Power issue (Voltage in Negative)

Nissan 300ZX technical discussion forum: Maintenance, performance, installations, modifications, how-to's and troubleshooting.
caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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So i'm stumped, recently my car started running bad ( would idle but shudder when gas was applied until about 2500 RPMs) then one day anytime i applied pressure to pedal my stereo and a/c controller would turn off. Did a lot of trouble-guessing and than found out my negative cable coming from my body to the battery had 8-12V coming to it. With car running i'm getting 15-18V from alternator, battery tested bad(not surprised there). I have checked and re-seated multiple grounds and event went thru car from when i changed hearer core a few years a ago to make sure i didn't pinch or break some wires in there. I did noticed my driver door had a clicking sound and found window motor was pretty hot when power was applied and is not inop (this is as new as stereo/a/c controller problem) posting.php?f=315&mode=post&sid=22c863e ... 668d7b436b# but with motor disconnected i'm still getting voltage to negative cable. I know its a Short either component or wire but am lost at where to look next. I have tested negative cable to other chassis grounds and don't have voltage, only from cable to battery do i have voltage. side not, when i puled negative from battery while car was running(to test alternator) my wipers went full speed even-though they were off, with motor disconnected still have voltage on negative. Please let me know what you guys think i should do next my brain is tapped! Thanks!!posting.php?f=315&mode=post&sid=22c863e ... 668d7b436b#


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mdmellott
Posts: 1147
Joined: Mon Sep 02, 2019 3:32 pm
Car: '13 Kia Soul+ 2.0L AT
'02 Pathfinder SE 3.5L AT P/4WD
Location: SF Bay Area, CA

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Yikes! I would zero in on the alternator and its cables. It may be a fusible link that burned and shorted to ground. With the car running (don't do that anymore until you find this gremlin) getting 15-18V from the alternator is troubling. Output voltage should not be variable unless other things are being turned on while you are measuring the voltage and it should never be higher than 15V. I'd be hot too if I were a 12V device juiced with 18V. The regulator in the alternator may have failed in the worst way, opening the voltage flood gates with uncontrolled power. Remove the battery cables and do not start the engine again until the short circuit is found and corrected. I would also remove the alternator and take it in for a diagnostic check at an O'Reilly or AutoZone store. They'll do it free of charge.

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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so the variable voltage is from me giving gas and letting off. Idle load is around 14.5v (i rounded up). Alternator was rebuilt about a year ago, but i prob will take it in as i already wanted to. I will check fuseable link, thanks for the info!

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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caseyfool wrote:
Wed Nov 06, 2019 11:46 am
So i'm stumped, recently my car started running bad ( would idle but shudder when gas was applied until about 2500 RPMs) then one day anytime i applied pressure to pedal my stereo and a/c controller would turn off. Did a lot of trouble-guessing and than found out my negative cable coming from my body to the battery had 8-12V coming to it. With car running i'm getting 15-18V from alternator, battery tested bad(not surprised there). I have checked and re-seated multiple grounds and event went thru car from when i changed hearer core a few years a ago to make sure i didn't pinch or break some wires in there. I did noticed my driver door had a clicking sound and found window motor was pretty hot when power was applied and is not inop (this is as new as stereo/a/c controller problem) posting.php?f=315&mode=post&sid=22c863e ... 668d7b436b# but with motor disconnected i'm still getting voltage to negative cable. I know its a Short either component or wire but am lost at where to look next. I have tested negative cable to other chassis grounds and don't have voltage, only from cable to battery do i have voltage. side not, when i puled negative from battery while car was running(to test alternator) my wipers went full speed even-though they were off, with motor disconnected still have voltage on negative. Please let me know what you guys think i should do next my brain is tapped! Thanks!!posting.php?f=315&mode=post&sid=22c863e ... 668d7b436b#
Driver window motor is Inoperable***

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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also, the volatage is at the negative cable with car off, not running and key in any position to include out of solenoid

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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so i did some checking and removed fusable links and still had voltage on negative cable. I hooked negative back up to see if i could find warm (to touch) wire or component but didnt find any, on a side note after i disconnected negative and re-hooked up miltimeter there was only 3 volts present but it began to climb steadily until it got to its 12v and plateaued. going to try and get car moved, as its on a sloped driveway and dont trust it being jacked up on it to remove alternator. i also do not know how to remove alternator with everything still installed, last time i removed it i was changing water pump/thermostat and all that so i had everything in front removed and went that way...

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mdmellott
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Joined: Mon Sep 02, 2019 3:32 pm
Car: '13 Kia Soul+ 2.0L AT
'02 Pathfinder SE 3.5L AT P/4WD
Location: SF Bay Area, CA

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Check your relays as well. I was thinking the ignition relay may be stuck and or burned and that thing with your wipers turning on when they were off tells me there is a stuck of sticking relay or two somewhere.

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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mdmellott wrote:
Thu Nov 07, 2019 7:38 pm
Check your relays as well. I was thinking the ignition relay may be stuck and or burned and that thing with your wipers turning on when they were off tells me there is a stuck of sticking relay or two somewhere.
the wipers only came on when i disconnected negative from battery with car running, correct me if im wrong but wouldnt a stuck relay make them be on constantly, or until power is cut off? When im disconnecting the negative im breaking a circuit and im thinking its creating another thru the negative (so a short) somewhere....but i am so lost. i have my dash out and everything unpluged that comes with it, disconnected fuel pump. A/C brain and am still getting 12v on negative cable. :werd:

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mdmellott
Posts: 1147
Joined: Mon Sep 02, 2019 3:32 pm
Car: '13 Kia Soul+ 2.0L AT
'02 Pathfinder SE 3.5L AT P/4WD
Location: SF Bay Area, CA

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Hard to say for sure what a bad relay will do. Yeah, stuck usually means relay contacts have arced so badly the contacts have micro-welded together or the relay internals could have melted and fused positive to negative contacts. Ignition relays have been known to melt down so badly they fuse to the entire fuse/relay block making the whole block worthless. Very few engine components get voltage directly from the battery. A voltage reading at ground when your battery is disconnected could simply be stored capacitance in your coils feeding back through a grounded short circuit.

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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A voltage reading at ground when your battery is disconnected could simply be stored capacitance in your coils feeding back through a grounded short circuit.
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The reading would go down though wouldn’t it? If it was stored? When I disconnected they other day it was at 3v and then started climbing. I’m not very electrical savvy I know basics and some plane electrical but that doesn’t help me with car

caseyfool
Posts: 8
Joined: Sat Aug 15, 2015 8:41 am
Car: White 91 300zx

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So I did some more trouble guessing and have come to the conclusion that my battery is shorted, I get voltage everywhere on car from negative on battery, even radiator hose clamps, also got alternator tested (rebuilt about a year ago) and it is instant failing, more than likely voltage regulator. So I guess I’ll try those out and see what happens. Also going to ohm out positive and negative cables to ensure there’s no reading there, something I thought about today 🤔


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