Low voltage with key "ON"

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silverkaturbo
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Joined: Thu Oct 11, 2007 1:00 pm
Car: 91 Nissan 240sx

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So I just got done rebuilding my motor after a rod knock at 230k mi with my last turbo setup and now there is some kind of electrical issue that Im having trouble finding. At first I was having trouble getting it to even crank with a battery jumper and an extra battery powering it but now with a fresh charge the battery will slowly crank it on its own if I jump the starter solenoid straight to the battery cable.

Ive checked the voltage at the battery and it shows about 11.8 but as soon as I turn the key to the on position it drops down to around 9.3. While turning it to crank it stays the same at 9.3. While its actually running it does keep around 14 though. This is all with a relatively new alternator and a brand new starter.

I have checked the wiring harness dozens of times and checked and cleaned ground several times also. One thing I have recently noticed though is that every now and then, after having the key at the on position, I can turn it off and take it out, but the dash lights stay on until I put the key back in, turn it back to on and off again, then they will go off as usual. Would this indicate a bad ignition switch drawing the power? Even before rebuilding anything I noticed something drawing power because if I let the car sit for maybe a week, it wouldnt be able to start again, which led me to putting my big original battery in the trunk instead of using just the small one next to the cold pipe.

Any ideas or suggestions? This is driving me crazy.


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C-Kwik
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With most electrical issues, the key is to methodically test to rule out problems until you find the problem. While the two issues might be related, I would start out assuming its not until you test every other possibility first. I would also start with the starting system as it is more accessible and a much simpler circuit. I doubt the ignition switch is drawing enough power to prevent the car from starting btw. With that kind of amperage, you'ld likely have a blown fuse. Your battery may be low. IIRC, most of my batteries are usually at something like 12.10V. Not a big difference, but perhaps coupled to a smaller battery, the voltage drop is more severe. What is it physically doing when it fails to crank? Perhaps try hooking up the bigger battery and see if there is a difference. Try swapping the starter relay with another component that uses the same relay to rule that out.

Once you resolve the starting issue or rule out all possibilities, then move onto the lighting issue. Unless there are some wires between the two systems shorting, I doubt they are related. You may need access to the FSM so you can follow the troubleshooting workflow (recommended for both issues if you're uncertain about how to proceed). Generally, when you follow the workflow it will become clear what needs to be fixed.

Lastly, I'd check the power draw with a multi-meter. If its higher than it should be, then you will need to find that. But if it has normal readings, its likely that your battery is too small for the application. Smaller batteries have less Watt-Hours (energy). For a given amount of energy used by the car, the voltage will drop more in a smaller battery than with a bigger battery. For a given amount of current, less voltage means less power is provided (Power = Volts x Current).

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silverkaturbo
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Car: 91 Nissan 240sx

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Well I feel like power supply shuldn't be a problem because Im actually running both batteries now; the small one up front and a bigger size truck battery in the trunk. Ive tried swapping out relays with no changes so the starter relay should be good too.

But when I turn the key, (well touch my jumper wire to the battery now) all i hear is one click and then nothing. If there is a little power like if Ive recently had the car running, it takes a second, but then it slowly starts cranking then gets fast enough to start up.

I did some more testing today. Checked voltage at the front battery with key off and read 12.1, with key on it dropped down to 9.8. I started pulling relays and fuses to see if it would change and when I pull the grey alternator fuse voltage bumped up to 10.1v. So I put it back and tried the rest and when I pull the green 30 amp IGN fuse it bumped up from the 9.8 to 10.3v. I know that this should help narrow it down but Im still unsure what to do from here, especially since im still almost 2 volts too low even with these 2 discoveries. Not really sure how to do the power draw test so i need to look that up and get back to you.

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zerepdivad
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Car: '90 240sx . '02 Lexus IS300
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How new is this battery? You shouldn't see that significant of a drop with the key on. Have you had the battery load tested?
If the voltage AT THE BATTERY is getting that low then that points to a battery issue so long as you're metering right at the battery posts.

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silverkaturbo
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Car: 91 Nissan 240sx

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The small battery that I have up front is probably a year old, the bigger battery I have mounted in the trunk is prob around 2. I just find it hard to believe it to be a battery issue unless they can suck power into them if they are going bad. I say this because the first time I started the car on the rebuild I had the big trunk mounted battery hooked up, the battery charger set to jumpstart hooked up, and it still wouldn't crank until I also hooked up my running truck with jumper cables. With 3 different sources it still would just barely turn over.

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zerepdivad
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Well just turning on accessories real quickly should not drop the voltage of any good battery to 10 volts. I could blast my stereo system for a good while before getting that low. I'd get the batteries load tested personally. Or try another KNOW good battery like out of a car that's driven on the regular.
Most parts stores do free battery testing if you bring the battery in to them.

