Help! I have a 1992 Geo Tracker With a check engine light that wont go away....When obd1 reader is plugged in it wont communicate....removed ground cable and light came on as soon as key turned....jumpered terminals B and C ---no blink codes....replaced ECM with used unit--same problem....any ideas???

Do a search for check engine light. It's probably the nag switch behind the driver side speaker in the dash. Just flip it whichever way and it'll shut the light off until the next service interval. Or better yet, I've read you can disconnect it from the odometer and it'll never nag you again. I must say though, the nag "check engine" light saved me some money when I bought mine from the previous owner so at least it served some function. I was overjoyed to kill the CEL with the flip of a switch rather than a diagnostic.

Which OBD1 reader are you using? it should still communicate with the nag light on.

Yep, I'd say the nag switch under the dash needs to be flipped to the opposite position.  If the CEL is on because of an odometer interval, then you will not be able to see the CEL flash, when you have the B & C pins shorted together.  The CEL does actually flash, but it just does a bright/dim/bright/dim flash and is very hard to see (you can usually see it "flash" in a dark garage or after dark).  But just flip that nag switch to the other position and see if you can get the CEL to flash normally (on/off/on/off) with the B & C pins shorted together.

Thanks, guys, for trying to help! I tried several times to flip the switch...the only change I could detect was a slight difference in the CEL intensity...what next? I dont know what OBD1 tester was used, but two different garages had the same result...I have not used a tester on my latest ECM...Ideas???

I seem to remember that it takes a very specific OBD1 reader to work on trackicks.
A reader/tester is generally not used on the OBD1 vehicles, to read the error codes.  Have you tried simply shorting the "B" and "C" pins in the 4 pin connector up by the battery?  If you do that, and turn the key to run, then the CEL should flash any stored error codes.
A constantly illuminated CEL generally indicates a bad ECM.  Do you know for sure that the replacement ECM is good?  Also, is the replacement ECM an exact match for the old ECM (the 33920-xxxxx numbers should be identical).
What is the back story of the CEL being lit up.  Has it been lit up ever since you owned the trackick?  Did it just come on one day and now won't go off?  Has this trackick ever ran?  Did it run when the CEL was lit up constantly?

The CEL was lit when I got the car.....I jumped B & C...no blink code...constant on....replaced ECM ( w/matching #s) from running salvage...car seems to be running ok...how can I tell if it is in limp mode? Could the wire between the light and the ECM be grounded?

Even if the CEL were shorted to ground that wouldn't explain why the code reader wouldn't work. Can you find out what tool they used? a Tech 1 or Tech 2 is the usual tool, although there are a couple of othes that will work. What error message did they get from the tool?
You could check the voltages on the diagnostic connector, you should have 12V on pin H and 5V on pin M with 0V on pin A
The usual problem with the CEL is trying to jump the wrong connector, it should have a rubber boot over it, what wire colours are on the connector you are using? I don't understnd the first bit about removing the ground cable and the light going on? did it go off?
There are many different 'limp modes' you need to pull the codes or examine the live data to determine whats going on.

Thanks for hanging with me, Rhinoman...I removed ground cable for few minutes ( to allow ecm to reset), re-attached the grnd cable, turned on the key, had an immediate CEL that never went away. I will try to determine what tester was used, and exactly what the tester says about no comm.

OK....where to start??? Took the Tracker to a GM dealer who had a mechanic old enough to recognize an older emission set-up....He read a code 51 with his tester...said the modulator foe the EGR system would not hold vacuum...replaced EGR vacuum modulator...still showed code 51 after reset...he decided the EGR vsv (vacuum solenoid) after removing the EGR valve and verifying it's operation...the dealer wanted $200 for the vsv...I found one (nib) for $30.00..installed, removed grnd. cable to reset ECM, started up and drove for a few miles and CEL came back on....checked blink code..51 (EGR)....my Chilton manual says code 51 is EGR(Claif. emission only)...California emission TracKick's have a EGRT sensor mounted in EGR valve to prove to the ECM that system is working...I have NO EGRT sensor....no loose wires hamging around (I looked for loose wires thinking someone may have replaced EGR valve with a non-California valve)....the GM mechanic thinks the ECM might look at the O2 sensor....everything I have says no...any ideas? Jim

Code 51 is the EGR system and includes an EGR failure as well as an open circuit or short circuit error of the EGR temperature sensor if its a california car. A scan tool can differentiate between the three errors, the CEL method cannot. The ECU determines if it is a California ECU or not by checking pin A4, if it is grounded its a California car. The usual problem with the EGR is simply blocked pipes, you need to try and ensure that they are all clear, including the hole through the head to the exhaust manifold. has your mechanic confirmed that the replacement VSV works?

Thanks, Rhino, for hanging in. Yes, we removed the EGR valve and all was clear....the vsv seems to work fine...but, where is the EGRT sensor? Is it possible A4 is grounded when it shoudn't be? Jim

You won't have an EGR temperature sensor, they were only fitted on California spec models. You should check that A4 isn't grounded but thats very unlikely. I still think that you have a blockage or maybe a split/porous vacuum line. Is your exhaust in good condition with an intact cat?

Yes, we removed the EGR valve and all was clear....

I have to ask how you determined "all was clear"?

The EGR valve itself can get caked up (but you said you replaced it)

There is a passage through the head to the exhaust manifold that can cake up (and can be a biatch to get clean, but doabale with lots of carb cleaner and stiff wire)

There is a passage from the EGR to the intake (some have a tube some through the intake manifold). This can also be a bear to get cleaned (disassembly is usually best).

If ANY of these are plugged, bammo code 51.

All the ports are clean....Remember, this is a 1992 Tracker....my info says that the code 51 only applies to calif. emission vehicles which have a temperature sensor in the EGR valve....if the temp doesnt change with valve operation, the ECM sets code 51 and turns CEL on......Later vehicles (1994/1995) were on Fed regs that also required EGR operation verification....but not my 1992.....I have no Temp sensor on my EGR valve....what does the ECM look at to set a code 51?