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voltage drop

with no lights on my voltage says at 13.9 once I put my lights on it drops to 12.9.
I checked all the grounds up by the battery and cleaned and replaced them, I also changed the battery. No change, could I have a bad alternator. Are there any grounds to check in the engine compartment. Thanks John
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83 sc cab turbo widebody. 3.5 liter supertec motor, B&B headers, EFI electromotive TecGT, TurboKraft GT35R billet turbo, 964 cams, WM inj, Bell full bay intercooler, Short bell housing turbo 4 speed tranny, roll bar, hargett shifter,
Old 11-17-2018, 05:56 AM
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Quote:
Originally Posted by jbrown View Post
Are there any grounds to check in the engine compartment.
There's like 17 grounds on the entire car, some hidden away in strange places (like inside the dash). Recall seeing a diagram on the 911 forum.

Primary ones to check after the battery would be
  • body/transmission ground strap (totally exposed and can get horrible after only a few years - but cheap and relatively easy to get at) and
  • alternator/motor case ground lead. Inside the fan shroud, so not especially convenient to check/replace.

IIRC, cable from the battery positive post runs back to a terminal on the starter used to connect it to the alternator power feed; clean/tight connection there couldn't hurt either.
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Old 11-17-2018, 08:12 AM
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Quote:
Originally Posted by spuggy View Post
There's like 17 grounds on the entire car, some hidden away in strange places (like inside the dash). Recall seeing a diagram on the 911 forum.

Primary ones to check after the battery would be
  • body/transmission ground strap (totally exposed and can get horrible after only a few years - but cheap and relatively easy to get at) and
  • alternator/motor case ground lead. Inside the fan shroud, so not especially convenient to check/replace.

IIRC, cable from the battery positive post runs back to a terminal on the starter used to connect it to the alternator power feed; clean/tight connection there couldn't hurt either.
Thanks
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Old 11-17-2018, 10:43 AM
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In addition to grounds also check electrical connections by cdi box. I did all my grounds then found I had a bad female connector that was causing my charging issues. belmetric sells a lot of the oem barrel connectors for the electrical plugs in our cars.
Old 11-18-2018, 03:23 AM
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Not sure if it makes sense, but I recently had low voltage (12.x) while running, as well as rapid drain while parked. I found I had a few bad aftermarket relays in the trunk. Replaced them and now get about 14 volts while running. Of course AC on and lights on still cause some drop.
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Old 11-18-2018, 05:43 AM
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Where is the 12.9 being measured from? Battery terminals? A module somewhere in the vehicle? Directly from the B+ on the alt? The light fixture itself? Important as to where the drop is occurring across.
Old 11-18-2018, 06:46 AM
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Originally Posted by 993Speedster View Post
Where is the 12.9 being measured from? Battery terminals? A module somewhere in the vehicle? Directly from the B+ on the alt? The light fixture itself? Important as to where the drop is occurring across.
My car is EF fied, Iíve been checking the voltage at the ECU and also directly at the battery Iím trying not to take these fan shroud off so I wanted to tryAll other ground first
Old 11-18-2018, 07:31 AM
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I cleaned and changed all ground wires under front bonnet where the battery is now I was just curious where are the grounds are that I could check
Old 11-18-2018, 07:33 AM
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My car is EF fied, I’ve been checking the voltage at the ECU and also directly at the battery
What RPM are you measuring this at?

You do know the standard alternators in these cars are fairly pathetic at idle/low RPM, right?

Your 3.5 is SC-based? Do you have the original SC (75A) alternator, or did you update to the 3.2 (90A) alternator?


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Originally Posted by jbrown View Post
I’m trying not to take these fan shroud off so I wanted to tryAll other ground first
It would be hard to get the fan shroud off with the motor in the car - and impossible with the induction manifold on.

Taking the alternator out is fairly straightforward - this exposes the connections to the alternator at the back, as well as the ground strap that connects the alternator electrically to the motor case.

Both that, and the transmission->body strap need to be good.
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Old 11-18-2018, 07:54 AM
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I cleaned and changed all ground wires under front bonnet where the battery is now I was just curious where are the grounds are that I could check
12.9V with the lights on at idle doesn't sound far off "normal" for a stock alt, although it's been years since I stuck the fluke on mine.

My oil pressure gauge would change slightly when I switched on the lights at idle due to the voltage drop (those gauges are just voltmeters), and that was running a 90A alt after the external voltage regular on the 75A died. Slightly less pronounced with a 90A, but still did it.

I'd double check for ya - but much of my motor ancillaries currently dismantled to fit Classic Retrofit's 170A alternator (google "Nippon Denso 6-phase hairpin") and various other delayed/deferred upgrades....

IIRC, 2 ground "stars" in the frunk (by each headlight/marker light), 2 in the rear/engine bay by the rear light sockets, usually 1 or 2 by the old external voltage regulator on older cars, several inside the dash (accessed through the instrument holes and through the frunk after removing the squirrel cage) and various others dotted around the car as traps for the unwary (doing something unrelated, I was surprised to find one on the rear bumper shock that I had no idea was there...)

Follow the brown wires. Eventually, they'll meet up with some buddies and get bolted to the body.

Voltage drop has many possible causes on these cars. Some is to be expected, the trick is to figure out what's "normal". Issues could be weak alternator, bad grounds, bad connectors, old wiring. etc. etc. Higher-wattage bulbs cause/show more voltage drop too.

If you're serious about lighting, use headlight relays, ATO fuse blocks, heavier bulb harness etc etc. All of which maximize the light output of stock wattage bulbs by reducing voltage drop across the bulb terminals.

Voltage drop at the battery terminals smacks of an alternator that either isn't well-connected - or simply doesn't make enough opmh to keep up. If voltage rises with RPM and you've got a stock alt, probably isn't that unusual.

A modern alternator will put out double the amperage at idle purely from improved performance from better design. An "old" alternator might not put out as much as it should because it's tired - but even in perfect condition can't put out as much at low RPM as a more modern design.
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Old 11-18-2018, 08:56 AM
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Quote:
Originally Posted by spuggy View Post
What RPM are you measuring this at?

You do know the standard alternators in these cars are fairly pathetic at idle/low RPM, right?

Your 3.5 is SC-based? Do you have the original SC (75A) alternator, or did you update to the 3.2 (90A) alternator?



It would be hard to get the fan shroud off with the motor in the car - and impossible with the induction manifold on.

Taking the alternator out is fairly straightforward - this exposes the connections to the alternator at the back, as well as the ground strap that connects the alternator electrically to the motor case.
Both that, and the transmission->body strap need to be good.
Nothing is original on this car lol... The alternator was upgraded about 5 yrs ago to a 120amp alternator. I bought it from a vendor that was recommended on this site. I don't remember there name but they came highly recommended.
The reason I know I have a problem is because it never did this before. I had the motor out to do a clutch last year. I should have had all the grounds in the back check and replaced then. I am just going to upgrade all the grounds and take out the alternator and upgrade all wires..
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Old 11-18-2018, 03:04 PM
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