First time posting! My goal of this thread is to describe the symptoms my car is experiencing and get input on what the issue might be, and then document the fix for the future.

I recently purchased my 1996 sc300. The car had so many weird issues that were very quickly resolved with the repair of the wiring harness in the front left wheel well. The only issues the car has now are the lack of starting with the key, the headlights, the taillight courtesy light on the dash, the dash backlighting under certain circumstances, and the trunk not opening. The car was hit on the drivers side door, right by the handle, so that will be the first thing I check when I get home later today.

Starting issue- Turning the key to the start position does not crank the car over, but does turn the fuel pump on. First I will start with the relay. Keep in mind the factory electronic wiring diagram has pin 1 and 2 swapped compared to the indicators on the relay.

The starter relay is confirmed operational by removing the relay. Testing continuity between 3 and 5 should read open/unlimited resistance. With voltage applied to pin 1 and ground applied to pin 2, continuity between 3 and 5 changes to 0, or continuous. The relay is good. Lets move onto the inputs and outputs of the relay.

Pin 3 should read the same voltage as the battery, we will say 12 volts. Test pin 3 by putting one lead to body/ground and one lead to pin three on the fuse block side. I got 12v, no problem there. Testing pin 5, Starter should spin when applying momentary 12 volts. I did this with a power probe (get one!), Pin 5 is good, I can start the car this way and it will run like new with the key in the on position. Testing Pin 1 is as easy as Pin 3. With the key in the ST/start position (may need a hand here) The voltage across this pin 3 and ground should be ~12 volts. This in my case was confirmed and when the key went back to the on position, dropped down to 2v. IDK if this is good or not, but it should at least trigger the starter relay. Pin 2 should read continuous with ground when, at the minimum the key is in the start position, most likely all the time. Mine does not under any circumstance.

Pin 2 is our issue, you SOB. Lets look at the pinout for the theft deterrent ECU. I see connections to a headlight relay, taillight relay, ecu ignition, starter relay, trunk unlocking thing, wireless door locks, etc. Weird every single issue I have is connected to this thing.

If anyone has anything I am missing, let me know!

Homework for tonight- make sure the inputs for this ecu are working. I'll recheck my fuses and go through the service hints section of the electrical wiring diagram, copied below. I should also rip apart the door. I shouldn't be shocked if something was messed but by getting hit by a car, lol.

D13, D14 DOOR KEY LOCK AND UNLOCK SW LH, RH
1−2 : CLOSED WITH DOOR LOCK CYLINDER UNLOCKED WITH KEY
3−2 : CLOSED WITH DOOR LOCK CYLINDER LOCKED WITH KEY
D16, D17 DOOR LOCK MOTOR, DOOR UNLOCK DETECTION SW AND DOOR OPEN DETECTION SW LH, RH
1−4 : CLOSED WITH UNLOCK POSITION
E 6 ENGINE HOOD COURTESY SW
1−2 : CLOSED WITH ENGINE HOOD OPEN
I15 UNLOCK WARNING SW [IGNITION SW]
9−10 : CLOSED WITH IGNITION KEY IN CYLINDER
L 5 LUGGAGE COMPARTMENT KEY UNLOCK SW
1−3 : CLOSED WITH DOOR LOCK CYLINDER UNLOCK WITH KEY
L 7 LUGGAGE COMPARTMENT LIGHT SW
1−GROUND : CLOSED WITH DOOR OPEN
T 7 (B), T 8 (A) THEFT DETERRENT AND DOOR LOCK CONTROL ECU
9 (A) −GROUND : CONTINUITY WITH LUGGAGE COMPARTMENT DOOR TO UNLOCK POSITION
14 (A) −GROUND : CONTINUITY WITH ENGINE HOOD OPEN
4 (A) −GROUND : CONTINUITY WITH LUGGAGE COMPARTMENT DOOR OPEN
8 (B) −GROUND : APPROX. 12 VOLTS WITH IGNITION SW AT ACC OR ON POSITION
25 (A) −GROUND : APPROX. 12 VOLTS WITH SHIFT LEVER IN N OR P POSITION AND IGNITION SW AT ST POSITION
7 (B) −GROUND : APPROX. 12 VOLTS WITH IGNITION SW AT ON POSITION
20 (A) −GROUND : CONTINUITY WITH IGNITION KEY IN CYLINDER
21 (A) −GROUND : CONTINUITY WITH FRONT RH DOOR TO UNLOCK POSITION
9 (B) −GROUND : ALWAYS APPROX. 12 VOLTS
6 (A) −GROUND : CONTINUITY WITH DOOR LOCK KEY SW LH TO UNLOCK POSITION
15 (A) −GROUND : CONTINUITY WITH FRONT LH DOOR TO UNLOCK POSITION
19 (A) −GROUND : CONTINUITY WITH FRONT RH DOOR OPENED
7 (A) −GROUND : CONTINUITY WITH FRONT LH DOOR OPENED
22 (A) −GROUND : CONTINUITY WITH DOOR LOCK KEY SW RH TO LOCK POSITION
16 (A) −GROUND : CONTINUITY WITH DOOR LOCK KEY SW RH TO UNLOCK POSITION
1 (A) −GROUND : ALWAYS APPROX. 12 VOLTS

