ECM pulling voltage when off

How's it going guys, sorry for the up coming long thread but I need some help. As the title says I think my ECM is pulling voltage when the truck is off. History of the truck, its an 86' MJ with a 92' 4.0 and AX15 inside. Lately I have been having an issue of a dead battery over night, which is worse than usual for this truck, usually its at least a week. But anyway after finally getting fed up with it I pulled the negative cable, and jumped my multimeter between the negative terminal and negative cable.

I pulled every fuse under the dash, only one had an issue, and that is the dome fuse, but it only pulled 1 volt. So I went to the distribution center under the hood, pulled everything until I got to the Engine Control fuse, unplugged it and the voltage dropped to only 1 volt being pulled total now.

Plugged fuse back in and unplugged the computer, still was pulling 9 volts, got my service manual out to try find what else may be on the Engine Control fuse, found the transmission control module, the truck use to have an Aw4 behind the 4.0. So I unplugged that while the ECM was unplugged and voltage dropped to 1 volt total pull again. Then I plugged the ECM back in and it was back up to 9 volts.

Anyway my question is, does anyone know what an ECM should pull when off? Could my ECM be junk?

Try it again using the ammeter function in Amps. Do not turn on any other devices like the fan or blower and try to keep the doors closed. The fuel pump alone pulls 3A when the ign is in the ACC position. The current draw is what kills the batteries.

The standby current in my 99XJ is in the mA range.

Just like you were doing, you will hook the meter in series with the negative cable to the negative post of your batt. Your meter should have another socket for Amps. Just use caution as most meters can only handle 10A at most and you will blow a fuse.

Thanks for the reply. I went out and tried that and I get 0. Kinda weird to me. Tried it on the 10A setting and the 200ma setting. I will double check tomorrow afternoon though.

You have two power sources for the PCM and the TCU, one is constant power and the other is switched power from the ignition switch. The switched power is simply an electronic switch activated by the ignition switch that turns on the main power to the control modules.

Grounds can be funny (weird), when the current can't find it's way through the proper path, it is likely to take any path, the path with the lowest resistance.

Your issue kid of sounds like you are grounding or cross shorting (between two different circuits) through moisture. You can measure voltage through moisture, but the amps are usually low and the resistance is high.

When I pulled the carpet up on the drivers side in my 87, the splices in the dome lamp and some other circuits were a clump of green and white corrosion and rust on the floor boards. My battery had a significant draw with the vehicle shut down, my battery would drain in a few days.

You might also want to check out the fuse block (inside) for moisture leaking in the connector or brake fluid dripping on it from the clutch piston.

8Mud said:

You have two power sources for the PCM and the TCU, one is constant power and the other is switched power from the ignition switch....

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Exactly, zero voltage just tells you the power to the wire has been switched off.
But voltage on a wire does NOT mean something is on, or current is even flowing, it just means the wire is live.

If the PCM is switched on/off internally by the ignition switch lead, with the main power line connected directly to the battery always live, finding voltage on that wire doesn't mean its consuming electricity. It just means the wire to the PCM is live.

Someone mentioned before using an ammeter instead of a volt meter to check.

Voltage is the pressure (technically "potential") in the wire. Pressure in a water pipe doesn't mean water is flowing, it just means that a valve to the pipe hasn't been closed to cut off pressure to the pipe.

Amps is the flow, actual electrons moving in the wire. You have to put a flow meter in a water pipe to tell if the water is actually flowing, someone opened the valve at the other end. And the flow meter will tell you if its an acceptable trickle of water flow, of if its a rushing gush of water going through the pipe.

You can have voltage in a wire, but it might NOT have any amps (current) moving at all, and thus no power is being consumed by the PCM. That is why you check amps, and you have to put the meter leads in series, i.e. disconnect it and put a lead on the pin of each connecter. If you see amps moving, its consuming electricity and it will tell you have much, less than 40milliamps is nothing to be concerned with. More than 200milliamps, you might have a problem and battery going dead early.

Do the check at the battery terminal first, everything off, no under hood or dome lights, etc, disconnect the positive battery cable and put the ammeter in series with the terminal and battery post. If the ignition off current draw is greater than spec you know you have problem somewhere, if its less than spec, you know you don't have a problem. Usually less than 200milliamps is acceptable, but most vehicles are 40milliamps or less.

Some PCM's turn themselves on to do self-checks or emission checks while the vehicle is off, or stay on for another 20 minutes, after the vehicle is turned off to do those checks. I believe "but don't know for sure", those are all 1996 or later PCM's that had to be OBDII compliant. But its always possible that earlier PCM's had some sort of feature to do this for a self-check.

