ECM power draw...

[mytmouz]

I finally got my project out of transmission jail. It has an electrical gremlin or 3, one of which is the alternator not charging... I think? I got a handle on that now... The main one is the battery drain with the truck sitting, and the key off. As a test I unplugged the ECM and the voltage went up approximately .03 amps. I am going to leave it unplugged to see if this is the issue,but thought I would eek some input. Now to my question... Is that something you experienced swap guys normally see? The vehicle does have a radio that has the power wire to keep the clock up to date on it, and was in the truck for years before I did the conversion...

[Six_Shooter]

You mean .03 volts, amps are different thing and in this case if you were reading current (amps), the current would drop when a draw is removed, not go up. ;)

What you need to do is measure the key off battery drain drain by using the current setting on a DMM. Disconnect the battery (either lead is fine, though I prefer using the negative terminal for this in most cases), and connect the DMM in series between the battery terminal you just disconnected and the battery lead you disconnected from it. Wait for everything electrically to settle, usually about 5 minutes and see what the current draw is then. It should be less than .010 of an amp, preferably, less than .008A The lower the better here.

On the voltage reading side I have seen batteries with nothing connected to them vary in voltage more than that .03V that you mentioned just sitting.

[mytmouz]

You are correct volts. My bad. Test was done with the battery connected, and with the multimeter in series with the cable and battery. (Lead on one battery terminal, other on cable, cable not connected to battery). Voltage reading was the same in both instances. I got the voltage uptick checking the battery with the cable disconnected after doing the above test. I am using the factory wiring and fuel system on the chassis that had the original engine, and integrated the tbi harness to the truck, The only thing that has been added besides the tbi ECM that was not there originally is an electric fan/relay/thermostatic switch combo.

[Six_Shooter]

Whoa... wait a second....

You had the DMM in series with the connection to the battery but on a volts scale?

Not the correct way to do things...

You need to use the current side (amps) when connecting the the DMM in series. (There are reasons why someone would measure voltage in the way you did, but never for a current draw problem such as this, it's an advanced technique used in measuring current passed through a circuit using ohms law with a known shunt resistance.)

Do the test again, but use the DMM in a current scale and report what it reads. The current will read higher with the offending load connected, than when it it is removed. If you see less than .050 Amps, you're likely good, I like to see to less, down around .010 after all of the systems have settled. I give about 5 minutes usually for this happen, though some modern cars need upwards of a few hours before all of the systems go to "sleep".

[mytmouz]

Will do tomorrow if the rain holds off. I was using the meter fooling with the alternator, wasn't really looking for a current draw at the time, just decided to fool with it while I was there. I put a battery in it (it showed 12.58 for what it's worth yesterday after I hooked the cables up), and did not start it, just shut the hood. Almost exactly 24 hours later it was 12.05 volts with the cables hooked to the battery and no start attempt, doors opened for a dome light, etc. I did not charge the battery before immediately before I reinstalled it, but a week ago when I brought it from the trans shop. I have 2 different batteries that it has pulled down like this...

[mytmouz]

Will have to wait until I return to work in a couple of weeks for the amp draw check. I did get the alternator to charge and have been doing a few test drives.

[steveo]

As a test I unplugged the ECM and the voltage went up approximately .03 amps

did you wire this yourself?

your ecm has both constant battery and ignition switched power inputs. are you certain they're connected properly?

i'd expect that kind of behavior if everything was wired to constant power (or perhaps bat/ign wired backwards.. which would still run but draw a crapload of idle power)

[mytmouz]

Yes, I wired it using a modified stock harness. I used the pin outs available here, and I do have both keyed power and battery power. I unplugged the ECM to see if the battery goes down until I can get back to it. Currently, I have another project in the one stall garage.