Here's the situation:

My car has a constant SES light on (1987 L98 GTA), but I pulled the codes and only code 12 comes up. Everything is running great (she made an 14 hour trip yesterday just fine - pulling 20 mpg), but the light comes on randomly, flickering over bumps and eventually deciding to turn off for a good while only to come back on.

The car's been down since last May and this was the first big trip...so I'm curious what would cause the light to do this. Any ideas?

 

It all begins at the battery.. The battery is an electrochemical device. It consists of pairs of lead and zinc plates suspended in an acidic solution. The acid holds electrons from the metal plates in suspension, and delivers them to the zinc plates when the external connection between teh sets of plates is completed. This causes electrons in the outer shell of each metal molecule on the plates to be displaced, forcing each molecule's extra electron into adjacent molecules. Any molecules with an extra single electron in their outer shells will tend to have these electrons displaced, forcing them onto the next adjacent molecules. Copper is among the more common of these molecules, and is used for most of the wiring on your vehicle because of that trait. The external battery terminals are connected to the copper wires on your vehicle that route this electron flow to various parts, including the lamps and ECM. The lamp receives these free electrons through the GAGE fused circuit wiring, and the circuit is completed through the ECM. Electrons flowing through the lamp cause resistive heating in the filament conductor, causing it to glow (become incandescent). Electrons continue to flow through copper wires to the ECM, where an output transistor completes the circuit to the other side of the battery. The transistor is a silicon plate doped with chemicals that alter its surface, and cause it to become partially conductive, or semiconductive. When the ECM circuitry applies a sufficient bias voltage to the center (base) and end (emitter) of the transistor, electrons will tend to flow more freely from the emitter to the collector at the opposite end of the silicon plate. This end is wired to the negative, or grounded side of the battery, where the electrons return to the lead plates to replace those leached away by the acid solution.

In all reality, convention electron flow is just the opposite - from negative to positive, but it is generally easier to explain this way in a negative-grounded system, as your vehicle has.

Why the lamp lights intermittently as you describe is way beyond my capacity, however.

 

:hail:

 

Actually, among your problems may be an intermittent signal from the distributor's HEI switching module to the ECM. That would set a DTC 12, which is generally ignored as an error code reviewing the stored data.

That could be due to poor connections (including grounds), a failing HEI module (which would normally cause misfire, but may fail in such a way that the low level signal to the ECM is affected only), or a problem with the ECM power to the VRAM that stores your BLM / integrator data tables and error diagnosis codes.

(That just came to me...)

 

Thanks Vader. The car has intermittent stumbling at low rpm, so I will check all the things you listed. I must say your second response is much less informative than the first...but much more helpful. Thanks.

 

Well to rule out some things -

I just did a tuneup and did:
Spark plugs
Spark plug wires
Distributor and Rotor
Ignition coil

I still have the constant SES light and slight (alot less though) problems at idle. However, I took it to a local shop and the guy found a vacuum leak at the brake booster, moved the vacuum line around and suddenly my idle is fine. Car's running like a dog, but the idle is fine. Still the constant SES.

He says it most likely is the computer. When I had the accident, it probably shook the computer enough that something have have cracked, or just become loose inside. Which makes a good deal of sense. IF you kick the bottom of the passenger side dash - the light goes on and off.

Does this make sense and should I go ahead and spend the money on a new ecm?

Thanks guys.

 

OH1? We didn't read about "the accident" before. I'd suggest you remove the lower right trim panel and check the connectors at the ECM. You may also want to remove the ECM "window" cover and make sure the PROM amd MemCal are fully seated on the motherboard. Don't forget to check the ground wires, too.

EDIT: That was supposed to be "OH!?"

 

Will do. Don't know what 0H1 means...but I T-boned a guy doing about 50 mph. Car's just now back on the road and I had the shop swap to serpentine while it was down. So many gremlins I could film my own movie.

 

Hopefully you weren't rattled too badly. That would certainly be enough G-force to dislodge components, connectors, and possible even wiring. Definitely worth a look.