Frustrating And Elusive Electrical Drain

The normal draw with the car off is around 25 mA. Door open is 300 mA (any door or the hatch, and the dome light will be on).

Open the hatch, trip the latch with a screwdriver, close all the other doors, and lock the car. Wait at least 10 minutes (30 minutes would be better). Put a DC clamp ammeter on the ground cable from the battery to the back of the car. Tell us how much current you see. (Then turn on the car, push the hatch button to open the latch, and close the hatch. Don't try closing the hatch without first opening the latch!)

There is a known problem with the combination meter that can cause a 300 mA (if I remember correctly) drain. It has a circuit to keep it powered up for a short time after the car is turned off. If that circuit fails it can stay powered up all the time.

Have you tried removing each fuse from the fuse box, one at a time, while measuring the parasitic current that remains (as above)? With any luck the majority of it will be on a single circuit.

 

Similarly, there is a known issue where the JBL amp under the RH seat develops an always-on drain. The fix for this is to buy a good unit from a wrecker and replace the amp.

 

Also, when the radio problem is happening, does it still draw current if the OP turns off the radio (at its power button) before turning off the car?

Is there anything plugged into the OBD2 port? If so, unplug it and see if the problem goes away. Parts of the OBD2 port are wired to the battery (at least on most cars, which is why there are "charge the battery through the OBD2 port" accessories for many 12V battery chargers). Some cars which were purchased used have trackers wired in above the OBD2 port. There have been cases reported where those trackers fail in such a manner that they cause a large power draw. In other cases they did so from the time of purchase.

Are there any other known mods, especially an after market alarm system?

 

YouTube search "parasitic draw test" w a multimeter. It's usually something you installed

 

Yes, measure the voltage drop across the fuses in the fuse box. Most car fuses have little wires that extend from the blade up to the top of the fuse and are exposed there. Car fuses have defined (quite small) resistances and you can look up those values.
If the parasitic current I is going through a fuse with resistance R then it will produce a voltage V between those two points. When the current is an amp or more that will produce V of a few millivolts, which can be measured with a good quality voltmeter. However if the drain is on the order of 100 mA the voltage might be only 0.5 mV which is going to be just on the hairy edge of what one could measure with most voltmeter's 0.2V range. The multimeters which can read smaller voltages are bigger and tend to be really expensive. For instance, a Keithley 197, which would be over $200 for a used one, new it would be at least $1000.

One can use a table that shows the expected voltage drop for various currents for different size fuses:

https://caravanchronicles.com/wp-content/uploads/2018/10/fuse-voltage-drop-table.pdf

This is one of those instances where the source of a big current is much easier to find than the source of a small one.

Anyway, this sort of voltage measurement will not usually cause any change in the underlying circuit, so one could test every fuse in the box without causing the parasitic current to go away.

 

Measure voltage drop across fuses and put the readings into a chart

 

As others have mentioned, measure the voltage drop across each fuse. You need a multimeter that can read down to 0.1mV in the millivolt range.

I open the driver's door to access the underdash fuse box but you have to trip the latch shut (and hold any "door open" switch - or remove it). Also open the hood for the engine compartment fuse box. Let the car sit around 15 minutes to go "to sleep". No lights on the dash.

Now measure voltage drop across each fuse. You need to get good contact with the meter probes. (Yes, it's a complete PIA to contort yourself to reach under the dash - oh well) Look for a steady, repeatable voltage reading. Most everything should be 0.0mV. Write down your results.

There might be one or two at 0.1mV, really anything more than that is suspect.

Posted via the PriusChat mobile app.

 

I think there is a pretty good chance that 17 year old fuses would have at least a little dirt or corrosion on top of those two tiny metal pads. So it might be necessary to "dig" a little with the probe tips to get through that and down to clean metal to allow a measurement as small as 0.1 mV.

 

Although checking for a voltage drop of a few mV across a fuse will reveal circuits which have a substantial current drain, keep in mind that fuses are essentially "short circuits" until they blow (which transforms the fuse into an open circuit).

A much more sensitive measurement can be taken by removing the fuse and temporarily connecting a multimeter set to the "amps" scale in its place. With this setup all of the current flowing in the circuit goes through the multimeter. This makes a precise current measurement possible.
It's best to make the initial measurement using a multimeter with a 10A range (or larger).
Next, if the current drain measures <1A with the meter on the 10A range then it is safe to select a range of 2A or even 200mA on the multimeter.
For extra protection add a fuse of your own choice in series with the multimeter. I use a 1A external fuse.
Note: Many multimeters have a 2A fuse inside the case. So adding an external fuse of 1//2 that rating (1A) prevents accidently blowing the fuse inside the multimeter.

 

The OP noted that if the current stops it stays stopped. So pulling a fuse and putting in the ammeter instead will not work once the car is already off - the parasitic drain will always be off. I guess it would work though if this was done and then the car started. Since that would almost certainly blow the fuse on the meter one would have to put a shunt wire around it, start the vehicle, stop the vehicle, remove the shunt wire, and then look at the current. Kind of a lot of work to do one circuit at a time.

There are specialized devices for this sort of work. Pull the fuse, plug it into the device, plug the device in where the fuse was. Now it shows the current through that circuit - whether or not the car is on or off. But again, it must be installed before the car is turned off. Here is one made by Ansbell:

(go to Amazon, look up B0F4JZXRJB)

This particular one only goes down to 10 mA.

Another class of devices for this purpose look like:

https://www.aeswave.com/fuse-amp-loop-with-voltage-tap-p9653.html

The current can be measured by placing a clamp ammeter around one of the wires.

Finally, on some cars one can gain access to the wires under the fuse box. When that is possible one can put a clamp ammeter on them one at a time. I suspect that this isn't generally very useful though because the wiring harnesses will have been designed to make those wires as short as possible, so there may not be room enough to place the ammeter.