Passed smog, failed Check Engine Light!
 

My 1989 Convertible has never had any issues passing a CA smog dyno test, but, mysteriously, the Check Engine light socket does not supply current to light the bulb, at any time!

The last time I posted about this, a rebuilt multi-sensor bar was recommended, but none were available. Any ideas on why this would be happening? All other functions work perfectly.

Have you given it a reason to activate the light? Like pull the wire on the O2 sensor, and see if it lights up?

Check Engine Light
 

No, but in normal functioning, the light illuminates briefly whenever the ignition is turned on, then extinguishes. I've failed the light visual test because it's not lighting when the key is turned, proving that it is functioning! Changed the bulb, it was fine.

If the bulb is good, then look at the attached jpgs and you can see the bulb has pwr from the METER fuse (if gauges work..fuse is good), and it gets turned on by a gnd from pin 1E of the ECU.

So if you change the bulb and you know the gauges work, then pull the plug off the ECU and with key ON, put a gnd to the wire in the plug that has 1E. LIght should come on. NOT the pin in the ECU itself, but the WIRE in the plug where 1E is.

If the wire in 1E is grounded and the light does come on, eeeek, seems to be a ECU problem to me, as in not putting a gnd on the wire in 1E when the key is put to on or if a sensor like the 02 is disconnected.

Hailers check is excellent. But, if it fails, I would check one more thing before trashing the ECU. The ECU side of the circuit is a pull-to-ground. If the light doesnt come on when you gound that circuit at the ECU connector, there is one more possible reason: an open in the wiring between the fuse and the lamp socket.

If the ground check fails, I would recommend checking to see that you actually have 12v at the socket.

Anyone ever mention that the idiot light cluster is known to have bad cold solder joints?
I am assuming that is what you mean by "multi sensor bar."??

I just took out my idiot light clust, only 3 lights worked and opened it up almost all the connectors had cold solder joints, resoldered them and they all work now. I'd check that it only takes a few minutes to resolder them all.

Great information!
 

Two things have never worked in the two years I've owned the car: the Low Fuel Level light and the Check Engine Light. All gauges work perfectly. Ive replaced all of the bulbs and even obtained another Multi-Sensor Bar, with the same result.
O2 sensor MUST be working, as I've successfully passed two smog dyno tests. There's an '88 Vert. in the junk yard: same ECU?

No. 88 has a series four ECU and the plugs won''t mate with the harness in the harness in the series five.

Low level light not working is because of a shot thermistor on the fuel pump. If you've ever seen a fuel pump, you'd notice a thick wire about six inches long going from top to bottom of the fuel pump assy with a bulb on the bottom of it. An electrical wire winds around the thick wire just mentioned and goes into the bulb.

There is a section of the FSM called Body Electrical that tells you how to check it out. Basically you pull the fuel pumps plug apart. The you put a ground on the wire that is dedicated to the low fuel level. That would be on the Rear harness side of the elect plug. With key ON, the low level light in the Warning Unit WILL come on.

Update: Failed Check Engine Light Diagnosis
 

Since the only real issue with my 1989 Convertible 5-speed is the non-functioning Check Engine Light, I finally dropped the car off at a rotary specialist shop for diagnosis and repair. By sheer chance, the owner's son also has a '89 5-speed Convertible, so it was easy to compare parts. It turns out that my ECU is burned! Car runs and drives perfectly, and easily passed the CA smog check, except for the Check Engine Light functionality test. I've been looking for an identical ECU, "N352," and have seen every OTHER possible number, including N351 and N353, used in the same cars. Apparently my car was made just before airbags were added (it has the light, but no installed airbag). Is my ECU completely unique to these last non-airbag Convertibles? The specialist wants to replace with the identical N352 ECU to make 100% certain this is the problem, but from what I'm learning, the dealer may be my only source. Has anyone ever seen an N352 ECU? Car is 100% original, so this is the unit it left the factory with.

how can you tell the difference from the ecus with out the ecu having any stickers?

The N352 is an ECU specific to verts sold in CA. Any other N35X ECU will drop in and the car will run, although I dunno how it will effect your emissions.

Heres an ECU application chart for reference, from RotaryResurrection.com

http://img59.imageshack.us/img59/5813/ecuc.jpg


Try posting in the classifieds. A new ECU will likely run you multiple hundreds of dollars.

But the thing is that my ecumenical doesn't have any stickers on it. Is there some way to tell from looking at it from the inside? Some kind of number or something?

Are you trying to discern bewteen S4 and S5 or the difference between the S4 ECUs or the difference between the S5 ECUs?