Control Module Voltage

[monte4ever]

I've been monitoring volts since I've noticed during WOT the volts would drop to 12.5ish. I've done the "Big 3" upgrade and even added a 1/0 gauge ground wire from the 270 amp Alternator mounting bolt to the engine block and it still drops volts according to the scanner. So today I checked the voltage reading off the Alternator mounting stud with the car idling and it read 14.6ish. Then I checked the battery, which is mounted in the trunk and it read 14.5ish. This is where I'm concussed, the scanner is logging Control Module Voltage and it reads almost exactly 1 volt less at 13.5ish. I also have a Autometer volt gauge and it reads 14.5.

My question is why does the scanner (ECM) read less than all the other test locations? I know it's not because of how it's wired because the ECM gets it constant power from the Alternator stud. The ECM is a P10 411 with a 12216125 Flex Fuel OS.

[blyne3]

a bad ground will show low voltage on the module.

[monte4ever]

I've checked all my grounds but maybe I missed something. Ima recheck them to be sure. What you're referring to is the ECM ground right?

[blyne3]

Yep, there should be a couple of them.

[blyne3]

also assuming the ecm is grounded to engine and there is an adequate ground to the block from the battery.

[monte4ever]

Yep. I'm running 1/0 pos and neg cables to and from the battery that's why I'm lost as to what's going on. I'm sure the ground on the back of the cylinder head is good and tight too. Also have 1/0 gauge going from the block to the frame horn and another running from the battery to a stud I welded to the trunk floor and frame. By the way, this is a LS swapped into an 82 Monte.

[monte4ever]

I check the ohms of the ECM ground wire location with a DMM and came up with .06. I did this by touching the ECM case with one probe and the gounding screw with the other. Is .06 ohms a good reading or should it be closer to .01? The ground I'm referring to is the chassis ground. The cylinder head engine ground is good.

[blyne3]

Voltage drop them by checking voltage on the ground wire to ground with either key on or engine running. It will tell you a lot more than ohming it. Voltage drop should be .2 volt or lower.

[blyne3]

you will have to back probe at the ecm to check it.

[monte4ever]

Ok, I checked and rechecked all the grounds and they all checked out to be good. I then ran a separate ground wire directly from the battery to the chassis ECM ground wire and still it shows approx 1 volt less than what the Alternator and battery shows (14.5). I also probed the engine block and ECM case with my DMM and it showed .01 ohms which is good. So my test show that there isn't any grounding issues.

[monte4ever]

After seeing your post blyne3 I went and started the car and performed the voltage drop test on the chassis grounding location to the ECM case and it showed .028.

EDIT: Went back out and probed pin 1 and 40 on both connectors and got around .002 volts.

[Mr. Smith]

I've always had the same problem.. and thought it was a problem with my OBD2 wiring.. like I used old wires which caused a bit of resistance or something.. or I didn't hook up all the little PCM grounds. Idk.

I just want to know how many volts my injectors and f.pump really get at WOT. Like when the scanner says 12.5v, is it 12.5v at the motor, or just the OBD2?

Have you figured out the problem or did you give up?
Anyone else with the same problem?

[Jason B]

Anyone else with the same problem?

Maybe. Is this voltage causing anyone a problem?
I noticed when I got hptuners 10 or so years ago the voltage read low on a bone stock never touched car. So I checked a few other cars- same thing, low voltage.

So instead of going through all the bs above I just got my popular not cheap dvom out and read the voltage at the pcm, battery, alternator, cig lighter, aldl connector, etc.
All 14.x volts. plug hptuners right in quick and it reads usually (and strangely) 11.x

I don't think I've ever seen it read out of the 11's. Even with the mpvi pro i bought 6 months ago.

[Mr. Smith]

Idk if it's causing me problems, that's the thing. What I do know is that I want 14+ volts when I'm at 6,300rpm and not 12.4-12.7v which shows on the scanner.

I think this might have to do with the ignition..... resistance from the ignition area (wiring, contacts...) to the OBD2 plug on older cars.
My car is also from the 80's (like monte4ever) and the stock battery gauge shows about the same as what the scanner shows.
Mine also shows around 11.6v with the ignition on, engine off.

My mates LS1-powered 13yo car (which comes with an LS1 from factory) shows around 14.4v at idle on the scanner (where it should be at and what I want).

[DrLoch]

I know this is an old thread, but I'm having the same issue. This is a 411 convert of a 1994 Impala SS with a modified old style LT-1 which now displaces 383 cuin. What go me to look at the voltages is I just changes my cooling fans to Derale 2 speed fans and I wanted to see if it would have any impact on my charging system when both fans were @ high speed.

In one scan the car had been sitting so everything was cool/cold. IAT ~ 57 Coolant ~ 72, had to move car off lift to attach exhaust hoses to run outside before I started logging. In sanner with Key On Engine Off voltage showed 11.8. Started engine began to log, voltage was @ 14.10. 9 minutes in voltage dropped to 13.9/13.8. After 40 minutes and coolant @ 175 I was down to 13.5 volts. In this scan when the fans did come on in 1st speed the voltage would briefly dip and the recover.

I'm suspecting the voltage regulator in the alternator. I tried the diode trick and the output voltage of the alternator never changed.

I'm going to do more testing where I take voltage reading from various places to see if I am having voltage drop issues because of electrical heat, i.e. ignition switch. BTW when I first noticed this I thought I had grounding issues because my frame and core support inner inner wheel wells are powder coated. My battery is mounted in the trunk with 1/0 POS and NEG welding cable running to the front. POS to the starter POS terminal, NEG to the engine block. So I did the big 4 upgrade and I used a small belt sander to sand through the powder coating to get to bare sheet metal as well as add additional #2 grounds to the body, frame rails and core support and housing of the ECM. (#12 one end connected to the block) I may change the ground pin ECM terminals andt the house all to the same engine block ground location since I can't remember where BL-1, BL-40, RD-40 and RD-1 terminate.

Just curious, does anyone know which voltage pin is being scanned and being reported on the scanner?