I own a 93 Del Sol Si a/t and I'll try to be as descriptive as possible with what is going on.

The car will start and run but the charging system is not working properly. Both the alternator and battery checked out as working properly by themselves. When the car is running with no accessories on, there is a constant drain on the battery at idle. Maybe a hundredth of a volt a second.
With anything on, blower motor/headlights it drops like a rock.

I went thru the steps in the manual I have (Haynes) and these checks have all passed:
Drivebelt, alt. bolts, all fuses, no shorts found (used test light with battery neg. to ground), no noises from alt, charging warning light is ok, cleaned all corrosion on battery, replaced (trans, battery, water pump) grounds with 4g wire (old ones were in questionable condition), continuity of black/yellow and white/blue wires from alt. (showed battery voltage with key on the II position), continuity of white/yellow and blue alt. wires with test light to the ecu (pin a16 and d9).

The only other parts of the system i can think to be a problem are the alt. wire harness (cleaned visible corrosion but very hard to see into and clean) or the ecu its self.

Anything else i should look into before chopping cables for a new harness and/or in the last moments of desperation, buying a ecu?

 

are you 100% posative that its not the alt? with a fully charged battery, it will be enough to start the car but without the alt charging the battery, it could be giving you those problomes. i would double check the alt and make sure it is working properly again.

 

Sounds like parasitic drain. How many milliamps of drain do you measure with the key off? Have you tried removing fuses one by one to pinpoint the source of the drain?

 

I've taken it in and it passed a bench test.

This is my second alt. and it gave me a battery warning light when the first one crapped out. Now I only get a light when the battery is close to dead.

I just tried the paperclip blink code method and got nothing.

Something has to be showing a a warning or error if the alt was faulty or doing nothing which leads me to believe the ecu has a issue. I may be wrong but everything else in the line system checks out as ok.

 

With the key off it never gets below 12.4-5v. Also when I start it and turn it off over 11v it will clime back to around 12.4 in a short amount of time.

I did the test light to the neg post to ground. The light comes on but fades quickly.

 

Read information here:

http://batterychargerstore.com/what-...tic-drain.html

You need a digital multimeter to measure DC milliamps from the disconnected negative battery post.

 

Got the meter and I'll check it now.

 

What is the voltage at the battery with the engine running?

You seem to have checked most everything, but I didn't see checking the continuity from the big white output wire back to battery +. It goes through the underhood fuse box. You should measure battery voltage at the output post of alternator all the time, and with the engine running should get 13.5 volts or more, which should match the voltage at the battery other than a few millivolts drop in the wiring.

Take the belt off and turn the key on, no need to start engine. The alternator should get noticeably harder to turn by hand. If it doesn't it is defective or not powering up. The I wire from the key powers it up, and it seems you have already checked that.

The ECU has the ability to cut the regulated output voltage from the normal 14 or so down to 12.5, but it can't turn it off completely. One of the wires from the ECU does that (presumably at WOT or near stall), the other one is an input to the ECU telling it how much drive the regulator is putting on the field coil. The ECU uses that to estimate load on the engine and stabilize the idle speed.

 

Seemed normal in the 30's

If I charge it to around 13.7v then turn it on it drops to about 12.5v then a steady .01v every few seconds until I shut it down around 11v. Id rather not damage the battery by draining it further than that.

I did check the fuse but not continuity. After the rain I will see to that.

This very good to know. Thank you.

 

You alternator is clearly not charging the battery. I agree with mk that you should first focus on the large white wire running between the alternator and battery.