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Credit goes to Jbronson. We hooked up the multimeter to the diagnostic port cooling fan and determined it was grounding. Removed the ECU and grounded it again and it wasn't grounding. That was the last part I suspected but kind of makes sense since the only relays I had tripped on were the two relays that get signals from the ECU.
For reference its T544 on the board. Im not entirely sure how we did it since I wasnt the person that found it. But for the most part we connected multimeter to the cooling fan pin on the ECU and just searched for continuity I think all over the board until we got a signal. We were able to reference from broken ECU to verify things.
The pin that sends the signal to the 2 fan relays is pin 3D on the 16 pin connector on the ECU. It is the second pin from the right on the bottom row. The ground for the ECU comes in through pins 4A and 4B on the 26 pin connecter. They are the top and bottom rightmost pins. I put an ohmmeter between pin 3D and pin 4A and it showed a short circuit. All the power outputs are controlled by power transistors which all have 3 pins (G D S), so I connected one lead to the pin 3D and checked all of the 3 pin transistors on the board until I found the one that had continuity. I tested the resistance between the pins and it was a direct short. Once removed from the circuit it is easy to see if the drain and the source are shorted out by using the diode range on the multimeter. It has to show open line in one direction and a voltage drop of 300 to 600 millivolts in the other direction. As for the transistor, I swapped one over from a scrap ECU board I had held onto, but a cross referenced part could most likely be bought at Radio Shack if you don't have one. After installing the replacement transistor the continuity between pins 3D band 4A was gone and the fans run as designed. Forgive the lengthy post it's just it could possibly help someone in the future with the same problem. We didn't have a spare ECU on hand and were eager to get the car going so we had to improvise lol.
The pin that sends the signal to the 2 fan relays is pin 3D on the 16 pin connector on the ECU. It is the second pin from the right on the bottom row. The ground for the ECU comes in through pins 4A and 4B on the 26 pin connecter. They are the top and bottom rightmost pins. I put an ohmmeter between pin 3D and pin 4A and it showed a short circuit. All the power outputs are controlled by power transistors which all have 3 pins (G D S), so I connected one lead to the pin 3D and checked all of the 3 pin transistors on the board until I found the one that had continuity. I tested the resistance between the pins and it was a direct short. Once removed from the circuit it is easy to see if the drain and the source are shorted out by using the diode range on the multimeter. It has to show open line in one direction and a voltage drop of 300 to 600 millivolts in the other direction. As for the transistor, I swapped one over from a scrap ECU board I had held onto, but a cross referenced part could most likely be bought at Radio Shack if you don't have one. After installing the replacement transistor the continuity between pins 3D band 4A was gone and the fans run as designed. Forgive the lengthy post it's just it could possibly help someone in the future with the same problem. We didn't have a spare ECU on hand and were eager to get the car going so we had to improvise lol.
thanks for helping your buddy out and describing the how to. with the transistor removed, to measure the 300 to 600 millivolt drop do you plug the connectors back into the ECU and turn the key to the on position?
your installation involved resoldered a replacement transistor to the board?
important grounds for the ECU are wires off pins 4A and 4B, hope I'm asking a pertinent question, would you know the important power up pins? thanks.
Replacement was soldering new transistor from a scrap board. Measured the drop on the transistor itself. Old one was bad and showed 0 millivolts. Replacement showed 554 millivolts
On page Z28 in wiring manual it shows pin A1 is the fuse power to the ECU. Its the top right pin on the 22 pin connector. If you suspect a transistor is bad you desolder it and find the two pins that correspond to the drain and the source or the emitter and collector depending on the transistor type. In this case it uses a type of power field effect transistor. Just read the number on the transistor and Google the datasheet. With the multimeter on the diode setting you connect the positive to the drain and the negative to the source. You will get either OL reading or a millivolt reading somewhere in between 300 and 600. Now reverse the leads and you will get the opposite of what you got before (OL first then 300-600mV or 300-600mV first then OL). If at any time you see 0mV, which means the pins are shorted, the transistor is bad.
So just an addition to the thread, I had the same issue with fans staying on when ever the key was turned to "on" (running or not), and had no black box (but did have two spare connectors hanging loose in the passenger floorboard). I didn't know if something was needing to be grounded or not but after investigating I did indeed have the engine bay/relay side of the harness present. Removing the added harness and plugging the relays directly back into their original connectors seems to have done the trick and now the fans are operating (as best as I can tell) according to the PowerFC settings.
Thanks for the info in this thread! I instantly realized I had the harness in place because of the comment about it only plugging into 3 of the 4 relays.
Mazda Recall Harness installed(with red paint on it).
Once I pulled out the fan recall harness it seems back to normal. Plugs were corroded and the ones in my relays from the front harness has some corrosion as well. Might have to find replacement plugs for it.
I don't know if this is normal but the fans are coming on whenever key is in ON position now. AC and everything is off.
i just want to mention , that my friend had the same problem of the fan always turning on , it the culprit was the extension harness from the recall by the fan relays.
01-02-13 | 01:19 PM
