U5 Fried

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U5 Fried

Postby KAT PWR » Fri Dec 15, 2017 7:31 pm

Today my MS3X ECU cut out on me 60 seconds into a drive. After pulling the car home I pulled the ECU and hooked up my Jimstim. upon inspection, I found that U5 was receiving 10.98V but only outputting 225mV. it's getting fairly hot to the touch so I know its fried but is there anything I should look at before just ordering and replacing the voltage regulator? I also found that the nut for Q9 vibrated off (this car runs solid motor mounts) and was next to its transistor on the heat sink rail.

I've been running this ECU for a couple of months now maybe have 60hrs run time on the ECU.
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Re: U5 Fried

Postby billr » Fri Dec 15, 2017 8:10 pm

Remove the MS3 daughterboard and check the mainboard again, connected to the JimStim. U5 may be overloaded by a bad MS3, not fried.
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Re: U5 Fried

Postby KAT PWR » Fri Dec 15, 2017 8:14 pm

that was tested with the CPU and MS3x cards removed. sorry I didn't specify.
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Re: U5 Fried

Postby DaveEFI » Sat Dec 16, 2017 2:24 am

If there is a short circuit on the 5v rail, U5 will shut down and get hot. I've repaired a few like this. Usually the tantalum cap C17 failed short circuit. Less likely D19 or C18.
If the unit is subject to excess vibration, I'd use a flexible mounting.
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Re: U5 Fried

Postby KAT PWR » Sat Dec 16, 2017 8:59 am

I went ahead and tested all the diodes and caps on the power schematic while on the board, ignoring the cpu clock. the diode check was easy all but D19 read infinite on one side and gave me 0 or a resistance valve the other way (cheap old meter). now the caps I'm unsure of c17, c18, c23, and c22 all read the same going from 0 Ohm to about 1 Ohm regardless of polarity. c15 and c16 gave different valves but still quickly raised Ohm valve when contacted. thinking about it will d19 being shorted will that skew my cap readings? should I remove d19 and retest?

D19 would give the 5v of U5 lead a direct short to ground. should I start with U5 (to be safe) and D19?
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Re: U5 Fried

Postby DaveEFI » Sat Dec 16, 2017 9:13 am

Since C17,C18, D19 are all in parallel across the output of U5 to ground, not easy to check if one has failed short circuit without disconnecting one leg of each.

U5 could, of course, have failed. But electrolytics are usually the first thing to suspect.
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Re: U5 Fried

Postby KAT PWR » Sat Dec 16, 2017 9:57 am

Thank you, I will recheck them later tonight. Hopefully, I can get a parts list together to tonight, so I can get these parts soon.
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Re: U5 Fried

Postby 142 guy » Sat Dec 16, 2017 3:45 pm

I am not particularly familiar with MS3; but, I believe the mother board and power supply are identical so here goes. I suggest removing the voltage regulator from the board and bench testing it. You will need a 100 - 1000 ohm resistor which you connect from the output to the ground pin. Connect a 9 volt battery between the input and ground pin and measure the output voltage. If you are getting 5 volts the regulator is probably OK and just went in to shut down. You need the resistors to apply some load on the regulator because linear regulators don't work very well without some load applied and having load current confirms that the regulator is actually able to track the control voltage. The regulator application guide says that you need to apply a 10 uF capacitor across the output also. I have tested LM series regulators without the capacitor so give it a try.

For testing, you can find pin outs for the voltage regulator on the web.

Whether or not the voltage regulator is cooked (and if it is, you definitely need to do this), with the regulator removed from the board and with the daughter board removed from the main board, use your ohmmeter to measure the resistance between a sanctioned 5 V point on the board and board ground. It should definitely be higher than 10 ohm (5v / 10 ohm = 500 ma which is the limit on the regulator). Do the same measurement at the point where the regulator output solders to the board. Again, if the resistance is 10 ohms or less that is a sign that you have a short or something else is toasted on the board overloading the regulator. If the resistance measurements check out, try plugging the daughter board back in and do the resistance measurements on the sanctioned 5 V bus. If the resistance measurements are still above 10 ohms, then there are no static shorts on the system.

If you fail the resistance tests on the motherboard try removing the polyfuse. This will allow you to narrow down on the 5 volt bus where the short is.

If the static tests check out, solder the reg (or a new reg) into the board. Disconnect the daughter board and power up the board from your jim stim. Check the voltage on the sanctioned 5 volt point. If you passed the resistance tests, you should be getting 5 Volts and the reg should not be getting hot. Assuming you pass this test then plug the daughter board back in and repeat the test.

If D19 (its a zener diode) was shorted to ground the polyfuse should F1 should have opened protecting the regulator. Maybe check the polyfuse to make sure that it is not permanently open! They do that sometimes if you repeatedly overload them. Being a zener diode, D19 can be tricky to test. It will read close to 0 ohms in the forward direction. D19 has a cut point of 5.6 volts. Below 5.6 volts it looks like an open circuit in the reverse direction and above 5.6 volts it will start to conduct current. Depending on the bias voltage (if it is over 5.6 volts) in your ohmmeter D19 will conduct in the reverse direction making it look like it might be shorted. Lacking the specifics, the best way to check for a failed D19 may be to remove it.
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Re: U5 Fried

Postby KAT PWR » Sat Dec 16, 2017 6:09 pm

thank you, 142 guy for the very informative step by step guide on how to diagnose the power supply circuit. I do, well used to do, a good bit of automotive electrical diag work as a mechanic but I've never tried to diagnose a complex electronics circuit like this before. I was testing voltage driectly at U5 pins last night so I know the ploy fuse is still good.

I started by removing U5 and after I removed each component I tested for >10Ohms at the 5V and GND pads by the proto area. Starting with DaveEFIs suggestions pulling C17 then C18 after that I pulled F1. Still no dice. Retested at U5 pin 2 GND and U5 pin 3 5V output, got 1M Ohm and rising. Now I'm on the right track. I'm about to test U5 after that going back to pull D19, C19, C23, C22 retesting at the proto area.
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Re: U5 Fried

Postby KAT PWR » Sat Dec 16, 2017 7:02 pm

after removing everything for the 5v rail ( on schematic sheet 6), I now have 802 Ohm. :D
now to test all the bits. i do have to order C19 as i was a bit rough on it trying to remove it.

tested U5 i works got a clean 5v with a 1uf Cap and led as a load.
Image
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Re: U5 Fried

Postby KAT PWR » Sat Dec 16, 2017 9:13 pm

found the diode and farad settings on my newer multimeter. results are in.

D19 0.00v forward bias, 0.00v reverse bias
C19 I broke
of C18 and C23 (.1uf): one is good within tolarence 95nf and the other is weak at 31nf
F1 is doulbe its post trip value at 2.2 Ohm


going to order C23, C19, D19, F1 and should be back in action.

thank you everyone for your help.
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