Megasquirt fuse keeps blowing

[Mavrick]

I was burning some different values into the VE table when the ECU fuse blew, no big deal I thought, I changed it but out, but now everytime I turn the key to accessory the fuse blows. So I checked continuity between the fuse and megasquirt, the resistance is .3 so I don't think there is an open on the line, the only other variable is when I unplug the megasquirt the fuse doesn't blow. So my question is, could there be a short somewhere internally in the megasquirt, I did a quick visual inspection and didn't notice anything out of the ordinary. Thanx for your help in advance.

[TheMonkey]

where did you tap in for power?

i had a similar experience at one point. i can't recall the details, but something else was on that circuit when powered off, it would blow the fuse. i ended up going directly to battery, which is better practice anyway as it acts as a big capacitor. try it temporary anyway to see if it stops, then at least you can rule out the MS.

[hugoazt]

Hi there!

I had the same issue last month, I discovered that the mica isullator was not isullating properly and was making a short beetween the transistor and the grounding.
Check your transistors and change the screws, use only the nylon ones...
Hope this can help you!

Regards!

Hugo Z.

[Mavrick]

How do you tell if the mica insulator is bad? I tested resistance between the two transistors that had mica insulators and the heat sink, and on both the middle prong had 60 ohms of resistance, either outside prong was over limit on my DVOM. So then I disconnected both transistors from the heat sink, and tested the resistance again and got the same numbers. I also ran a seperate fused wire to the megasquirt to no avail, the fuse keeps poping. One more tidbit of info I forgot on the original post was the last time the car started it ran fine, but when I turned off the key the car kept running untill I killed the fuel pump, do those transistors that have mica insulators on them controll the fuel injectors? Because after I killed the fuel pump and noticed the reason why I couldn't burn to the computer was because the fuse had blown, the fuel injector fuse was also blown. Does that change what I should be looking at, or am I still at square one?

[hugoazt]

What was causing my fuses to get blown, was that the metal screws that holds the transistors Q12 and Q9 were making contact with the heat sink, causing a short between the base of the transistors and the grounding of the heat sink.
So, I just replaced the metal screws with the nylon ones and my problem disappeard... No more fuses blown...
Just do that test and see what happens...

Regards!

Hugo Z.
Alajuela Costa Rica
200sx SE-R 1998
MSII, PCB 3.0

[Mavrick]

so I was finally able to change the metal screws with nylon ones, and it didn't change anything, fuse keeps blowing. Any other ideas, I would like to finish tuning. thanx in advance.

[Matt Cramer]

Is there anything else powered from the same wire?

[hugoazt]

if nothing else is connected to the same wire, and the MS is connected directly from the battery (using a Relay), you need to check those transistors. One of them can be damaged.

[Mavrick]

the fuel pump was getting power from the the same line, but the fuel pump is also on a kill switch inside the car, which has been turned off, I also made a new circuit that just powered the computer and the fuse still pops. How do you check the transistors to see if they are bad? resistance, the way they look??? thanx for the help guys.

[hugoazt]

On this URL is the info on how to test transistors with a multimeter.

http://www.electronics-radio.com/articl ... r-test.php

[Mavrick]

well I finally had a chance to test the transistors, and according to the link you provided they were both blown, but when I got the new ones from digikey, I tested the new ones and all 4 I bought were blown (according the the link)...... so I am thinking it is not the transistors at all. So........ any other ideas??? I am very frustrated right now, and am just ready to get a completly different EMS. Any help right now is much appriecated!!!!

[8974Ollie]

Check for bridged solder joints?

[Mavrick]

yeah, that was the first thing I checked. I bought the unit pre-assembled and the only thing I have replaced is the two injector transistors. Thanx for the post.

[Mavrick]

bump..... I hooked up a stimulator to the megasquirt today, and it does the same thing it does in the car, blows the fuse. Anybody??????? Ideas???????

[Mavrick]

This should be a relitivly easy fix, no? does anybody know the power distribution of the board? you know what recieves power first, second, and so on? any help is much apprieciated!

[Matt Cramer]

A fuse? On the Stimulator? How are you powering the Stim?

[S.Bretz]

Sounds like you might have a problem with the voltage regulator....that's where I would start.