blew highside driver no st sign. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 06-14-2012, 01:38 AM   #1
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Default blew highside driver no st sign.

diypnp
driving down road then dead

long story short my tip125 transistor on the highside driver circuit is black and has a crack in it.

the odd part is I never have made the st sign fuse mistake especially not while travelling at 60m/h on a highway.

so what coulda caused the transistor to blow?

also is the highside bypass good for a permanent solution or just till I can get a new tip125?

cheers
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Old 06-14-2012, 10:58 AM   #2
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jump the FP wire to 2O. then jump the corresponding pin in the AFM connector to 2O to the pin the jumps the Fuel Pump Relay.
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Old 06-14-2012, 11:38 AM   #3
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yeah I found that fix was just wondering whether if the circuit overloaded would it damage something else in the ecu?
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Old 06-14-2012, 12:03 PM   #4
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probably not.
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Old 06-14-2012, 12:23 PM   #5
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cheers for the advise I have ordered a few transistors but ill use the bypass till I get em.

I think the reason the highside driver mighta blown could be I may have wired the new fuel pump relay wrong and sent 12v back to the ecu.

if thats the case though then I still have problems.

I'll do the bypass tomorrow if that doesnt work I will then jump fp to gnd in the diag box and see whether that turns on the pump.
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Old 06-25-2012, 10:40 PM   #6
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did the bypass. that didnt work.
but when I got the transistor I soldered that in and everything worked.

cheers
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Old 07-01-2012, 05:47 AM   #7
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ok so I get about 100km from home and I notice electrical burning smell in the car. I pull over and sniff the ecu to find it is the culprit. I give it 15mins to cool down and turn around and head for home.

what could possibly be causing the transistor to get so hot? taking into account I am a noob at electronics.

my thoughts are I have a problem somewhere else in the system rather than the transistor isnt getting cooled well enough. my only known electrical issue is an almost dead battery.

if anyone can shed some light on what could be making it break or even what breaks a transistor id greatly appreciate.
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Old 07-02-2012, 11:42 AM   #8
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the st_fuse is pulled right? you must have a short somewhere on that wire.

can you check to make sure the screw holding down the FET isn't touching 5vref? there's a small via hole right were you bolt it down that can touch...
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Old 07-24-2012, 12:08 PM   #9
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thought I found the short but apparently not as I got to a track and my car died again blowing the same transistor.

sadly as I have replaced the transistor a few tome and dont have a solder sucker I have overheated the pcb and a couple of traces for the transistor have peeled a little which has led to the new transistor not working...

can anyone tell me where each of the pins for the transistor connect to? its a diypnp

I know the middle pin connects to the (out) hole on the highside driver circuit and the diode. the pin closest to the in/out holes connects to the (in) hole
but I cannot figure out what the pin on the otherside connects to... my guess would be a 5v powersource but I cannot see any trace for it.

once again if anyone can offer advice I would be very happy.
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Old 07-24-2012, 12:13 PM   #10
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looking at the driver, with the screw hole on top, then black body, then pins. from left to right. 1, 2, 3.

#1 - R41 - IN
#2 - OUT
#3 - raw 12v.

R40 connects to #1 and #3.
the Diode connects from #2 to #3, with the banded end on #3.

Basically what's happening is the FP ground trigger on #1 tells the driver to switch on and send 12v from #3 to #2 and back through your harness.

(please double check this yourself, I'm going off my head here)


Honesly, run FP to 2O and jump the AFM connector like I suggested and just simply do away with this driver all together. That's what I would do and how I run my fuel pump. So I'm going to suggest it again.




pro tip: Next time just cut the legs of the FET and solder the new FET back to the old legs within the PCB still...
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Old 07-24-2012, 12:30 PM   #11
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I'll try running the highside bypass lasttime I tried it didnt work but I must have done something wrong.

thankyou so much for explaining what goes where I will do that if the bypass doesnt work again.

and that protip is a very good tip I will do that in the future.

thanks once again
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Old 07-26-2012, 12:32 PM   #12
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I have now fixed my issues.

brain the bypass you suggested did not work for me whether it was I was supposed to get more than .2v on the 2O wire or because of some bodgy alarm install the 2O wire didnt have continuity to the afm connector or something else.

I decided to bypass the whole lot and wire the red/white (switched 12v) strait to the fuel pump and lose the fuel pump relay.
I do realise this is a potential hazard in the event of a crash but I am more than happy to take the calculated risk.

cheers for your help brain.
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Old 07-26-2012, 12:42 PM   #13
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do not like this approach. if anything, run a wire from FP to the db15. then from the DB15 run that wire directly to the lt. green wire on the FPR under the steering column.
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Old 07-26-2012, 12:53 PM   #14
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I tried something almost the same but basically got the 2o wire and cut that and soldered it to the light green wire.

it did not work im not sure if its cause the ecu doesnt send enough volts or because I have a bosch relay in place of the stock fuel pump relay (the old relay broke and I had to wire in a replacement on the side of the road).

is the only reason you dont like my approach because the fuel pump stays on even when the cars not running and the fire risks that are involved in the event of a crash?
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Old 07-26-2012, 12:56 PM   #15
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it's a ground to activate the relay, so there shouldn't be any volts

might be why you keep blowing the high side driver. good luck with your car fire
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Old 07-26-2012, 01:03 PM   #16
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Quote:
Originally Posted by Braineack View Post
it's a ground to activate the relay, so there shouldn't be any volts
This changes everything (I now think I may have wired the relay wrong) and I feel the slight voltage must be coming from said alarm wiring which happens to all be the same colour and relatively hard to figure out whats going on.

I'll go to bed now and have another look at the wiring tomorrow with the fresh knoweledge of its supposed to be a ground...

cheers
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Old 07-26-2012, 01:07 PM   #17
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yeah, the normal fuel pump output is a ground.

in most applications, the ground activated the fuel pump relay.

in the 90-93, the AFM does this when the flapper door opens. But Mazda wired it in such a way that the pump can prime. When you use the high side driver, you're sending 12v back through a different starting circuit that allows the relay to activate when it sees 12v on that side.

this is why you pull the ST_SIGN fuse (start signal) because then you'll have 12v always on the driver and it burns up from too much load/current/magic.


i prefer turning on the relay with a ground like normal, so I send the ground back through the AFM connector to the lt. green wire. so the MS controls the pump instead of the AFM flapper.



hope that helps.
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