1989 Suburban Wiring harness

Good Morning,

If someone has already talked about his, I'm sorry for doubling up. I am restoring a 1989 Suburban and there is an issue somewhere in my wiring harness that is giving me an 042 code. It'll run right for couple days and then just fall on its face randomly. The mechanic helping me can't seem to find the issue and we figured we would just put a new wiring harness in. I haven't been able to find any online. I checked with Painless and they only made up to the 1987 with the squarebody's. Any recommendation where I may be able to find this? It is for a 5.7 350 TBI.

I just finished re doing my 89 suburban harness myself. Four main things I found to be bad.

One was bad grounds on the back of the block and up by temp sending unit for ecm on intake by thermostat.

Two firewall bulkhead connection was full of tar stuff and possibly bad connections there.

Third was a faulty ignition switch. The pink wire with the black stripe was have a really back connection while just jiggling the key ever so slightly.

Forth was bad ignition module in distributor. Went through 3 of them in one year. Dont know what was taking them out but spent big bucks on last distributor and haven't had any problems since.

This ^^ and a coil sometimes fading will set it

If the distributor/ ignition module aren't properly grounded, you'll burn thru several in a short span.

When the module starts dying, so does the coil


Edited again,

Sometimes if you overtorque the knock sensor above the starter, it'll set it (because if it's too deep in the block, it becomes overly sensitive to valve train noise and such)

But that's only usually if you turn the truck on and the code sets like, a couple minutes after turning it on when it performs a self test of the ESC system

Here's my journey thru code 42 struggles on my 89 burban.

in addition to the above, also check the connectors for the ICM and make sure they are good. seems i remember a fella recently here and that was his issue....hard to track down that kind of stuff for sure.

This is all great information, I've already replaced the ignition module and it is a brand new engine. I wish we were more thorough with the harness when we had everything out. I'm going to mess with it some more this week and I'll let you know if we can find the culprit.

Again, very much a beginner at this, does anyone know what the major differences would be with a wiring harness from a 1987 compared to a 1989? Painless Wiring still makes one for an '87, but not an '89.

87beachburb said:

This is all great information, I've already replaced the ignition module and it is a brand new engine. I wish we were more thorough with the harness when we had everything out. I'm going to mess with it some more this week and I'll let you know if we can find the culprit.

Again, very much a beginner at this, does anyone know what the major differences would be with a wiring harness from a 1987 compared to a 1989? Painless Wiring still makes one for an '87, but not an '89.

Click to expand...

OK, so I JUST literally did this.

My 89 suburban had an engine bay fire and I used a uncut 1987 suburban harness that had rear ac and smog equipment.

The primary bulkhead is not the same shape.
You will have to cut it about a foot away from bulkhead and solder and splice I'm afraid.

I deleted all the smog stuff from the harness.

The alternator wire had to be lengthened because 87 is pass side. 89 is driver side .

The 87 alternator was 2 wire. My suburban is just 1 wire.

Same with the compressor for the ac.

The oil pressure switch is a 2 wire on the 87, the 89 is a 3 wire. I had to get a new plug + pigtail since mine got burnt.
That was the "major" changes I had to make.

I traced down every single wire I cut at the bulkhead so I knew where they went.

DO YOURSELF A FAVOR and label/tag each wire at the bulkhead before you cut, that way you can trace it all back should you go this route....


I'm currently driving the truck with no ses lights, and everything functions as it should because I took my time and spent several days making sure it was all right and done correctly.

That's the unfortunate part.... the cutting the bulkhead.

If you need information/ help, it's all still fairly fresh on my mind and I could probably help you via telephone.

I really appreciate all the insight, you guys are definitely way more knowledgeable than me and maybe more than my mechanic. Long story short, it still isn't right and yesterday I lost power going 30mph down a side road. It smelled like burning metal, but once I let it sit for 5-10 minutes it started right back up and I drove it to the mechanics. I'm still getting the same code. Basically from 0-35 the truck runs rough. I'm thinking it is multiple issues, maybe transmission because around 35mph, it shifts in and and out and loses power, maybe torque converter? It sometimes needs me to pump gas for it to start and stumbles from 0-10mph. We redid the grounds and ignition module along with the distributor. I've attached a pic of what I found under the dash, the mice were living lavish. Is there anything these mice could have done to cause this?

Going crazy, ready for this to be mechanically sound to be redone and painted. Could it be the TBI? Anything that I could throw parts at?

mice could have chewed wires under the dash....I'd check things over thoroughly while you have the dash pad out.

Isnt code 42 the distributor code that the OEM distributors were problematic and caused it to set erroneously?

That's to bad the mice got in there. I would be definitely checking the harness. Especially by the ecm theres a pretty good spot for them buggers to make a nice home up there.