Potential parasitic draw 2004 rx330 AWD 3mz-fe

So I took on an engine swap because we love this car and didn’t want to scrap it after only putting 19k miles on it in a 4 month period. It’s a gorgeous car.

Anyway I got pretty far on my own but I had to get help mating the transaxle to the engine then buttoning it up before the first turn over.

The new engine sounds great abs at first the only code I had was bank 1 camshaft position sensor and I figured out that I had not plugged that connector in.

The first day we turned it over and took it for a ride the only issue was that the power steering pump that. Came with the salvaged engine was bad so I had to swap that out. The battery charged and we were able to turn it over 5 times with no issue then the next morning it turned over but the battery light came on and the instrument cluster lights started to go in and out intermittently.

Had aaa come and jump the car checking the alternator which is good then I plugged my little but useful craftsman obd tool in and saw the battery was jumping between 6.8-7.1v the aaa driver said the battery was just under 10v when he checked it.

So I almost tried to recondition the battery but just decided to get a new one from Walmart for $100. Now I tested the fuses for a draw with the bad battery and there was a draw from the dome and ecu-b fuses.

Yesterday I did some more stuff I took the water inlet housing off and put a new gasket on the thermostat(at least I think it’s a thermostat but idk because there is another thermostat on the top passenger side of the engine where the outflow hose is) and in my wrenching I found 1 ground not tightly secured on a bellhousing bolt then I found 2 grounds not even bolted on. One was in between the transmission housing and the wire harness clip that attaches to the housing and was touching the metal but not secured on the other was just hanging around and i had a hard time figuring out where it went but I connected it to the chassis above the wheel well / behind the air box because it was the only place that made sense.

So I tried to test for a draw yesterday after connecting the grounds but I think I did it wrong not letting the car go to sleep. The dome draw was gone but the ecu b fuse was drawing 500ma and it spiked to 7-800 at first before holding steady at 495-500 I am going to try again today i was told I need to hold the leads on between the negative terminal and cable for 10-15 minutes and see if the draw drops down to 25-50ma I had my key in my pocket but I was told that shouldn’t cause the ecu to be active.

Anyways if the ecu b does have a parasitic draw what should I do?

What’s the cheapest tool I can use to pinpoint a short without cutting the loom open and fussing around with wires? The $27 cable tracker at harbor freight says it tests for continuity so shouldn’t that be sufficient?

Is there a way to see if the ecu is good by disconnecting the harness from the ecu?

I did have a small fuel fire on the top of the engine that melted the wires to the coil packs and injectors. We had it extinguished in about 40 seconds. And I initially thought that was where the short was but now I don’t think that’s the case.

Any tips tricks and or guidance is greatly appreciated.

Could you please explain your parasitic draw test set up and how you measured the current draw you reported at the fuses?
It is very important that the test be performed with an amp meter between the negative terminal and battery post, that all the doors are latched for the entire duration of the test, the key and fob are away from the vehicle, and it must sit for at least 30min (many vehicles may take 45min-1hr) for all modules to go to sleep BEFORE you record the ignition of draw or begin any testing at the fuse panels.
It is a very tedious test that must be performed correctly, I see many techs cut corners or just not fully understand the process, and they never find the draw.

Once you confirm a parasitic draw is present in the way Noah suggests the most effective technique imo to locate the origin of the draw is to measure voltage drop across fuses. To do so a simple voltmeter with a mV scale will be sufficient.

So I think there may not be a parasitic draw, I’m going to confirm tomorow.

This is what I did to discover the ecu was drawing. I closed all the doors and latched the hood with a a screw driver and then connected the mm in line negative lead to negative battery terminal then positive lead to the cable that connects to the neg terminal. I had the mm set to ammeter function and then I pulled the fuses individually till I found the one(s) that were drawing. There was a draw on both ecub and dome light but after finding the grounds and connecting them the dome light draw is gone and now the sunroof window works. However the horn and the back door power open / close do not function. I also still have p0037 which is low voltage in the heater circuit if the bank 1 position 2 o2 sensor.

I found out today that the alternator cable was not tightened enough when I jiggled it and the alternator came on and then went off. I tried turning the car over today and it wouldn’t turn over even with gas applied. But then I plugged in the obd scanner I have and the engine turned over but it took a little gas to get it there. Then when I unplugged the obd scan tool the engine went dead. It said the battery was abnormal I was getting 11.46v at the battery and then 10.7v on the obd meter battery scan function. After tightening the nut on the alternator the car now starts up with no issues.

However I still made the decision and disconnected the negative battery cable just in case there is a draw from ecu but I think that’s unlikely the battery is at 12.7v when I disconnected the cable. Now I just need to figure out why the horn and back hatch are not working and what’s causing the heated circuit failure on the bank1 position 2 o2 sensor.

Man engine swaps are not as straightforward as I thought. I knew it would be tough but I thought it would be mostly labor intensive not an enigmatic game of whack a mole with electronics.

I think I did the test wrong initially and the draw I was seeing was from the ecu still communicating to multiple components. I had the doors closed and the hood latched but the key was in my pocket. However this is an 04 Toyota product and since it doesn’t have a push button ignition I was told on Lexus specific forums the key being nearby shouldn’t be an issue also that it should only take 15 minutes for the ecu to go to sleep but now I understand as soon as you connect the meter in line the ecu awakens and will be drawing a current. So from what I have learned and am understanding you need to have the multimeter / ammeter connected long enough for the ecu to enter hibernation mode before you can know wether or not there is a parasitic draw on the battery or a short in the harness in that specific circuit.

But the test you described was the test I did however I didn’t wait long enough for the ecu to go to sleep

If the car starts only with a scan tool plugged in you may have a bad/missing ground connection. On the 16 pin obd2 plug terminal 4 and 5 are chassis and sensor grounds and most scanners just bridge them together. If this makes the engine start you may want to verify these two ground connections individually.

The car now starts and turns off with no issue. Is there a ground wire that connects through the wiring that fastens onto the alternator? The answer should be yes so if the wiring wasn’t attached properly on the alternator wouldn’t that create the issue you’re describing?

What about the horn and the back power hatch? Is there a way to reset the back hatch power function on Toyota vehicles?

Typically the alternator is connected thru its case to B neg. The case contacts directly to the engine block/head, then it's strapped to the body and finally to the battery post.

I would have to think really hard to imagine I had seen a ground wire connected to the alternator case to provide a ground connection for the alternator on an original installation. Though I've seen improper repairs done that way.

I have all grounds connected properly at the moment. I get what you’re saying about the obd scanner and the issue I had with ignition earlier being caused by a ground. But now the engine starts and shuts off with no issue and it stays on when the obd is disconnected.
The alternator is grounded, so if the electrical connection was not properly connected then wouldn’t that mean it was not properly grounded or am I a total doofus?