 

Hello,

Check that the Shifter indication on the Instrument Cluster, that it matches the actual shifter position.

The thing is that the car starts and runs, so the Theft Deterrent ECU recognizes the key and allows the start, so the ECU itself is working. Makes me wonder if there is simply a break in the connection between the ECU and a Relay.

Access the ECU and check if you get a Ground on the ECU itself. If so, there is a break in the connection, if not, the ECU is cooked. If you don't really want to replace it just yet, there are a few fully reversible ways to temporarily bypass it - you can either take apart the fuse box, or access the Theft Deterrent ECU. The gist is pretty similar in either case, de-pin the connector as it is described here and here, then make yourself and adapter wire as it is described in the latter post and connect the wire that you took out straight to Ground, that way you will bypass the ECU and rely on just the Ignition Switch to start the car.

Hope this helps and best of luck!

 

Arsenii, Thank you very much! The shift indicator is correct, but I get 12V to the starter relay from the ECU, I get no switched ground to the relay from the theft deterrent. I have assumed (good or bad thing, not sure yet) that the theft deterrent is the issue, more on that below.
Update time, I had a wild weekend.

First things first, I ripped into my door and got it all fixed up after finding the cause of the door not working was not due to being hit by a car, but PO had lost/ broken one of those rotating clip things. Found another and it all works. The key fob now works after a shoddy soldering job too! And for some reason, the trunk now opens via the unlock button thing in the car and the fob, so no key needed. OH, and the alarm works! the horns on this thing are louder than a freaking train, especially in a garage when you can find the key, lmao. Maybe I was heading in the wrong direction with the theft deterrent ecu?

Unfortunately, this is where things go south. I had started to test the inputs to the theft deterrent ECU and was getting very weird results. That, in combination with the terrible position and hot weather I decided that was good enough evidence to replace it. A new one is ordered and the plastics are put back in place. The wheels are taken off and suspension adjusted (everything's tight and no rust at all, awesome!), and the car is put back on the ground. I still need to jump the relay, and the cluster back lighting and headlights work, but oddly. In park, the headlights work on auto when it's dark out, but do not work when the switch is on, unless you are in reverse/neutral/drive. The cluster lights work when the car is in reverse or neutral, but only when the lights are off. I think, I could have these mixed up a bit.

Anyways, the car is back together so I jump the starter and go for a drive. Spirited of course. Then not-so-spirited when the car completely shuts down. oh god help me.

