Battery Voltage
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
Battery Voltage My 95 Formula was sitting for several years before I replaced the engine a year or so ago. Ever since I did the engine swap, the battery voltage at the gauge/scan tool reads low. It will read 13.5 at the battery via volt meter, but only 11.5-12.5 at the gauge. Yesterday I probed the wire for the gauge coming off the back of the alternator, and its showing the same as the gauge. That seems odd to me since its reading 13.5 at the battery post of the alternator. I replaced the alternator about 3 months ago because the old one had a bearing squealing in it, but it made no difference in the voltages.
Does anyone have any idea why the gauge/computer is reading so low?
Does anyone have any idea why the gauge/computer is reading so low?
Replace the 3 cables (Battery to Alternator, Battery to Starter & Battery to Fusebox) and grounds with 0 gauge battery cable.
You really don't need that big of wire if you aren't running after market amps or electronics, but it doesn't hurt.
Simply checking the cables and connections for corrosion, and repairing/replacing bad connections will probably do the job.
You really don't need that big of wire if you aren't running after market amps or electronics, but it doesn't hurt.
Simply checking the cables and connections for corrosion, and repairing/replacing bad connections will probably do the job.
Trending Topics
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
Okay, so I took the cables loose from the battery and cleaned them. Problem still exists. The car has been sitting for about a week now.
Engine off, battery voltage: 12.5.
Engine on, immediately after start, battery voltage: 14.5
Voltage at back of alternator: 14.5
Voltage at distribution block: 14.5
Voltage on gauge: 13
Still seems that the gauge/computer consistently read about 1.5 lower than what is at the batter/alternator.
Engine off, battery voltage: 12.5.
Engine on, immediately after start, battery voltage: 14.5
Voltage at back of alternator: 14.5
Voltage at distribution block: 14.5
Voltage on gauge: 13
Still seems that the gauge/computer consistently read about 1.5 lower than what is at the batter/alternator.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Topdon ONE vs. Artidiag 800 BT2: Which is the Diagnostic Tablet For You?
Pouria Savadkouei
Gas Monkey Built a 6-Wheel Ferrari Testarossa With a Corvette LT4 Engine
Verdad Gallardo
7 Most Reliable High-Performance Engines GM Has Ever Built
Verdad Gallardo
Amazing '71 Camaro Restomod Is Modern Muscle Car Under the Skin
Verdad Gallardo
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
This is the volt meter I'm using http://www.summitracing.com/parts/ANM-CP7677/
It sees 13 volts right now, because there is 14.5 at the battery. But after it warms up and the alternator is only putting out 13.5, I only see 11.5 to 12 at the computer via scan-tool. The gauge inside the car reads the same thing the scan-tool does.
It sees 13 volts right now, because there is 14.5 at the battery. But after it warms up and the alternator is only putting out 13.5, I only see 11.5 to 12 at the computer via scan-tool. The gauge inside the car reads the same thing the scan-tool does.
check out this wiring diagram from Shoebox, it's for a '95. Look at the top of the diagram, the power for the pcm goes from the battery (B+) through connector C100 cavity D and then to the pcm. Try to locate the C100 connector and check voltage at cavity D. Or check resistance back to the battery after you disconnect both ends of the wire....look high resistance or corroded terminals.
I don't have access to a '95 service manual otherwise I could tell you where the C100 connector is.
http://shbox.com/1/1995_pcm3.jpg
I don't have access to a '95 service manual otherwise I could tell you where the C100 connector is.
http://shbox.com/1/1995_pcm3.jpg
C100 is in the circle with the PCM connectors in this pic. click on pic for detail.
http://shbox.com/1/harness.htm
Thx shoebox for ref.
http://shbox.com/1/harness.htm
Thx shoebox for ref.
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
I was actually thinking about it this morning and I was going to pull up that diagram and see if I could figure out where the PCM power was coming from, but looks like you beat me to the idea 
As it happens, I think I still have the right side kick panel off as I was having to track down an intermittent short in the PCM fused wire. Turns out one of the fuel pump relay wires was rubbing up against some metal and shorting out.
As it happens, I think I still have the right side kick panel off as I was having to track down an intermittent short in the PCM fused wire. Turns out one of the fuel pump relay wires was rubbing up against some metal and shorting out.
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
Whoops, wrong connector, I was looking at C210 in the diagram
Check and lube the grounds for the wire harness. Any paint or rust will cause a low voltage reading. You could use a jumper to jump the batt ground to the harness grounds and see if there is a change down the line. If so, Bad grounds.
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
I just got done looking at it. Battery voltage is 12.25 at the battery. The C100 connector is showing 12.25. I also checked using the sensor ground off the drivers side head and it is also showing 12.25. The scan tool shows 11.6. Are there any other sensor grounds? PCM ground itself?
Disconnect the BLACK connector from the PCM and check the resistance from connector C100 to cavities 15 and 31 in the Black Pcm connector. Or check from the positive battery cable (with it disconnected) the the black pcm connector to check the whole wire run.
http://shbox.com/1/1995_pcm_conn_b.jpg
http://shbox.com/1/1995_pcm_conn_b.jpg
Last edited by guppymech; Aug 18, 2012 at 12:06 PM.
Thread Starter
Registered User
Joined: Aug 2012
Posts: 8
Likes: 0
From: Deltona, FL
I checked the resistance to those to pins, and it was 0.01. However, I did notice that pin 15 for the sensor ground had some damage to the wire and when I checked its resistance to ground, it was high, and intermittently even higher. I tried jumping it to a good ground, but my voltage issue still remains. So I suppose I need to track down that ground wire and see why its not making a good ground.
