Anyone ever had erratic voltage? I just pulled a datalog on my MegaSquirt and it shows that my battery voltage is fluctuating from 12.2 to 13.9v. The dash voltmeter shows pretty constant, but I guess it's not really all that accurate. I'm pretty sure the rectifier is in good shape, it's a solid DC output. I guess that just the regulator took a dump on me?

I upgraded to the FD alternator around 3 or 4 years ago, and have since driven the car probably less than 5k miles. Battery is fairly new and in a good state of charge. Is there any other reason that the voltage would jump around so much?

 

Does your log include voltage w/key to on ,engine off as part of the data ?

 

It could, but I only took a log of it cruising. When sitting at idle, the voltage is pretty steady. Around 13.2, only flickering to 13.1. I was watching the AFR's jump all around and wanted to know what was causing that. Megasquirt has a battery voltage correction that adds/removes fuel based on a change in voltage (mostly used at idle). I had it set to add around .2ms of fuel for every .1v. So, that adds up to around a PW swing of 3.4ms. That's a significant amount, considering that the cruising PW is around 3.6, the battery voltage correction can nearly double the PW.

 

Could be a loose cable at the alternator, or a bad ground for the megasquirt system or bad engine ground. It may not be that the alternator --> battery negative voltage is swinging, but rather the ground of another part of the system is coming up. Do you have electric fans or something else that may draw a large amount of current installed?

 

I don't know how accurate the alternator tester at O'Reilly auto parts is, but I just had it tested there. It passed the regulator and rectifier (and whatever that other test was, I didn't see it) tests, so I guess it's time to start re-wiring everything.

It COULD be going out when the underhood temps get high, so that could also be an non-testable factor. I'm just going to get some wire and heavy gauge cable, and start re-wiring the battery, starter, and alternator, and then put down as many ground straps as possible.

 

Mine has been doing the same thing. At idle, I will see it anywhere from 13.1V to 13.8V. At cruise, 13.3V. At cruise and idle while lowering windows with lights on, 13.1V down to 12.1V. My alternator and starter are fairly new. Battery is brand new.

Let me know how your fix goes, man. I've been considering a rewire myself.

 

check the belt tension?

 

What do you think this is? Amateur hour!??

 

Do you happen to have a quality multimeter capable of reading a high degree of amps being created by the alternator?

 

Unfortunately, no. I looked back at the datalog, it seems that the voltage is fluctuating so rapidly that I don't think the dash meter could even catch it. It's pretty random, and can be at 12.2v, then less than a second later, it'll be at 13.9. It's REALLY fast, but I'm worried that it's causing problems in the ECU.

Still no problems with starting or charging.