Remember when testing for amperage that you need the meter to be the only path for current to flow through. Amperage is the number of electrons passing through the meter and the meter naturally has a small amount of resistance so electrons would rather run through the available circuit instead of the meter. to test for battery draw disconnect the battery cable and put the meter in as part of the circuit, the only path for those electrons to flow.

joejeep92 said:

...to test for battery draw disconnect the battery cable and put the meter in as part of the circuit, the only path for those electrons to flow.

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Exactly, also the battery would be easier to test than trying to put the meter in series with the PCM and it's connectors.

If the Ignition Off Draw (IOD) on the battery is within spec, you don't have a problem, no need to keep messing with the PCM. If its too high, then you start isolating circuits and trying to find the offending one and fix it.

It might be easier to keep measuring at the battery and just pull fuses, until you see the IOD drop, instead of trying to measure everything at connector to the device.

Remember, everything has to be off, don't miss the fact that by having the hood open the under hood light is on, or leaving a door open so a dome light is on. Close everything up and make sure its off, like if the car was shut off and parked, disconnect the light under the hood or pull the bulb.

Several years ago I checked the switch off current draw
on my 89 (Renix), and it was about 14-16mA. My 96
draw is about 8mA, half of what the Renix was. These
should be normal values for the corresponding models.

The underhood light has been mentioned. Another light
to check is the glove box bulb. It can be on constantly
and will never be seen (with the door closed).

Thanks for all the advice.

I double checked it again. Like before, multimeter in series with negative battery terminal and negative cable.On the 200ma setting I get 0. When I switch to the 10A setting it will jump to .05 but then slowly drop to 0 after a couple seconds. Doors shut, key off, no under hood light on this truck, ECM still connected, glove box light bulb removed (it burnt out a while ago). Has me stumped....

you may have an open fuse in your meter, with everything else off (especially blower), radio, and you put the ign into acc the fuel pump should come on and draw a few amps. (do this in the 10A range). If you still have zero, then check the fuse in your meter. Some meters have separate sockets for Amps and volts, so make sure you are plugged into the correct jack.

Copy will try again with stuff on and see.

I tried both the 10A and comm socket a couple times. I have one of those cheaper craftsman multimeters with the 3 sockets at the bottom, 10A, comm, and the red one for volts/ohms

I wouldn't do it with ignition in accessory...could blow your low amp fuse. If you are looking for battery draw then you are looking for key off amperage anyway. Normal ranges are under 50 mA.

Ok so went back at it again. Charged the battery, multimeter in 10A direct current, using the 10A slot and comm slot I get .15 amps. Pull the dome fuse and I get 0. Doors closed, no lights on or anything. So I guess track down either how it is getting power or what is shorting to ground.

Yes, .150 is 150 milliamps, that is more than 4 times what should be drawing on standby. Even pulling the dome fuse, you should still have a little bit of residual. Somewhere around .020 - .040 .

Considering you were only drawing .150, you should be able to safely use the 200mA range on your meter. Time to get a helper and start pulling fuses.

If that's a 10 amp fuse, there's a ton of stuff on that
circuit. Including radio memory, courtesy light (under
dash) pwr antenna, glove box, vanity mirror, etc.

If vanity mirror equipped, check that. I once had that
light stay on with a Mustang, drove me insane trying
to find the draw...:confused1

Roggom, I believe youre right about there should be residual. But I think all that residual is on this fuse, clock memory, radio memory, dome lights (if door opens) etc. But something in this circuit is probably shorted to ground and pulling excessive power than needed. So I think instead of pulling fuses, its time to pull whats on this circuit.

RunninOnEmpty, luckily this truck isnt like a cherokee with all those lights and what not. There should be only a handful of things on this circuit.

I did notice the stereo cycle on like it just got hooked to battery power as I messed with the connection at the multimeter ends, ended up pulling .22 when it did that and back down to .15. So Ill start with the radio and work my way around.

Found the draw....the stereo. Now to figure out how it is doing that.

Fixed it, 12v ignition was hooked to 12v memory, that was my bad in initial install of stereo I bet. Hooked it back up right and now I get .02. From what you all have mentioned that is within spec and makes sense for the 12v memory side of the radio.

Thank you guys, I really appreciate the help. Hopefully now I can fire this thing up after not running it for a couple days and go hit the trails on the weekends!

Good job!
Glad you got it fixed.