headlight wiring question

[JonoTurbo]

My friend has a problem with his car where the headlights don't turn off. No matter what the switch is set at they are always on. We have gone through and changed the switch, the headlight relay, and the integration #2 relay. None of these had any effect. I have determined that it must be the headlight relay wire going to the ecu that's causing the problem. My question is why does the ecu need to be wired to the headlight relay in the first place, what will it do to cut that wire?

He has an 87 ecu and harness in an 89 car and I think that is the original cause of the problem. I talked with another supra owner who said he has the same setup and the same problem. Does anyone have any more information as to why this happens?

 

[GrimJack]

Some information for you, anyhow.

When you leave your lights on, and turn the car off, something shuts the lights off. I've always assumed this is the ECU. You can turn the lights off manually and then back on, and they will stay off even without the car being on.

 

[chriso]

Not sure why either but if you have an 87 ecu/harness in an 89 car there is your problem. You have to match the ECU/Harness to your frame year. The dash,interior (firewall harness) and engine bay harness are all year specific.

 

[Jeff Lange]

Some information for you, anyhow.

When you leave your lights on, and turn the car off, something shuts the lights off. I've always assumed this is the ECU. You can turn the lights off manually and then back on, and they will stay off even without the car being on.

It's when you turn the car off, then open the driver's door, it turns the lights off. It's the auto-turn off system, and it's controlled by the integration relay no. 2.

http://www.cygnusx1.net/supra/library/Wiring/pages/Page_072.html

 

[JonoTurbo]

Not sure why either but if you have an 87 ecu/harness in an 89 car there is your problem. You have to match the ECU/Harness to your frame year. The dash,interior (firewall harness) and engine bay harness are all year specific.

Everything else works just fine. So if this one issue can be overcome there's no reason to have to year-match everything.


In regards to what you said Jeff, that is interesting. Pin 10 on Integration relay #2 is connected into the ECU. (According to the headlight wiring diagram) Nothing in that diagram describes any way in which the relay would know the car is not running unless it uses the connection to the ECU to tell. So perhaps both you and Grim are right.

 

[Jeff Lange]

In regards to what you said Jeff, that is interesting. Pin 10 on Integration relay #2 is connected into the ECU. (According to the headlight wiring diagram) Nothing in that diagram describes any way in which the relay would know the car is not running unless it uses the connection to the ECU to tell. So perhaps both you and Grim are right.

Nope, it knows the car is not on because it gets power from a switched fuse.

The auto-off system doesn't show a connection to the ECU because it doesn't use it. If you look at the relay internal diagram on the headlight diagram, you'll notice it's different.

Also, as far as I can tell, the connection to the ECU is simply to tell the ECU when the headlights are on, to control the idle-up solenoid.

 

[dirtlord420]

i got a 89 harness in my 87 and the light never come on.

 

[daledoe]

Thats funny, I've never heard of that befor and now I have.

 

[JonoTurbo]

Nope, it knows the car is not on because it gets power from a switched fuse.

The auto-off system doesn't show a connection to the ECU because it doesn't use it. If you look at the relay internal diagram on the headlight diagram, you'll notice it's different.

Also, as far as I can tell, the connection to the ECU is simply to tell the ECU when the headlights are on, to control the idle-up solenoid.

Alright, I see what you are saying. Unfortunately since that appears to be true then the ecu uses that connection as an input and not an output, that doesn't explain why this headlight issue occurs. Unless, the 87 ecu uses it differently than the newer ecu's, or it is actually connected to a different thing on the 87 ecu.