Sunnii

I just did the timing belt recently and when I finished I accidentally put the battery on backwards and it made a big spark now the car just cranks and sounds like it wants to start but won’t I’m pretty positive it’s getting fuel I can smell it in the car and I have replaced all the fuses and relays I’m thinking it’s the ecm I ordered a new one so I’m hoping it’s that any help?

CLLEXUSS

The same thing happened to my car with reverse polarity by the previous owner that owned it over 10 years ago. She took it to the dealership towed it in to them. From what I can gather in the records what the dealership did is replace fuses, test wiring, and various connections, and they also re-flashed the ecu at the dealership. Apparently the ecu seemed to be okay from what they determined but they decided to do what they did anyways just in case. I'd have to look at the records again but from what I remember that was the extent of it pretty much. So I guess the point is testing the ecu first before making the assumption is optimal. But you already ordered one, so now you have two I guess and I hope that fixes it.

CLLEXUSS

So it looks like you have a 1999, it won't be as simple as say plug n play on a 1999. So if you are planning on simply taking the old ecu out, and simply popping the replacement in, it's more involved than that, your car won't accept it with just the ecu alone. There is programming involved.

CLLEXUSS

You can obviously get the new ecu to work but there is a process you need to go through that involves programming. I've never done it on my car but lots of people have here in this forum.

Sunnii

even if I buy a already programmed one

CLLEXUSS

Well, we have 2nd Generation LS 400's. (1997-2000) First generations were different in this regard. The keyword you want to look at is "immobilizer". I am hesitant to say too much as mentioned I have not personally replaced one myself on a 2nd Gen. But for sure that's the word and the issue that needs to be addressed in case you were not aware already which you might already know about.

paulo57509

I believe the immobilizer in the ECU must match the transponder in the key(s). Therein lies the rub.

Before attempting to swap the ECU, check the 7 fusible links in the underhood fuse box in this area:

CLLEXUSS

I found a little bit of additional extra information on my car way back in 2001 when the car had less than 10K miles on it on the Lexus Owners website. It's a bit spotty as service write ups and reports can be but I found this in my cars archives from over 20 years ago. (Goodwill Assistance Reimbursement request also) In addition it looks like they had to repair wiring also. This was back in 05/01/2001. She would would not drive it that much apparently and the battery went dead, that's when the problem happened they way she jumped it. I've owned the car for about 5 years now, no issues. Below is a copy and paste from 2001 Lexus Dealership;

(REPLACE BATTERY,120A ALTERNATOR MAIN FUSE, AND ALL OTHER FUSES BLOWN DUE TO POSS REVERSED BATTERY POLARITY SERVICE:
NO POWER ON DASH FOUND 120AMP FUSE BLOWN ALONG WITH 7.5A EUC-B FUSE, 10A DOME LIGHT FUSE, 15A ECTS FUSE, AND 20A RADIO FUSE
GUEST REQUEST GOODWILL ASSISTANCE WITH REIMBURSMENT OF REPAIRS DONE WHILE UNDER WARR-100 AMP FUSE BLOWN-BATTERY WENT DEAD-JUMPED BACKWARDS MILE 8711 AMOUNT-484.86
GUEST REQUEST GOODWILL ASSISTANC WITH REPAIRS TOWED IN WONT START CAN NOT JUMP 100 AMP FUSE BLOWN/BATTERY DEAD REPAIR WIRING/REPLACE 100 AMP FUSE AND BATTERY ** WARRANTY OPCODE **)

So you can see they replaced the battery too, and found an issue with some wire or wiring but did not specify further. Only other thing I found besides this in another section was they checked the ecu software and I believe the terminology was re-flashing iirc. And it checked out okay. Maybe that will give some additional helpful information in your case.

8/8/2001 (9511 miles)

Sunnii

yes this helps out because that’s also what I was maybe thinking to that it could be wiring or maybe a broken sensor? Thank you guys for the help I much appreciate

Yamae

The ECU seldom fails when the power supply is reversed due to the fool proof protection circuit inside. The important first step is to check all fuses. At least the alternator fuse 120A is blown.