# Voltage drops when lights turn on + voltage drops when radio plugged in



## miro1234 (Feb 20, 2021)

hi there,
i just bought 2004 xtrail (petrol) (t30). i am installing a new stereo and noticed it keeps restarting when keys are turned on (ACC).
so i measured voltage on the ACC vs GND and BAT vs GND and noticed it cycles from approx 12V or 13V down to 7V or 9V which is why my stereo keeps restarting (without engine running). Checking this further, I tried turning on the parking lights and for that, voltage drops completely down to mV (again engine not running). when lights are off or my stereo is unplugged, i get perfectly stable voltage (engine not running).

i am suspecting the light circuit, this however does not explain the drop when lights are off and load is put on the radio side. can anyone point me to any direction on what to check?


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## rogoman (Dec 16, 2004)

Could be due to a bad alternator or the battery is shot. A properly working charging system puts out about 13.2 to 15.0 volts, but this is a general spec, and the factory service manual should be referenced for the correct charging system voltage specifications for a particular vehicle. A battery should have a static charge of 12.2-12.6 volts. If a battery is not good, the charging system may not be able to charge properly. If a vehicle is not charging properly and the battery is good, the first thing to do is to turn the ignition switch to the "ON" position *without starting the engine* and make sure the charging system warning light is operating. If the bulb is burnt out, the charging system will not charge. If the bulb is OK but still does not illuminate, the circuit must be tested. If the warning lamp does illuminate, then the next thing to check is to make sure the circuit between the battery positive post (+), or fusible link, to the connection in back of the alternator is good. On Nissans, this should be a thick (approx. 10 gauge) white wire to the "BAT" post on the back of the alternator. With the negative cable (-) disconnected from the battery, measure the resistance between the "BAT" post on the back of the alternator and the battery positive (+) post; the resistance should not be greater then 0.2 Ohms. It's not uncommon for this wire to get corroded and burn up, creating resistance in the circuit. So, before assuming an alternator is bad, make sure this circuit is good and battery voltage is getting to the alternator. It's also important to make sure the alternator belt is tight and not slipping and the battery connections are clean and tight.


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## miro1234 (Feb 20, 2021)

I've measured voltage on alternator + and - and I am getting 14.8V. I will do further diagnostic on the charging system tomorrow. 
Battery has 12.7V everything off and 12.3V with ACC on and lights on. So to me, this sounds like acceptable values. 

The charging light is off, however comes up every now and then during drive which I have to investigate as well. So I'll do more diag on the charging light and the circuit as you described and will see what comes out of that. Thanks for your suggestions so far.


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## VStar650CL (Nov 12, 2020)

See if there's voltage drop across the Accessory Relay contacts and normal resistance through the relay coil (around 100 ohms for most Nissan relays). You may have a bad relay. Are you running an amp along with the radio, and if so, how is it powered? If it's wired off Acc and not the battery, you may be overloading the circuit. Amplifiers should generally be wired direct to battery with their own fuse, with only the "on" or "enable" signal wired to Acc. The intermittent generator light is more concerning, is your belt and tensioner healthy?


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## miro1234 (Feb 20, 2021)

No, there is no amp. Everything is original (at least by looking at the car) except for the stereo I am installing. 

The intermittent light looks more like if it was coming from vibrations or bumps. I am about to check things. I will provide more info.


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## VStar650CL (Nov 12, 2020)

miro1234 said:


> The intermittent light looks more like if it was coming from vibrations or bumps. I am about to check things. I will provide more info.


That would be consistent with a high-resistance connection somewhere, and if it causes voltage drop then it's also consistent with resetting the radio. Check your wiring, something is either loose or a bad pin-fit.


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## miro1234 (Feb 20, 2021)

So for the relays, I have checked all 3 on the fuse box (on the back) and 2 of them are giving me 75 ohms and one (IGN) 71.8 ohms. So since they are all 3 nearly the same, I am guessing that is ok.

Charging light is on when key is in ACC and once engine starts, it switches off.
Tested resistance on alternator + vs battery + and I was getting cca 0.9 ohm.

I had removed all fuses and put back to the fuse box + all the connectors were removed and put back and all looking good there.


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## miro1234 (Feb 20, 2021)

VStar650CL said:


> That would be consistent with a high-resistance connection somewhere, and if it causes voltage drop then it's also consistent with resetting the radio. Check your wiring, something is either loose or a bad pin-fit.


But in this case, the radio resets only when the engine is not running. This puts the alternator out of the game. Once the engine starts, I am getting stable voltage in the radio side.


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## VStar650CL (Nov 12, 2020)

miro1234 said:


> Once the engine starts, I am getting stable voltage in the radio side.


That puts almost everything out of the game except for your battery and the main supply path to the radio. The best way to find out what's happening is with voltage drop. Power up the Acc and put one leg of your voltmeter on battery+, then trace down-circuit with the other leg. The readings will tell you the voltage _differential_, i.e.,how much loss you're getting from the battery to a given point. With healthy connections you should lose no more than about 100 millivolts. If you find a point where the reading spikes, you know something between there and the battery has excessive resistance. The same trick applies to the ground side, just measure from battery-minus.


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## miro1234 (Feb 20, 2021)

Ok, so I just tried bypassing GND from baterry and that keeps it up. The voltage still drops 0.4 V on the radio side comparing to baterry, but at least it does not go down to 9V or so. So I am going to try to trace this down for now and we will see. Thanks for the suggestion.


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## VStar650CL (Nov 12, 2020)

You're most welcome. If the problem is in the main ground cable to either the chassis or engine, it will be easiest to find with the engine running and the headlights on. The extra current will maximize the voltage differential across the bad spot and make it easier to locate.


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## miro1234 (Feb 20, 2021)

Thanks for the tips and sorry for the reply. It was something with the adapter cable from original Nissan connection to the radio socket. There was some mixed up wiring which was causing this. I did not notice until going directly from battery/chassis. So all good now. Thanks for all your tips.


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