explains PWM setting.
As regards squirts per cycle I assumed a more normal type of engine, since you didn't say what it was but mentioned car. With yours 1 is correct.

708803 · Post by **708803** » Mon Jan 27, 2014 4:10 pm

I set the parameters to PWM 30% and Threshold 1.0 ms. The motor idles fine.
I noticed when I was lowering the injector Threshold and the Opening time that I could go to almost nothing without effecting the idle much. At a point it bogs when the throttle is opened. Do I turn both the Threshold and the Opening time back up or just the Opening time to correct this? The adjustment in the link was not clear to me on that.

DonTZ125 · Post by **DonTZ125** » Tue Jan 28, 2014 5:33 pm

Opening time for Lo-Z is generally around 0.6ms, but can be less. If you turn the threshold down too far, you really aren't opening the injector at all. It manages to idle, but anything beyond that runs out of fuel.

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