This is an add-on to this post: https://rennlist.com/forums/964-foru...ine-light.html

In California it is mandatory that you bring your car every 2 years to an emissions check (smog check).
Amongst other things the checklist includes a test of the function of the check engine light (CEL).
964 with model year 1989 and 1990 do not have a working CEL, however they have an indicator "window" labeled CEL in the instrument cluster (and that is not good as you soon will see).

I and several other fellows with 964 of that vintage ran into issues when we either failed the smog check altogether or had to do a LOT of convincing with the technician who thought that the CEL in our cars was broken.
Here is what usually happens: the technician shines a flashlight into the instrument cluster, he sees a label "Check Engine" and at this point all logic and arguing usually is futile (I was successful twice in changing the technician's mind, others were not)
This is getting very frustrating.

That brought me to an idea:
Maybe we can trick our way out here?
In a 964 of years 1991 and higher the CEL comes on when the ignition is turned on and goes off when the engine runs.
Is there a way to simulate this behavior?

Quick remark: you probably should disconnect the battery while working on the electric/electronic parts of the car. Well, I didn't because: "safety last" :-)
And remark #2: If you do this, it's all at your own risk, if you break something, don't blame me ;-)

If you look at the back of the instrument cluster you will find a blank (instead of a lamp) behind the CEL "window" - no surprise. (sorry in the picture the blank is already removed, but you get the idea).




Test 1: Put a working lamp into the slot and see if the CEL light comes on. Result: nada, it's not that easy.

OK, let's figure out something else. How about we put in a light source which behaves just like the CEL?
OK, good idea but I don't want to create an over-engineered circuit for this simple task.

But doesn't the Oil pressure warning light do exactly that: comes on when the ignition is on and turns off as soon as the engine starts running.

So how about we tap into the wiring for the oil pressure lamp and connect our light source to it.

And here is how I did it:
parts needed:
- white mini LED (3mm)
- 2.2 kOhm resistor
- a few inches of wire
- a thin drill
- soldering iron
- Tesa adhesive wiring cloth tape - or you can also use electrical tape, doesn't look as OEM as cloth tape, though

- I took the blank cover and drilled two small holes in it to route the two pins of the LED




- Now solder the 2.2kOhm resistor to the positive pin of the LED (it's the pin which is slightly longer). Before you do that cut the pin so that the resistor rests against the elongated part of the blank. (We need the resistor so that we don't fry the LED - which wants 2V - with the 12V we have in the cluster)
- solder two wires (with different colors) to the pin of the LED and the resistor (remember which color goes to the resistor). also cut the other pin of the led so that the soldering point between the pin and the wire also rests against the elongated part of the blank.
- Wrap some of the cloth tape (or electric tape) around the pins of the LED to secure the LED and the resistor and the pin with cloth tape (or electrical tape), make sure that you use enough tape so that nothing can accidentally get bent and break.



Now we need to find the electrical connectors to tap into:
- At the back of the cluster you find two black plugs, remove the plug on the right side of the cluster by inserting a small screwdriver between the far ends of the plug and the plastic of the cluster, slowly wiggle it free - it easily will come off (if in doubt, for reference: in my first picture of the cluster it's already removed)

Before we go any further, let's do a quick test, for reference, here's the back of the plug:



Insert your positive (the one which goes to the resistor) wire into "3" and your other wire to "5".
Turn on the ignition and verify that the LED is lit.
If not, do some troubleshooting:
- swap the wires (maybe you got the polarity wrong)
- check your soldering

If that test worked, you know that you have a working LED, Now you need to open the plug: very carefully insert a very thin screwdriver at the places I marked with a red arrow (4 in total)




the plug will flip open and you will see this:



The two pins you need are marked with arrows:
- the pin in the middle with the orange wire is +12V
- the pin to the right with red/green is for the oil pressure lamp

Now (also very carefully) solder your two wires to these two pins
- the one which goes to the resistor goes to the +12V pin
- the other one goes to the OIL pressure pin




Do NOT solder the wires on top of the pins, otherwise the plug housing will no longer close, you need to solder them to the side of the pins (see picture)

Do not plug in the plug yet, let's do a test first: Close the plug housing and give it a try: turn on the ignition and the LED should light up, start the engine and it should turn off again
I then wrapped the whole length of the wires with the cloth tape (to give it an OEM look) even though nobody will ever see this
- Put the LED back in where the blank was
- re-attach the black plug



Give it another test, just to be sure
If that was successful, put your cluster back into the dashboard

Voila you have a simulated CEL

 

Thanks for the great documentation on this. It is probably the better long term solution to easily get a smog test done anywhere. I prefer this over touching my ECU and dealing with Eproms.

 

blowing a little fresh life into this thread:
It was time for the bi-annual smog check again and my little hack worked like a charm
This was the FIRST EVER smog check, where I didn't have to have a long discussing with the SMOG technician that my car does not have a check engine light.
very happy!

 

I don't have anything in front of me to check however what does the emissions sticker on the deck lid say? I know my turbo has the same CEL light without a bulb and just says Catalyst OBD exempt. I am assuming that the sticker should say if a CEL light is fitted to the car or does it just say Catalyst like on my 93?

By me 25 years or older don't require smog checks this fix seems quite creative but will it be an issue if they figure it out one day and find the light does nothing? Do they plug into the OBD I port?

 

Looks like the emission laws are a bit stricter in CA...
The only thing I know is that during the last two SMOG checks I ended up arguing with the technician about the fact that there seems to be a CEL light in the dashboard that "doesn't work".
And now the problem's solved
And I don't think that any SMOG station here has an OBD1 to OBD2 adapter.

 

Clearly CA is the toughest. I am just surprised anyone would argue with a federal sticker that should show if a CEL is required or not. Good solution. I am curious if there are any ramifications if someone figures out what you did.

 

Actually, the 964 DME ECM provides provides a functional CEL output on pin 22, i.e. a ground with key-on and off when the engine starts.
Some 964s may even have the wiring from the ECM to instrument cluster pin 10. One can also jumper the oil light ground pin to the CEL's
ground pin in the instrument cluster. The oil pressure switch can easily provide current to two bulbs at once.

 

Thanks for this heads up. I ended up using the oil pressure switch ground to control the CEL on ignition. Works a charm. I hope to remove this in the future, but this will help with smog.

 

The internet is a wonderful public media source!! Hopefully no CA smog officer trolls!

 

By law they can't make you activate the CEL just because there is a provision for it on the dash when the DoT sticker on the back deck lid clearly states "OBD exempt".

 

Yeah, I passed smog on Friday. First thing he checked was my CEL.