Open the hood to find the battery tie down was broken (It had been hanging on by a thread) and the battery had slid into the power steering reservoir. Of course no insulation on the terminals so the battery shorted. A fuse blew? Hopefully?!? NOPE! Just melted ground wire insulation and MOLTEN battery terminals.

So I look for any broken fuses in the engine bay, nothing. After a minute the car was resurrecting itself though, So I called a roommate to bring my my power probe so I could start the damn thing. It started just find but died immediately. So I got in, we started it, and I got it home without letting the rpms drop. As I was taking my drive of shame I just happen to notice how everything is working... Odd.

If it wasn't the ecu before, it certainly is now. I started poking around and got the ECU(ECM?) out, and decided to open it up. Sure enough one of the capacitors is leaking. Whether this was the issue all along and I was just wanting it to be the theft deterrent ecu, or this was wrong AND the theft deterrent ecu was bad, OR I just blew the ecu on my spirited drive, This is where I am at.

The car doesn't crank, and when I jump the starter relay it will start right up but not idle.

Homework for this week- replace some capacitors and start removing the harness loom, I'll try to find any burnt wires and replace them. I have no idea how far into the harness this damage goes.

 

As written above, you can simply de-pin the terminal for the Relay from the Theft Deterrent ECU and check what the ECU itself is doing. If it won't give you any Ground, it is likely some key in the ECU that got fried, so you can either replace it, or bypass it.

Reminds me of some of the hurdles I had to face after the previous owner back in the day, here is a link..

Would the headlights simply shut off when you switch from Auto to On in Park?
What about the Parking Brake, is it working?

I don't have the wiring diagram for a 1996 SC, but from the 1993 model, the headlight circuit is a fairly simple one, there aren't even any daytime running lights, so the headlight should turn on and function even with no key inserted. As for the cluster, sounds like a misadjusted brightness dial to the left of the wheel, if it is turned all the way down, the cluster backlit will turn off, but only when the headlights are on.

Ouch, sounds like you got all prerequisites for a very bad day..

Please don't tell you were driving the car on the battery booster.. That can cause all kinds of consequences, since the battery is designed to absorb any voltage spikes in the system, preventing other components from frying. On the subject, so many electrical issues in the car can actually be a result of a bad alternator, if it were to overcharge the system, like if the voltage regulator were to fail.

One way to test it is to disconnect and isolate the B+ terminal on the alternator, start up the car and check the voltage on the alternator's B+ terminal, it should not be above 14-15V, if it is, the alternator has to be replaced.

ECU repair is a pretty vast topic, if you have never done that before, it may be wise to find a company that will do it for you, it is not just about taking caps out and throwing others in. Here is a very detailed thread about it, if you decide to find a company, a lot of people on this forum really suggest Tanin Auto Electronix, they not only replace the caps, but also have a fleet of vehicles they use to test those ECUs before shipping. I personally never tried going to them, but heard a lot of good things about them.

Hope this helps and best of luck!

 

Quick update on this, it is back on the road!

The previous incident had done the ecu in, so I ordered one off ebay and plugged it in. Started right up with jumping the starter and ran great. I decided to get the key working the half assed way, so I provided ground to the one half of the starter relay circuit. They key now turns the car on and everything works except for the headlights. They work in automatic mode when it is dark enough.

With the car running (Really well I might say), I took it on a 2,000 mile trip to deals gap and stopped by slammedenuff in gatlinburg. Everything went wayyyy too smooth. So smooth in fact that I am not going to sell the car and will be daily driving it for a while. Which means that my temporary fix should be looked into, along with a couple other things.

I did order a theft deterrent ecu, but tried to be lazy and ordered the wrong part number. I'll try this again to see if it fixes my problems which at the moment are only the headlights and key not turning the car on.

In the meantime, I'll adjust the ****ty coilovers and put new shoes on. The interior needs some work, the coolant looks nasty, and the fenders were rolled by a smoothbrain, so I have plenty to do.