1987 Chevy 3/4 Ton Pickup Code 42

[chika989]

Since December 2011 my '87 3/4 ton Chevy pickup has been giving Code 42. I've done a bit of online research but don't completely understand the potential fixes for this problem. Below is some info I discovered on the internet but now I don't know how to implement my new found info



Typical causes for code 42 include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM


I have checked #3 and the PROM is properly seated in the ECM
I have checked several times #4 and connections are fine
#5 factory wiring harness looks like it has never been disturbed
#6 ignition module is new
#7 up to this point I have not replaced ECM because I don't know how to test it to see if it is faulty

#1 cause for code 42 appears to be BYPASS line is open or grounded.....could someone please explain exactly what that means and how do I check it. I don't know how to identify the BYPASS line and I don't know how to determine if it is open

#2 cause of code 42 appears to be EST line is open or grounded. sorry but again I don't know how to identify the EST line and I don't know how to determine if it is open

Any help would be much appreciated.

[beamn7]

Bypass line is the brown wire with a black tracer that has a in-line connector near the distributor and firewall that you disconnect to check the timing. This code will set by either unplugging the wire or if the EST module is faulty.... Check to see if that wire is unplugged....

[chika989]

Checked the BYPASS....it was plugged in so I unplugged it and plugged it back in....it seems fine. I cleared the code 42 and took 'er for a test run. Code 42 came back.

I'd like to move on to check the EST line to see if it is open or grounded but but don't know how.....

Any help in identifying the EST line and how I would go about checking to see if it is open or grounded would be much appreciated.

Thanks!

[beamn7]

EST is Electronic Spark Timing. The EST wire is the white wire that goes to pin D on the ign module at the distributor.

You may want to check the two bolts holding the module to the distributor. The bolt next to the 4 pin connector is also the ground for the module. Seen more than one truck that had a loose or stripped bolt causing a performance issue etc....

Here is a schematic showing the EST line.

The rest is available at:
http://www.chevytalk.org/fusionbb/sh...hp?tid/212353/

[chika989]

Still working on this problem!

Regarding the seven causes of Code 42 listed in my earlier message, I have followed up on all of these, replacing parts as necessary.

1. By pass line is neither open nor grounded - checked this with OHM meter
2. Have checked EST line with OHM meter and checked out good
3. PROM and CAL pack are properly seated in the ECM
4. Have replaced the connections between the control module and ECM
5. Have checked routing of EST harness and have determined that ignition wires
are in good shape
6. Replaced ignition module twice
7. Replaced ECM with new ECM

I have also retimed and adjusted the idle back to factory specs.

Truck still stalls about every 1/2 mile, will immediately start up but throws a code 42 again!

I am at a complete loss here and have no idea where to go from here!
Any suggestions would be much appreciated.

[chika989]

Just an additional thought: Is there actually any connection between the truck stalling and the Code 42?

[beamn7]

Good point.........
Question though, have you actually cleared the code after setting the timing and plugging the bypass connector back in? The code wont clear by itself....
Might want to take a close look at what is called the "pole piece" aka the magnet and the pickup coil on the dist itself. Could be swollen and / or corroded. Might not be a bad idea to just replace both of them....

[chika989]

Hi and thanks for your reply.
I disconnected the battery and let the truck sit for couple of hours - I believe this will clear the code. If I'm wrong here, please let me know.
I've replaced the pickup coil on the dist but I'm not sure what the "magnet" is. ????

Thanks again!

[Weasel Diesel]

Originally Posted by chika989

I've replaced the pickup coil on the dist but I'm not sure what the "magnet" is. ????

Thanks again!

The magnet is made onto the distributor shaft and is serviced by replacing the shaft.

[Weasel Diesel]

Say,... just thought about it... Have you by any chance set the ignition timing without disconnecting the bypass? If you have, then maybe the timing being out of wack may be setting the code due to a false signal. The bypass must be disconnected when setting the base timing. There was also 2 TSB's that were put out on the problem back when I worked for GM..... 881806E for false engine codes, and 8842 for incorrect prom installed. my memory is a tad fuzzy, you'll have to look up the tsb's to be 100% positive.

[beamn7]

You should always replace the pole piece when replacing the pickup coil. Here is a picture of it. Application for your vehicle is GM part# 10495802

[beamn7]

Here are the attached aforementioned documents / Bulletins....

[chika989]

Thanks for posting the tsb's.....much appreciated.

Back at work now....4 12 hr shifts.....doesn't leave much time for working in the garage. I'll get back to it on Sunday. You guys have given me some good suggestions and I'm anxious to put them into action.

Thanks again!

[burb1085]

Did you ever get this fixed? I have the same issue.

[chika989]

Currently back to work.......four 12 hour shifts...doesn't leave much down time. I will post here over the weekend things that I have done. But to answer your question......I am still having intermittent code 42 issues. :-(