Help me with OBD1 function and meaning...

[musketeerracing]

Bit of a story to this one; 1990 Legacy 5MT LS sedan

1. Bought the car a few months ago and check engine light (CEL) was coming on intermittently but engine running fine except for pendulum effect on cold idle that goes away after a couple of minutes (although sometimes sufficiently serious that you have to hold the throttle steady yourself during warmup, which prevents the pendulum effect).

2. A shop friend hooked up a new Snap-On scanner and got a clear 35 code - the infamous purge solenoid.

3. Took a couple of used purge solenoids off parts cars but found that all were electronically open ie. failed, so a couple of days ago finally broke down and bought a new one. not yet received, not yet installed.

4. Today I happened to be under the dash to try to get to the (also infamous) heater fan relay. I noticed two pairs of unplugged plugs: one green with provision for two pins but carrying just one per side, and a black pair with a single pin.

5. I hooked up the green pair and (get this) with the ignition on but the car not running the two radiator fans started cycling on and off about as quickly as four-way flashers flash (or as quickly as old car alarms used to beep the horn...more later). Also the CEL would flash on as the fans fired, and off as they stopped. With the car running the fans would not run at all and the CEL was off (I think). But the fans didn't fire at all while I was idling the car, although the temperature was steady.

6. When I hooked up the black plug as well, there was no change.

7. When I disconnected the green plug, there was nothing going on with the fans but I saw patterns in the CEL flashes. Looked these up and lo and behold I got:

11 - crank angle sensor
17 - #4 injector
21 - coolant temp sensor
23 - air flow sensor
35 - purge solenoid

Lots of codes!

8. With the black plugs once again unplugged, I got no CEL.


So my questions are these:

A. Should I beleive the SnapOn diagnostic (code 35 only) or this U-read OBD?

B. Is the green plug the test sensor plug? should it normally be connected? And if so, what the heck is going on with the fans? Could it be related to an old alarm system that I see under dash and under hood but appears to be functionally disabled?

C. what is the black plug?

D. Do you see any pattern in these codes, given that the engine is running fine and strong but the idle pendulums on cold start?


When I disconnected both plugs i got no CEL at all, although I didn't drive for long enough to necessarily get my normal CEL that I've always thought was about the purge solenoid only.


Hope this provides someone with some useful distraction at work!

Best, ACP

[vrg3]

Read about the plugs here:

http://www.surrealmirage.com/subaru/engine.html

We don't know what the Snap-On tool was reading -- stored codes, active codes, something else...

It makes sense to clear the codes using the procedure on that page and then see which ones come back when you read the codes using the procedure on that page. Then you should only need to worry about the ones that come back.

[musketeerracing]

Great - just printed it out. Will do and report. Thanks.

ACP

[Legacy777]

yeah....i'd believe what the plugs tell you over the snapon tool

Clear codes..... both connectors and procedure on my site will work, or pulling the EGI/TCU fuse for 30 min also works....

[musketeerracing]

OK - I'll refresh for the codes. But tell me these two things:

1. What is the balck plug vs. the green plug?

2. Is there ANY reason the fans should fire when the green plug is connected?

3. How can there be a flaw in the crank angle sensor when the engine is fine, and has never stumbled on me?

A.

[evolutionmovement]

The ECU is supposed to cycle the fans when the green connectors are pugged. The different colors are for different check modes, I forget which is which, but one is a whole system diagnostic and the other is a stored codes diagnostic (for when the car is presently not acting up - the kind I just got last night).

You may just have a lose wire or something. Mine was running a little funny toward the end of a run and the CEL came on. Present check turned out all clear, but the stored codes pulled out: 21 (water temp), 22 (knock sensor), 24 (air control valve), 33 (vehicle speed sensor which I've gotten before). According to the failsafes listed, the car is presently seeing no issue with anything as none of the failsafes are operating and the car runs great. I'm going to sheck the connections to see.

Steve

[Matt Monson]

That's right. More than the fan should cycle through. The car should go through and self test most everything. You will hear little clicks in different solenoids as it runs through the test cycle. The flashing CEL you get at the end is the all clear...

[Legacy777]

Black plugs - read memory connectors. With these plugs connected, they simply pull any codes that may be stored in the ECU's memory.

Green plugs - Diagnostic mode connectors. With these plugs connected the ECU will do a full check on all the sensors to verify they are operating within tolerated specs. It is normal to hear the fans on with these plugged in. It is also normal to hear some clicking and such behind the dash. The instructions on my site give the proper procedure to utilize the d-mode connectors.

Black & Green plugs - clear memory mode - With both connected at the same time they will clear the stored codes from the ECU's memory. Again you need to follow the proper procedure, which is outlined on my site.

The crank angle sensor code may be erroneous....which is why we said clear the codes and check again. If it is erroneous....it won't display again. If there is a problem, it'll come back. It's simple troubleshooting technique.