Dainiaus webaz – elektronika ir ne tik !

This post will be in English as I promised to share the information with my colleagues at Chrysler forum. So, the problem was that my Chrysler Sebring LX Coupe with 3 l engine (2001) was not starting. I will not describe all the things I tested before the final solution, only the right way from start to end. So, I found that fuel pump relay is heating very much. Reasons can be different, but result is the same – a lot of heating means a lot of current being used. First of all I was going to replace the relay, part number MR312504. But at our parts store the price was around 40 EUR or 60 USD for a single relay! Yes, I thought its a joke too… But no. As the price in not in logically acceptable I made a perfect solution. It will be described later. So, the relay was faulty and it burned the controlling transistor in the ECU. Checked the cable from relay to ECU – good. The pinout is a very helpful thing to do that. On relay coil power pins I had +12V (the battery voltage), but very weak current – it is not enough even to turn on the relay. Checked with lamp, it was not burning, and the voltage goes to zero when connected and back to battery voltage when disconnected. So, as the cable to ECU pin 21 is good I suspected that its ECU fault. Now simple logic – why do I have that weak voltage on relay coil pins… As the relay is controlled by microprocessor via transistor (it all should be in ECU) one of possible explanations could be that transistor is burned so emitter – collector junction is damaged. And the voltage I see is control voltage from the base of transistor, so it is voltage by which microprocessor controls the transistor. So, now its the big question – what transistor is at place Q30 ? SMD, marking:

SOT89 package. PCB has marking JE331B37204A. Started checking the neighbor transistor with same marking to guess what it is. So, simple logic again on how to pick the right transistor for replacement (as the marking is unknown). Transistor controls the simple relay, so it does not need any hi frequency, hi current or hi Voltage capabilities. Next I guess that the transistor is field effect type, because (on the good neighbor) between Drain and Source I can see the diode (multimeter set on diode check shows 497,1 mV – pn junction voltage). Batween all other pins it was more that 1000 mV so, it is for sure not the PNP or NPN transistor. As the diode’s cathode is turned to Drain, the transistor should be N-type. Another way to determine transistor type – if tansistors one pin is connected to VCC (plus) – its a PNP ir P-MOSFET, if one leg is connected to GND (minus) its NPN or N-MOSFET. Gate is connected to uCPU pin 69 (biggest chip with Mitsubishi logo and QFP-80 housing) via 2.2KΩ resistor. MFI relay and controlling transistor is also connected to same uCPU. That damn resistor tried to blow up my theory about FET. Thing is, that simple PNP and NPN transistors are controlled by current – to make the current they need a resistor. FETs, on the other hand, are controlled by Voltage and does not need any resistor. So, FETs need to have Voltage supplied to Gate and simple transistors need to have current supplied to Base. And here is the question – resistor shows that transistor is NPN, but multimeter says it has a diode as FET. But time was passing and I was a bit lazy to play with determining exact model on the transistor. In automotive electronics nowadays often are used „smart” transistors. First to get to my mind was BSP76 (thanks guys @o.electronics for good advice). Its N channel vertical power FET in Smart SIPMOS technology. Fully protected by embedded protection functions. This transistor incorporates several features at once:

• Logic Level Input
• Input Protection (ESD)
• Thermal shutdown with auto restart
• Overload protection
• Short circuit protection
• Overvoltage protection
• Current limitation
• Analog driving possible

But will it fit the purpose ? Lets take a look at its description:

Drain source voltage VDS 42 V – OK
On-state resistance RDS(on) 200 mΩ – could be less but OK.

Nominal load current ID(Nom) 1.4 A – great, as the relay will only require around 100 mA.

And here you may see that diode I was talking about, between Source and Drain. So, I guess we have a winner 🙂 I ordered those transistors at our local electronics store. Got them in several days and prepared to put one to its place. I missed one thing – old transistor was in SO-89 package and new SOT-223. So, the new is slightly bigger, and will not fit directly in place. Damn. Somehow I should fit it, because its a very good transistor – virtually indestructible. Had to put some wire, solder the transistor in lifted position. So, I managed to fit it on the board. Empty place where the bad resistor was (at position Q30):

Warning – oversized picture inside 🙂 Next – the new transistor soldered in:

As you may see, first I soldered to PCB two wires for Gate and Source, left about 10 mm each. Then at appropriate angle soldered in the transistor’s biggest contact on the top (pad, connected to Drain). After that transistor is secured in its place, so last two wires that was prepared soldered to appropriate pins and cut to fit the length. Thats it. Testdrive 🙂 :

Its alive !!! And the last thing – those expensive relays… You can use regular automotive relays and heres how:

Take a look at the relay box. On the frond side of it there were 3 MR312504 relays. One of them is fuel pump relay, other is MFI relay and I dint remember what the third relay is for, but that does not matter. See that empty place with 4 holes ? Its where the Fuel pump relay used to be. Now it is replaced by that Siemens relay nearby. This is done very simple – in the place where Siemens relay is now there were only holes without any pins inside, so I just unscrewed the relay box (it was simple), took out the original pins from places. Small screwdriver will help you – need to bend the locking pin and its free:

In the holes you will have a female socket, this example is male, but the locking pin is the same. Its that lifted metal thing in the center. So, bend it inside, take our the F socket. As you have the regular relay holes without sockets – just put it in the holes and you will have a place to insert regular relay. Be sure to put the pins into the right places. As the Chrysler Serbring Coupe = Mitsubishi Eclipse Spyder and almost = Dodge Stratus this solution fits all those cars, ECU part number MR560034.

P.S. Sorry for a very long article – installing the OS on computer so have plenty time to kill 🙂

Posted in: Auto..., Elektronika by Dainius /