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Finding an electrical gremlin

Old 12-08-2018, 10:44 AM
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Default Finding an electrical gremlin

Background : 96 Miata
had a bad main ground connection from ppf to engine at MATG and melted COOLING FAN relay and wires beneath it. Fan 12v+ went to the ECU ground and toasted several functions on the MS3 Basic. Tach stopped working and AC compressor clutch stayed engaged regardless of pressure switch or anything else.

Upon returning home I added the missing engine ground strap, separated the wires to the cooling circuit that were melted and replaced the 2 OEM fan relays with one heavy duty 40 amp aftermarket relay controlled by either of those two fan signals, and replaced the MS3 Basic board.

The car cranks and runs, the cooling fan works correctly, the AC compressor clutch cycles on and off correctly, BUT there is power in the HOT IN RUN circuit with the key out of the ignition! I have what appears to be a small amperage lighting up my aftermarket gauges powered by the POWER WINDOWS circuit. The gauges go out if the window switches are toggled or if the blower fan switch is turned above zero.

Unplugging one of the three harness connectors from the ECU makes the gauges go out. It is the connector with the fuel pump and cooling fan signals on it.

I am looking for places my HOT IN RUN circuit could be receiving power.
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Old 12-08-2018, 10:56 AM
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The way I do a parasitic draw test is to get a mini fuse holder from Advance/Auto Zone and install an eyelet on both ends, then connect that in series to my multi meter at the negative post with a 10 amp fuse installed in the holder. The purpose of the fuse holder is to allow that fuse to pop before your meter fuse does in case of amperage spike.

After that i pull fuses one at a time while monitoring draw to determine which circuit is shorted to ground. Once the affected leg fuse is removed and the draw goes away I will then disconnect each component in the circuit to see if the component is shorted or if a wire is shorted.

Try that to see which circuit it is specifically. When my main relay melted/failed due to pump upgrade the fuel pump ran with the key out which would explain the ECU plug connection.

Last edited by L337TurboZ; 12-08-2018 at 11:58 AM.
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Old 12-08-2018, 12:12 PM
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Looking at all the wires on that particular ECU plug, a few of them are tied to the AC system. The AC switch button and the AC relay controlling the clutch are both sourced off of the WIPER FUSE which is hot in run. If I remove the WIPER FUSE the gauges go out. There is a diode on the AC switch button circuit. I wonder if I might have burned up the diode. Checking that next.
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Old 12-08-2018, 12:35 PM
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Disconnecting the temperature and pressure switches did not break the circuit so I know the diode on that circuit is not the culprit.

Interestingly enough removing the AC clutch relay breaks the circuit so I must be getting power backwards from the ECU on the blue/ black wire. About to verify.
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Old 12-08-2018, 01:42 PM
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There you go. I was getting 5 volts positive on that signal wire from the ECU back into the hot in run circuit. The board had been damaged during my testing of the fuel pump circuit I'm guessing.

Problem solved by reassigning spare output to run the compressor clutch.
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Old 12-08-2018, 01:59 PM
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Recap: When Reverent had sent me the replacement board the car would not crank. He did not tell me certain component had changed in his board design and that I needed to change my decoder for cam position sensor from falling Edge to Rising Edge. So I spent a few days chasing things around trying to figure it out and I was pretty sure I had accidentally sent 12 volts positive up the fuel pump signal ground to the ECU. That probably fried this other piece on the board related to the compressor clutch, allowing it to send 5 volts outbound instead of being a switchable ground.
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