My '90 RS 305 TBI is throwing a code 42, and it's a bit hard to start and idles a little rough. Other than that it runs fine.

I've been trying to determine the cause of the code, which states it's either an open or short in the EST circuit, a faulty ignition module, or faulty ECM.

I've tested the ignition module three times and all three times it came up good. I'm thinking there's a problem with wiring from the ECM to the ignition module.

Here's a reference I've been using

http://www.gearhead-efi.com/gearhead...c/TBIcodes.pdf

I'm not experienced at all with electrical circuits, though I have a multimeter.

Assuming I do find a problem with one of the circuits from the ECM to the ignition module, I'll have to tear through the wiring harness to locate this one wire and replace it, or what?


Any advice is appreciated, thanks.

 

With the hard to start problem, the problem is most likely a grounded EST line, circuit 423.

Check that first.
Also a PROM not fully seated in the ECM can cause a Code 42

 

Oh, just realized you said your not very good at electrical stuff.
To test if its grounded:

1. Turn ignition to OFF
2. Unplug ECM connectors
3. Turn ignition to ON
4. Set OHM Meter to 1000-2000 OHM Range
5, Probe ECM harness connector Circuit 423 with the RED lead on the OHM meter and the BLACK lead on a ground. It SHOULD read LESS than 500 OHMS

If it does not read less than 500 OHMs the you have an open circuit (no connection) or a bad ignition module (which you said tested good).

 

This is the part that's confusing me, when it says unplug ECM connectors, do I unplug the connection that is located AT the ECM, or the end that is located at the ignition module? Because the circuit runs from the ECM to the ignition module, so I'm confused as to whether I need to unplug the ECM connection at the module or the ECM.

Also, assuming I do find a problem in the circuit, because I know the module is good, then I know there's a short/open in the circuit then what should I do? Try and somehow track the short in the circuit down? That sounds very tedious.

 

Disconnect it from the ECU. Your testing what the ECU would see.

If you do find a short/open then yes you will have to follow that wire through the harness and find the break. Or you can cheat and cut the wire a few inches from each end and solder/splice your own. Then you wont have to open the harness and the crap wire is taken out of the circuit completely.

 

Good to go, I'll test it the day after tomorrow.

 

I tested it this morning, circuit 423 read .503 K Ohms. That's just outside of the 500 ohms it should be less than, what do you think?

 

Bump. Anyone??

Also, if I do replace the circuit 423 wire, what gauge wire should I replace it with?

 

 

Thanks for the reponse.

The circuit should read less than one ohm? So right now it's reading 503 ohms, thus it's reading 503 times as high as the normal resistance reading?

So there's most definitely a problem there, right?

 

 

No dice. I soldered in a new wire about two inches from the ECM harness connector running all the way to about two inches away from circuit 423's connection to the ignition module (D4).

SES light still comes on about five seconds after starting the engine.

 

 

Just figured I'd update you guys and anyone that has a code 42 problem.

I replaced circuit 424 (tan/black wire) and that fixed the problem.

So, if anyone experiences a code 42, check circuits 423 and 424, even though there might not be any visible damage at the ECM and ICM sides, there is likely a short somewhere.