My 87 Blazer is Possessed.

[CloneoftheMean]

I have an 87 Blazer that popped up a Code 42. I replaced pretty much every part related to the TBI system, multiple times even. I finally found a computer and prom from an 88 Blazer and that seemed to fix the problem.
Now it kicks and spits and shifts super hard when I accelerate. My son was riding with me and said to pull over and pop the hood. He cut the wire from the alternator to the battery and the truck ran flawlessly. Like it was brand new. We hooked the wire back up to the battery and the Blazer fell flat on its face again.
Has anyone had this happen, or anything similar?
I'm stumped and about to burn this thing to the ground...

 

[fast 99]

check AC [not DC] voltage or have the alternator bench tested.

 

[Sad Sack]

I once had an alternator go into an overcharge state of 18 volts and really screwed my car (71 Camaro) up drivability wise, ruined my battery and cost me a tow.

 

[gmbellew]

besides a bad ICM, which may or may not set a code 42, code 42 is an issue with the EST (white) or bypass (tan/black) wires of the ICM harness. they either see unexpected voltage or unexpected open or could be shorting out on something. the red/black wire is a ground, and if for some reason it isn't grounded it can cause some drivability issues and errant tach readings.

so by design, if you unplug the bypass to set timing property, a code 42 is set. codes are stored for approximately 30 starts i think.

the ECM is not as drivetrain specific, but the PROM is. you might try swapping the old PROM into the new ECM. the PROM is the coding/tune for the computer to control fueling, etc.

ECMs can go bad, but it seems to be rare in my limited experience.

did it pop code 42 while you were driving?

 

[CloneoftheMean]

We've tested the voltage and current. 2 previous alternators were bad, putting out wrong current. The newest is working corretlyand giving 13.8 volts. I even swapped out the battery for fun. No difference.

gmbellew, there's no new codes. The truck just isn't happy w the alternator hooked up. I'm going to install a new wire w a heavier jacket and try to keep it as far from the block as I can. I kind of want to replace the TPS...

 

[Matt69olds]

As someone mentioned, check A/C voltage. The alternator can have a leaky diode and still charge just fine. However, some of the electronics might not like the noisy A/C voltage.

As for the transmission shifting hard: your truck is a 87, with a TV cable. The transmission relies only on the tv cable to determine shift points and line pressure.

If it shifts hard, start by double checking the tv adjustment.

 

[CloneoftheMean]

It only shifts hard when the alternator is connected... Just more of the mystery.
I need Scooby Doo and the gang on this one!

 

[Sad Sack]

I had a weird alternator issue on my square, when I first got it the battery was dead overnight every night. Obvious draw and a big one at 4.5Amps, did the fuse and amp draw testing, I even dropped in a spare alternator, Tester showed it was charging correctly but once I removed the pigtail the draw stopped, but the dead battery problem remained but long story short the previous owner had evidently installed a replacement alternator pigtail, but wired it backwards, compared it to another pigtail then went AhHa and once I reversed the red/white wire, problem solved.

PO install:

You must be registered for see images attach

Then after I looked up the replacement pigtail, lightbulb moment:

You must be registered for see images attach

 

[Grit dog]

Idk, similar issue with a newer Ford. Bad alt diode was causing cam sensor coaxes and ran like schitt.
You’ve pretty definitively narrowed it to the alt. Try another new one, imo.

 

[Ricko1966]

We've tested the voltage and current. 2 previous alternators were bad, putting out wrong current. The newest is working corretlyand giving 13.8 volts. I even swapped out the battery for fun. No difference.

gmbellew, there's no new codes. The truck just isn't happy w the alternator hooked up. I'm going to install a new wire w a heavier jacket and try to keep it as far from the block as I can. I kind of want to replace the TPS...

Check AC voltage !!!! there shouldn't be any. If it has AC ripple that's a problem.

 

[Rusty Nail]

Placing my bet on ground cable to the alternator bracket.
Bonus points the ground ON THE FENDER is? M.I.A.
Or!
Maybe it's the oil pressure sensor TO FIREWALL..
AHH!
Does it have an amateur installed aftermarket radio?

 

[CloneoftheMean]

Rusty, yes I installed a radio, but that was 15 years ago. Directly to the battery (fusebox) Buuuuut just for S&G's next time I drive it I'm going to keep the radio off for testing purposes.
Everyone else, thanks for the great input. Some good advice for sure. Not the typical "check your tire pressure" kind of crap I see.

 

[Beast496]

But have you checked your tire pressures yet? lol

 

[Rusty Nail]

^ it's very important.

xc_hide_links_from_guests_guests_error_hide_media

 

[fast 99]

Really don't understand why people ask for advice, then don't follow it. You can have perfectly good DC output with excessive AC voltage.

There will always a very slight amount of AC. A good battery will absorb it and poor battery may not.

Bring the engine slightly above idle, check AC at battery with digital meter. If it is anywhere near .100 [one 10th volt] alternator is toast. A good alternator will be much lower in the .020 or lower range. There is not a hard and fast number because alternator quality, wire and battery condition varies.

If that checks out start cleaning and checking all the grounds.