Formy95

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?

DisasterFormula

iTrader: (1)

Probably some corrosion somewhere adding resistance.

katz_2003

iTrader: (5)

Mine does the same thing for some reason! I dont know what it is..mine shows as low as 11.3ish on datamaster (yellow on gauge). I would like to know why also!

ROCNDAV

iTrader: (100)

Mine was doing the same thing. I Ran the big three and it seemed to clear up all of those issues.

Formy95

What do you mean by "the big three"?

ROCNDAV

iTrader: (100)

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.

katz_2003

iTrader: (5)

I'm gonna try that in the next day or two and see if that cures my problem.

Formy95

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.

guppymech

iTrader: (7)

what kind of VOM are you using...a cheap one or something good like a Fluke meter? If you're seeing 13v on your scan tool with the engine running, I'd say you're doing OK.

Formy95

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.

guppymech

iTrader: (7)

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

guppymech

iTrader: (7)

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.

Formy95

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.

Formy95

Whoops, wrong connector, I was looking at C210 in the diagram

jaycenk

iTrader: (1)

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.

Formy95

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?

guppymech

iTrader: (7)

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

Formy95

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.