Subjug4tor

iTrader: (1)

Ok, I'll keep this as brief as possible while still being informative, please read the whole thread before giving input-

UPDATE: see post #6!


A couple days ago we went to go buy a '92 SC400 about two hours away. The car is rough, but at the time ran and drive great. The sale goes through, and we start the drive home, me in the Lexus, wife in the Honda we drove up there. About halfway home we stopped for just a minute, and I shut the car off. About 3-5 minutes later, I go to start the car, and it's just endless cranking but no fire. Fast forward to today after towing it home, and here's what I know:

1) Key on, engine off, there is no CEL on the dash. I swapped the bulb, still nothing. Now I know this means problems getting power to the ECU, or a bad ECU generally speaking, so I dug further.

2) At the ECU plug, it appears that there is 12v power input on the wires it should be except for the circuit that comes through the EFI main relay- the wire that goes to pin B31 on the ECU. This is the +B circuit, and sure enough, in the underhood diagnostic box, +B has no power with the key on.

3)After finding this, I swapped the relays thinking that was it- no dice. <strike>Even jumpered the correct pins where the relay plug into the fusebox, still nothing.</strike> There is power TO the relay, both on the trigger side and switched side, and the ground for the trigger side is solid.



Unless I'm missing a step in the wiring somewhere, what I have gathered is this- I have power through the EFI relay, but not on pin B31 (+B) at the ecu, therefore there is a problem somewhere between these points. The EFI, IGN, and all related fuses are good, this seems to be the only power source the ECU is not getting.

What struck me as odd is the way it failed- It didn't die or stall out, I shut it off normally with the key, and then a moment later it would not restart. Any Ideas?

Update- I must have jumped the wrong set of pins on the EFI relay the first time, because out of options I tried it again and I got a CEL on the dash. Turned the key. and she started right up. So, bad relay, right? I swapped the relay out with both of the two identical ones in the box, and even tried a new in package Hella brand relay that I wired up to spade pins for the task- still nothing at all. Put the jumper back in, works again. In my mind this can only point to the relay not getting trigger voltage- but there is voltage across the trigger pins (From the M-REL output on the ECU). The only thing wrong is that it's only reading about 11v across the trigger circuit. The +B side of the relay reads 12v across the pins with the key on, and the battery itself reads 12v.

The new question is, would the M-REL output being only 11v rather than 12v be enough to keep any relay I tried from working? Am I dealing with a bad ground, or a bad ecu?

Bankswood

probably a bad fuel pump ecu. check the forum for 12v bypass. Should help

Subjug4tor

iTrader: (1)

There is no power getting to the fuel pump ECU, so that would not affect this. Also, I already jumped it before I knew that it was not receiving power and that did not change the behavior, so I plugged it back in.

Note- The CEL does not come on with the key on, engine off. The +B circuit that is not getting power on the ECU side is also the one that goes through the circuit opening relay to become the FP voltage, so if I don't have +B voltage, there wouldn't be any going to the fuel pump ecu.

If I'm wrong then someone correct me, but this is how I'm reading the diagrams. Here is one I am referencing-

Subjug4tor

iTrader: (1)

UPDATE (above post has been edited but posting here to bump and make sure the update is noticed): I must have jumped the wrong set of pins on the EFI relay the first time, because out of options I tried it again and I got a CEL on the dash. Turned the key. and she started right up. So, bad relay, right? I swapped the relay out with both of the two identical ones in the box, and even tried a new in package Hella brand relay that I wired up to spade pins for the task- still nothing at all. Put the jumper back in, works again. In my mind this can only point to the relay not getting trigger voltage- but there is voltage across the trigger pins (From the M-REL output on the ECU). The only thing wrong is that it's only reading about 11v across the trigger circuit. The +B side of the relay reads 12v across the pins with the key on, and the battery itself reads 12v.

The new question is, would the M-REL output being only 11v rather than 12v be enough to keep any relay I tried from working? Am I dealing with a bad ground, or a bad ecu?

Cabbos

Hi what was the outcome? I'm have similar issues.

Subjug4tor

iTrader: (1)

There really hasn't been an outcome so far. The problem has actually gotten a lot weirder.
I took one of the stock relays and put it in the box, and out of desperation I Clarkson'ed it and smacked it with a wrench. Click! I hear the electronics turn on, and sure enough, CEL on the dash. Car starts up fine.

Thinking I had simply found a bad relay, I order a replacement online. Today it got here, and I took the old relay out, and put the new one in. Car won't start again, no CEL. Put the old relay back in (the one that I hit) and it works.

I think, "okay, surely this relay has gotten stuck closed and is acting as a jumper." So I tested it- no continuity on the switched side without voltage- with voltage applied, it closes like it should.



So that's where I'm at. The car runs and drives just fine, but only with that one specific relay or a jumper, not with a new relay that tested good, or any other good relay. I really think it's just cursed, and plan on replacing the whole fusebox whenever I happen to find one. No rush though, as the car runs and we have AAA.

