Strange CEL problem (Stays on and won't flash when jumpered)

[yankiwi]

OK, so the Check engine light is now on constantly, and won't flash when jumpered.... Sorry for the long post, but I'm stumped.

Car is a 91, originally NA now 1JZ however this system seems to be common throughout the models/engines.. The CEL has worked correctly previously, and I have not touched a thing lately..

I have made certain I'm jumpering E1 and TE1, tried both at the diagnostic port and again at the ECU pins, as well as grounding TE1 directly to Bat neg.

I have tested voltages as well. I have 12v coming into the M1 plug, pin 9 with M1 unplugged. So the light is getting voltage, and the 7.5A fuse in the kick panel is good. When the M1 plug is connected, ign on, I now get 1v reading in the circuit between the plug and W - check engine light pin, on the ECU..

My understanding is that normally the light is getting a constant 12v, and the ECU grounds this to turn the light off. Seeing as I now have a constant 1v coming from the ECU it is keeping the light on, so I assume this is because I have a code (although I would have thought it would be 12v??), which should be correct as it was carrying a MAP code last time I was able to check, but why won't it flash a code number when jumpered??? This all happens when the car is running, or not, and is still constant after Bat has been disconnected/EFI fuse pulled for 30 sec....

So I have no idea how to check the codes or solve this, at the moment I'm suspecting my ECU is the problem.

Any help would be great.

 

[hvyman]

Is the light on with the key out?

 

[yankiwi]

Nope, it's off with the key off/out... Comes on just like normal when the key is turned through 'ON'.... Then stays on when the car is started.

 

[yankiwi]

Any one got an idea???? I still can't figure this one out.

I need to understand how the ECU sends the 'flashes' to the CEL, so I can try and trace why it wont do it anymore....... I'm guessing it grounds the 12v coming through the light on/off to display the flashes, but does it ground through E1 or elsewhere?? Is there any other way to check codes????

 

[jetjock]

...My understanding is that normally the light is getting a constant 12v, and the ECU grounds this to turn the light off....

1) The ECU grounds pin W to turn the lamp on.

2) E1 is used as the ground source for the lamp.

3) My schematic and the online TEWD shows the lamp as being connected through M1 pin 8, not 9. Perhaps an artifact of your swap.

4) The voltage drop between M1 and pin W with the lamp on, assuming the ECU is supplying the ground to illuminate it, should be no more than a few millivolts. One volt points to a connection problem, alternate ground path, or faulty ECU lamp driver.

5) Problems with Vc can cause problems with W. Verify Vc is +5 vdc.

There are several ways to address the problem. For example push out the pin W and try another lamp on terminal W referenced to +12. Or meter terminal W after it's been floated. Either should tell you if the problem lies with the ECU or the vehicle wiring.

Vf can be used to determine if codes are set but can't be used to read them. Observing how Vf reacts in various modes can also be used to confirm the ECU is responding to a connection between TE1 and E1...

 

[yankiwi]

Thanks for the info jetjock. Looks like my issue is with the ECU.