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silverkaturbo
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Car: 91 Nissan 240sx

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Right, I drive that truck every day that I used to help jump the 240 with, and it still cranked super slowly. Battery in my truck is only a few months old... I seriously, seriously doubt this is a power supply issue. I mean how much more power can it possibly need to crank like normal if it was just a low battery since I've used a battery (which worked fine before I rebuilt everything), a running vehicle, and a jumper box all at the same time and it still wants to turn slow.

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zerepdivad
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Car: '90 240sx . '02 Lexus IS300
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There's no other real reason you would be reading 10 volts AT THE BATTERY itself if it was a good battery. It's not going to discharge that quickly....
Could be a combo of things really.

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lexcrob
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I would try doing a voltage drop test on your ignition. Whip out the multimeter and put it on positive terminal and on the starter where the positive connects. Have someone crank it over and WALA the amount of voltage that comes up is lost in the wiring you've got bridged or the positive circuit. You can do the same test for the ground side this should point you in right direction.

As far as the ignition switch thing your on the right track pulling fuses but id utilize the multimeter again. Its called parasidic draw test you may already know how to do it but ill refreash. I always hated doing it or forgot but anyways.

1-turn ignition switch off
2-disconnect negative batt. terminal
3-jump connection with meter
4-this is your "draw
5- .02 volts is normal anymore and sumthings suckin juice

tips: obviously pull fuses to eliminate circuits / most meters only go up to 10 ohms in other words if you open a door crank engine hit headlights POP goes your meters fuse / normally you have to give it a few minutes after first connection security systems bcm,pcm etc may draw for a bit

GOODLUCK !!!!

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zerepdivad
Posts: 2010
Joined: Thu Nov 24, 2005 5:39 pm
Car: '90 240sx . '02 Lexus IS300
Location: WI

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lexcrob wrote:I would try doing a voltage drop test on your ignition. Whip out the multimeter and put it on positive terminal and on the starter where the positive connects. Have someone crank it over and WALA the amount of voltage that comes up is lost in the wiring you've got bridged or the positive circuit. You can do the same test for the ground side this should point you in right direction.

As far as the ignition switch thing your on the right track pulling fuses but id utilize the multimeter again. Its called parasidic draw test you may already know how to do it but ill refreash. I always hated doing it or forgot but anyways.

1-turn ignition switch off
2-disconnect negative batt. terminal
3-jump connection with meter
4-this is your "draw
5- .02 volts is normal anymore and sumthings suckin juice

tips: obviously pull fuses to eliminate circuits / most meters only go up to 10 ohms in other words if you open a door crank engine hit headlights POP goes your meters fuse / normally you have to give it a few minutes after first connection security systems bcm,pcm etc may draw for a bit

GOODLUCK !!!!
so much misinformation/terminology i don't even know where to start....

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lexcrob
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Just trying to help

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silverkaturbo
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Car: 91 Nissan 240sx

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silverkaturbo wrote: unless they can suck power into them if they are going bad.
^^^Apparently something like this is true... I pulled out both batteries I was running, took the one from my truck and hooked it up in the trunk of my car and she fired right up. I just figured if I had a good battery hooked up too there was no reason for the power to still be low.Thank you guys for your help. Once you said 10v AT THE BATTERY i was like, oh god that HAS to be the problem. Im retarded... :facepalm:

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zerepdivad
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Car: '90 240sx . '02 Lexus IS300
Location: WI

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lexcrob wrote:Just trying to help
You had the right idea just lots of wrong wording.

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C-Kwik
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silverkaturbo wrote:
silverkaturbo wrote: unless they can suck power into them if they are going bad.
^^^Apparently something like this is true... I pulled out both batteries I was running, took the one from my truck and hooked it up in the trunk of my car and she fired right up. I just figured if I had a good battery hooked up too there was no reason for the power to still be low.Thank you guys for your help. Once you said 10v AT THE BATTERY i was like, oh god that HAS to be the problem. Im retarded... :facepalm:
Chiming back in a bit late here, but actually this is quite true. A battery draws power when a higher voltage is applied to it (generally). Its basically how any rechargable battery is charged. The greater voltage potential from the charger causes energy to go into the battery increasing its charge. Further, if you have a dying battery hooked up to a good battery, the dying battery will suck energy out of the good battery until the voltage for both batteries become equal. In some severe cases, I've seen where a car couldn't be jump started due to the dead battery drawing too much current (taking away the available current from the starter). We had to disconnect the battery entirely and jumpstart using just the terminals to get it started.

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WDRacing
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Similar to opening 2 air tanks that are connected, the pressure will equalize until each has the same. I learned that the hard way in welding shop.

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C-Kwik
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WDRacing wrote:Similar to opening 2 air tanks that are connected, the pressure will equalize until each has the same. I learned that the hard way in welding shop.
Pretty much. There is a reason why pressure and flow of a fluid are analogous to voltage and current flow of electricity.

Don't you learn everything the hard way? ;)

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WDRacing
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Yup, but I usually only need the lesson once.


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