Voltage issues

[fastie]

Hi there. I've just put my board together and getting some strange results. Occassionally when plugging power I get heat and smoke (unplug it instantly) - R15 burnt out so will need to be replaced - U4 was also warm to touch.
When testing the voltage on U1 I get on pins 40 & 20 = -4.36volts (negative value), on pins 2 & 20 = - 4.44volts. I have also seen -3.57 which shows a inconsistency.
Then I tested the Proto and U5 which reads 11.5volts and with a battery 8.5 volts ( it seems everything is half a volt down)

Can anyone give me some clues of what to do next to resolve this? Why would my multimeter be reading negative values?

Staf

[SymTech Laboratories]

You might have your positive and negative multimeter leads backward.

[fastie]

Ha - seems obvious - but yeah tried that over and over - always same result.
My first issue is the fried R15 resistor.. anyone know what could cause that?

[Matt Cramer]

My first issue is the fried R15 resistor.. anyone know what could cause that?

Usually, it's Q14 becoming shorted out and sticking on.

So, the next question is why Q14 is stuck on. This can be either a short from an assembly error (the pins are really closely spaced and easy to accidentally solder together) or trying to run too much current through the injectors.

[fastie]

Thanks - you may be right there. Q14 looks to possibly have a solder spill touching. Do you think it would have shorted or shall I try resoldering back in. Also, is this a part I can buy from electronics shop or reorder from DIYautotune if it has blown?

[dontz125]

Try wicking up the excess solder with desoldering braid. Q14 is a 2N3904, and can be had at any real electronics outlet.

[fastie]

I found that D19 had blown and R15. I replaced both and then the circuit tested fine.
I decided to test the circuit a 2nd time and D19 has blown once again. Can anyone advise to why this might be happening?

[slow_hemi6]

Your U4 might be dead and perhaps thats causing issues with the VCC 5V line which is being taken up(not well) by the 5.6V zenner. The zenner can only pass about 180mA before it is in big trouble. But that's my guess, higher than 5V getting onto the 5V line. If as you say, U4 got fairly hot it could well be toasted.
If you do have to change out U4 then you might consider a chip socket (decent one not a cheapie) for it at the same time.

[fastie]

Thanks. How can I test U4? The positive legs are reading 12v except the first leg which is reading 0v

[slow_hemi6]

1 & 8 do nothing on U4. What do you get on 2 and 4?

[fastie]

If you count anticlockwise like U1.
1 2 & 3 are reading 12V
4-8 reading 0V

[slow_hemi6]

This is the pinout. Notice the notch and or circle at the top/No1 pin.

Does this match up to the numbering and voltages you are measuring?

[fastie]

Thanks for that hemi,
5,6,7 are reading 12V. The rest nothing

[slow_hemi6]

That actually sounds correct for the U4 but not correct operationally. The way it is now with low on the U4 pins 2 and 4 the injector fets both will be on. Seems strange the 2 pullup resistors R14 and R17 don't seem to be pulling pins 2 and 4 up to 5V. Do you currently have 5V supply? What Volts do you get at each pin of F1?

[fastie]

No 5V supply. I'll replace D19 again and see what happens

S

[slow_hemi6]

Give this a read. http://www.megamanual.com/ms2/V3trouble.htm

[DaveEFI]

No 5V supply. I'll replace D19 again and see what happens

S

Diodes very rarely fail for no reason.

[klvnmykvl]

Did you ever figure out the cause of your R15 burning? I'm starting to experience the same issue.