Voltage issues: MHD/Dash reading low, but multimeter reads normal?

tofu? · 09-03-2024, 03:14 PM

I tested at the battery, at the alternator (grounded to the alternator housing), and at the aux power under the hood. I get 14.7v all around.

When i read mhd codes or check in-dash voltage (menu 9), I get a reading of 13.1-13.6

How do i troubleshoot this? Not sure which wires it could be

edit: IBS is unplugged to rule that out

dzid_ · 12-25-2024, 01:22 PM

Mine are also lower. But the difference is only 0.2V when driving. Probably similar diff when off.

On top of that I get battery discharge warning few minutes after shutdown (but the cranking works OK). I am thinking this voltage difference may contribute to the premature warning.

What is your voltage difference with ignition off and lights off? If the issue is the wire resistance then the delta should probably get lower.

Do you get low battery warnings as well?
For me even longer trips don't help much with low battery voltage, so I a think there might be a second issue there, but probably unrelated.

Btw, I noticed that in Park right after the ignition the charging current is the highest (56A @15.1V !!). I messure using current clamp. I may leave it in this state as a way to fully charge the battery.
After I start driving charging falls to 8A (@ around 14.4V). I need logs of what happens on longer trips, but I digress...

09-03-2024, 03:14 PM	#1
tofu? Second Lieutenant 89 Rep 245 Posts Drives: e92 n54 Join Date: Dec 2013 Location: NYC 11385 iTrader: (0)	Voltage issues: MHD/Dash reading low, but multimeter reads normal? I tested at the battery, at the alternator (grounded to the alternator housing), and at the aux power under the hood. I get 14.7v all around. When i read mhd codes or check in-dash voltage (menu 9), I get a reading of 13.1-13.6 How do i troubleshoot this? Not sure which wires it could be edit: IBS is unplugged to rule that out Last edited by tofu?; 09-03-2024 at 03:20 PM..
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12-25-2024, 01:22 PM	#2
dzid_ Enlisted Member 10 Rep 41 Posts Drives: 135i 2011 DCT Join Date: Mar 2016 Location: Mi iTrader: (0)	Mine are also lower. But the difference is only 0.2V when driving. Probably similar diff when off. On top of that I get battery discharge warning few minutes after shutdown (but the cranking works OK). I am thinking this voltage difference may contribute to the premature warning. What is your voltage difference with ignition off and lights off? If the issue is the wire resistance then the delta should probably get lower. Do you get low battery warnings as well? For me even longer trips don't help much with low battery voltage, so I a think there might be a second issue there, but probably unrelated. Btw, I noticed that in Park right after the ignition the charging current is the highest (56A @15.1V !!). I messure using current clamp. I may leave it in this state as a way to fully charge the battery. After I start driving charging falls to 8A (@ around 14.4V). I need logs of what happens on longer trips, but I digress...
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