Intermittent ECU voltage fluctuation while driving and headlights pulsing

[ScottT101]

I have been getting high ripple readings of ~300-400 when I test the charging system for a long time, but the car has always charged and run fine. On some nights, I notice the headlights and dome lights will pulse very noticeably while driving, but not every night. I watched my scanner three nights ago and the ECU voltage was fluctuating wildly at steady highway speed, headights on but no AC, from 14.2 to 16.8. The last 2 nights under similar conditions there was no pulsing in the lights, and last night the ECU voltage was rock steady at 14.325-14.375.

When I first noticed the high ripple reading, I tested the diodes and they passed. This is the original alternator on a 2009 Honda Civic with 307K miles, first noticed the ripple maybe 100K miles ago. It doesn't seem to affect the driveability of the car, athough I feel my mileage could be higher. 25-26 MPG with half highway and half start-stop driving, feully says the average is closer to 30.

Thanks in advance for any advice. I don't plan on driving the car much longer (been saying that for the last 100K LOL) so don't want to spend the money for a new alternator, just wondering what might be causing this. My thought is voltage regulator but I wouldn't expect it to be so intermittent.

 

[meep]

Make, model, year?

 

[CleanSump]

2009 Honda Civic with 307K miles

We don't know which engine, though.

 

[ScottT101]

We don't know which engine, though.

1.8 l gas.

 

[meep]

Yeah I totally missed that …

I mean, diode pack is what I’d look at too. Note that lower-end multimeters are tuned on the AC side for 60hz, unless it’s a true RMS meter, so the 300-400, which I assume are millivolts, may not be accurate.

Enough line noise like that and I could see the regulator getting thrown off with wild readings it’s endlessly compensating for. Other things to test would be grounds. Set to DC volts and measure the V between battery post ground and different points such as alternator frame, engine block, firewall. Do this with all accessories on. If you see much more than 0.3 volts or so (at full load) then I might start looking for bad grounds or adding jumpers. I had a Chrysler once that metered at 1.1V, with flickering lights. After adding a 4-point ground harness, the flickering was still there but throttle response was a little snappier and transmission shifting was unexpectedly quite improved. So I’d definitely check for ground quality. It won’t hurt to measure the positive side for V drop at load as well.

 

[ScottT101]

Yeah I totally missed that …

I mean, diode pack is what I’d look at too. Note that lower-end multimeters are tuned on the AC side for 60hz, unless it’s a true RMS meter, so the 300-400, which I assume are millivolts, may not be accurate.

Enough line noise like that and I could see the regulator getting thrown off with wild readings it’s endlessly compensating for. Other things to test would be grounds. Set to DC volts and measure the V between battery post ground and different points such as alternator frame, engine block, firewall. Do this with all accessories on. If you see much more than 0.3 volts or so (at full load) then I might start looking for bad grounds or adding jumpers. I had a Chrysler once that metered at 1.1V, with flickering lights. After adding a 4-point ground harness, the flickering was still there but throttle response was a little snappier and transmission shifting was unexpectedly quite improved. So I’d definitely check for ground quality. It won’t hurt to measure the positive side for V drop at load as well.

I got the ripple reading from a Topdon BT100 electronic battery / charging system tester.

 

[ScottT101]

Yeah I totally missed that …

I mean, diode pack is what I’d look at too. Note that lower-end multimeters are tuned on the AC side for 60hz, unless it’s a true RMS meter, so the 300-400, which I assume are millivolts, may not be accurate.

Enough line noise like that and I could see the regulator getting thrown off with wild readings it’s endlessly compensating for. Other things to test would be grounds. Set to DC volts and measure the V between battery post ground and different points such as alternator frame, engine block, firewall. Do this with all accessories on. If you see much more than 0.3 volts or so (at full load) then I might start looking for bad grounds or adding jumpers. I had a Chrysler once that metered at 1.1V, with flickering lights. After adding a 4-point ground harness, the flickering was still there but throttle response was a little snappier and transmission shifting was unexpectedly quite improved. So I’d definitely check for ground quality. It won’t hurt to measure the positive side for V drop at load as well.

Sounds like some good suggestions. I have been concerned that the voltage fluctuations could be throwing off the sensors and affecting the engine efficiency, but I have never really noticed any changes in gas mileage or drive feel in the car since it was almost new.

 

[f355spider]

Heat can definitely cause intermittent voltage regulator issues, high or low. Seen it before. I would be concerned with 16.8 vt taking out some electronics...Its not expecting to see higher than 15vt. But if the car is not worth it...just drive it and not worry about it.