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ecu reads 12v but battery and alternator read 14v?

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Any idea why this is? Or causes of this? this is on haltech elite 2500

Check where the power feed for the ECU is coming from and also ensure you have a good clean earth. You can always confirm the voltage at the ECU header plug with a multimeter too.

I was having a similar issue with my Performance Electronics ECU in my Cobra, noticing voltage drop spikes in the ECU. Using the logging feature, I determined the supply voltage was dropping and I traced it to a defective voltage regulator.

In a carbureted car, lose a couple volts for a second is not a big deal, you just aren't charging the battery correctly. But in an EFI setup, the ECU really begins to do strange things when it happens.

New voltage regulator and my voltage trace logs are now showing a flat line - and the issues went away.

Just a thought


I'm not sure on the Haltech but on the Link ECU's there are two different power options to monitor, the 12V internal power and the battery supply voltage.

I would check the voltage at the header plug first and then check your monitoring options.

Well boys I figured it out! Thanks for all the help you guys provided!

The issue was being the idiot I am, I wired the radiator fans into the mass air flow sensor switched 12v power. That mass air flow sensor gets its 12v from the mfi relay which also supplies power to the ecu. When I unhooked the negative of the fans I immediately seen 14v on the ecu. Whoops, I wonder how much better my car will run with proper dwell time, injector dead time etc etc.

Now I gotta figure out how to wire my fans in properly. Looking for a wiring diagram for 3000gt showing me what wires are what on the stock connector that goes on fans. Such a pain working on cars that have 0 support. 2jz and such have professionally made harnesses just plug it in and go, and they have no issues like this to worry about.

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