dailo

Hi

In my 1991, I ran a scan with my laptop. Comparing the DVM on the battery to the reading at the ECM there is more than a 2 volt difference.

What would cause the ECM to sense a reduced voltage, would this be from the ignition switch contacts in the steering column?
Could this cause the alternator to work harder than it needs to?

I also checked the wires coming from the alternator plug, and there is also about a 0.8 voltage drop across these.

There is less than 0.2 voltage drop from the alternator output wire to the battery.

Is there a specific test for the ignition switch?

Cheers!

Cliff Harris

The ECM measures the voltage from the ignition switch. It goes through the ignition switch, connectors, wiring, fuses and other obstacles to get to the ECM, so there is some voltage drop along the way.

The ECM uses this voltage to do several things, mostly to tell when the ignition switch is turned on or off. It also will shut down the ECM to protect itself if the voltage is too high (over 17 volts). If the voltage is too low it will shut off the engine. One other function is to adjust the pulse width of the injectors to compensate for voltage variations.

Try measuring the voltage at the ECM with your DVM and see how that compares to the voltage the scan software is reporting.

dailo

OK will give that a go, thanks.

I also spent some time today taking down the steering column, and unplugging the ignition switch plugs. Sprayed contact cleaner into the switch and into the plugs.

Just was wondering if the drop was normal, which it seems to be, and whether or not the ECM has anything to do regulating the voltage or amperage of the alternator.

I remeasured the plugs on the alternator after cleaning, the main output wire reads exact battery voltage.

The larger wire in the three wire plug reads battery voltage, with the key on, the smaller wire in the middle reads 0.5 volts lower than battery voltage.

I guess the ignition switch drops voltage by 0.3? So I'm only missing 0.2 which shouldnt be too much should it?

Cheers

vetteoz

What fuel pump volts is your scanner showing?
FP usually displays at a similar value to the Batt V as seen by the ECM

dailo

I'll have another look tomorrow and post back, thanks!

I take it you're in Aus? Do you have a RHD as well?
I'm thinking something may have happened during the conversion when the battery was moved over to the other side that is causing this to happen.

I also have a drain of 150-170mA, or .15 to .17 Amps after the key is out of the ignition.

I was out removing every fuse along with every relay that exists in the car, it was still there with the aftermarket alarm, radio and amplifiers unplugged.

Must be somewhere from the fusible link junction block, I'll also be checking that in the next few days once I have space to lift the car up.

vetteoz

PM sent
Start with the headlight motors;known drain spot
If the limit switch inside motor doesn't close the motor will continue to pull power
Try turning the manual adjuster ****; if motor is powered up it will fight against you