blown injector fuse -- help

[ChevelleFan]

I'm working on dialing my Saturn 1.9L DOHC. This is basically a stock engine, using the Saturn OBD-I Ign Control Module and the stock low-Z injectors. (19lb/hr) I'm running batch fire with 2 injectors on each channel.

I finally took it up in the revs (6500) the other night and it popped the injector fuse. I had a 10A fuse in there. Factory fuse is 7.5A. We've got literally thousands of racing miles on the factory setup, often going to 7000rpm and never had this problem. Admittedly, this is a scratch-built harness for the MS, but I think I did a decent job. Everything is soldered/heatshrinked and then inside split tubing. After replacing the fuse, I drove the car home about 3 miles without any issue, but I kept RPM under 3-4000.

Ruling out a simple short or bad injectors, is there anything in my config that could cause this? Car has run fine so far, but I have less than an hour or two of run time. You can see at the end of the log where this happens.

Thanks,

-Dave

[tryingbe]

Are you using a peak and hold board or resistors?

[ChevelleFan]

Not using resistors. It's a 3.57 board. I have the injector PWM Current Limit at 30%, which I think I found in the manual.

-Dave

[billr]

I would be tempted to just run it up again a few times with the new fuse; sometimes fuses just die, especially old ones.

Is the fuse in the stock Saturn UHJB? The contacts in there are known to be a bit dainty. With the double-ended female clips, there are four places for each fuse to have poor connectiont and abnormal heating of the fuse.

[ChevelleFan]

Not a stock fuse -- I have a little add-on fuse panel for the Megasquirt stuff. Went out for another tuning run tonight, but we didn't tempt fate since temps are down in the 20s. Max RPM was 5200, Not a good night to end up on the side of the road in case it's more than a fuse.

-Dave

[racingmini_mtl]

Your PWM time threshold is set to 25.6ms. That means you're running them at full battery voltage all the time, i.e., no PWM. Set that to about 1.0ms

Jean

[billr]

Ah, so simple to see when you are an expert like Jean!

[DaveEFI]

When I started out with Lo-Z injectors (8) I was using B&G code, and found instructions for how to set the PWM from scratch in the MegaManual, and have stuck with the settings I arrived at by trial and error since. I've looked for similar instructions in the Extra Manual - but con't find them. Perhaps someone who knows it better than me could give a link to them.
So for information, I've pasted the relevant MegaManual bit here:-

****************************************************************************************

To run low-impedance injectors with the PWM current limit mode, you need to set two parameters - the "PWM Current Limit (%)" and the "PWM Time Threshold (ms)" - both are on the “Constants” page. The current limit % is the percent duty cycle when the current limit is invoked. The time threshold is the amount of time from when the injector is first opened until the current limit is activated.


Start with:
PWM Time Threshold = 1.0 millisecond, and
PWM Current Limit 30% if you have the flyback board installed.
Once you get your engine idling, then first adjust the PWM duty cycle down in 1% increments until you notice a change in idle quality (be sure to hit the "send to ECU" button each time you change the value). This is the point where the current limit is too much and the injectors are not being held fully open.
Then move the value back up 3 - 5% (for example, if the idle falters at 45%, then put in a number of 48% to 50%).
Move on to adjusting the time threshold. Lower the time threshold by 0.1 milliseconds at a time until the idle quality deteriorates.
Then increase it 0.3 ms.
Now, repeat these steps (starting at #2). Adjust the duty cycle and time threshold alternately to get the optimum values for your set-up. You will converge on a set of numbers that work well for your set-up.

[ChevelleFan]

Thanks everyone.

Jean, your comment on PWM time threshold makes sense. I've changed it. I also changed dead-time from 1.0 to 0.7 based on the on-line help bubble in Tunerstudio. It seems like I'm going to have to re-tune -- fortunately I hadn't gotten too far.

Dave, I remember seeing those instructions a long time ago, and frankly, they always made me wary of using low-Z injectors because it always seemed... subjective. I say that because it easy enough to get the engine to idle on otherwise 'bad' settings, like I had. That wouldn't give me much confidence that the settings I get after going through that procedure would be sound. I'd switch to high-Z injectors here, but I have an initial requirement right now that I be able to switch back to the factory ECU until the MS is a proven setup, and the low-Z are the factory injectors.

The weather here in NEOhio has turned, so I won't get much road-tuning done in the near future.

thanks for the help!

-Dave

[DaveEFI]

Well, I used than method. And afterwards did some fine tuning of the idle settings, and did it again. And carried on until I was happy with both. And the car has been running with the same Lo-Z injectors for many a year.