Anybody have any insight???

DiRocco

My 1993 SC300 is acting very similarly. I have just begun researching though, haven't put the voltmeter or diagnostics to it yet. Will report back my findings. Thanks for the guidance.

1UZagoo

I have nearly the same problem on my LS400. Your wiring diagram is completely different though. It shows the trigger side of your EFI relay gets power from the IGN fuse from the ignition switch, but you mention the M-REL pin at the ECU, which is how mine is setup.

Based on my car, check with ignition switch on if coils, igniters, injectors, etc has battery power at the black/orange wire. IGN fuse should have power with key on as well. If all checks out well there, double check the ground pin for the efi relay (pin 3 on my diagram). Pins should be labeled right on the relay. If good ground, then it's something to do with the trigger wire which on my car comes from the ECU as a black/orange wire (completely separate from the ignition switch wire mentioned before). If you have 11v on that wire, I'd think the relay would still click. Since you smacked it and it started working, it might be just not quite enough voltage (or amps really) to pull the relay shut. My car gets no voltage and I've swapped the ECU out with a known good one and I have the same problem.

If I jump the efi relay and +B to Fp in the diag port my car runs. CEL never turns on with key on engine off which is weird for me.

Heres a pic from the wire diagram for my car (93 LS400).

1 - Black/Orange from ECU (MREL) Should be 12v with ignition switch on
2 - Black/Yellow from ECU (BATT) Should be 12v all the time
3 - White/Black Body Ground
4 - Black/Red +B which powers most computers and ECUs including Fuel Pump though a controller

Javier214

Can you share a picture of the paper clip inside fuse box so we know what pins to jump having same issues

1UZagoo

I haven't started my lexus in a while so dead battery and I don't remember which pins for sure, but the relay should say the pin numbers on it if I remember correctly. It's best to use a multi meter to do some testing before hand.

First step is do you have +12v at the B+ terminal in the diag port (under cap shows the pin out). Key on test B+ to negitive battery terminal, should have +12v dc. If not, then the EFI relay isn't connecting the battery to B+ for some reason (fuse, bad relay, not being triggered by ecu). Jumping the relay is a temp fix/diag step, the car shouldn't be driven with the band-aid setup. If you have 12v at B+, skip down a bit.

Pin 1 runs the the ECU and is the remote "trigger" for the relay (+12v when on)
Pin 2 should be battery positive terminal (ohm test to positive terminal with battery disconnected)
Pin 3 should be ground/negative battery terminal (ohm test to ground terminal with battery disconnected).
Pin 4 connects to everything labeled as "B+" which includes the fuel pump ecu.

The relay shorts pin 2 to pin 4 when triggered, jumping these two pins with a paper clip emulates the relay on.

============================================================

Now that you have B+ with 12V you know the ECU, Fuel pump ECU and such should have power if the harness is intact. I don't know a whole lot about the fuel pump ECU besides it has a high and low run setting. At the diag port, check Fp to battery negative to see if it has +12v while cracking (get a helper). If no voltage then I'd guess something's up with the fuel pump ecu or something that tells it to turn on (engine ecu?). To test the fuel pump (the actual motor that pumps fuel), you can jump B+ to Fp in the diag port and this makes the fuel pump run at max presure as soon as the key is on. This should be enough to get the engine to start if all else is in working order.


Locations:

Fuel PUmp ECU - Located in Trunk, passenger wall behind the carpet if I remember correctly
Fuel Pump/tank - Behind rear seats, the tank is also the back of the trunk area. This isn't the norm, most cars have the fuel tank under the car. Access is though the trunk
Engine ECU - passenger dash under glove box area
EFI Relay - Under hood on driver's side fender in the fuse box (lid tells you what everything is)
EFI Fuse - see EFI Relay
Diag port - On engine, driver's side if I remember correctly. It has a rounded top cap and is pretty small, might have to google images to get an idea what it looks like. Under the cap shows pinouts and the pins are generally filled with grease so they don't corrode.

Good luck and if you need pics of the jumper spots, I'll have to get it in a day or so once the battery is charged up.

Ebonysc400

1 pull hand brake
2 make sure you are in park
3 remove main fuse box cover on right side of fender
4 pull main efi relay out of socket
5 turn ignition on but do not crank yet
6 plug main efi relay back in
7 start your car!
**WORKED FOR ME **
my sc400 died at gas station after filling up. had a friend tow my car to his house my car just started its 218am now i can go home.

92sclawson

This worked for me. Did you ever determine what the issue was that caused it? Bad Relay or bad ECU? Thank you in advance for your feedback!

oldManTan

low battery causes the relay not to click, not enough voltage for relay to switch. new battery, or jumping it fixed it for me.

92sclawson

Thank you for the info!

ratSpaW

this worked for me, thanks !