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General 1g ECU no idle control (board repair)

dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
I'm testing out a stock 1g ECU (1993 EPROM) and discovered it has no idle control. I know it's the ECU since my original ECU works fine. It doesn't fast idle on startup, compensate for any electrical load, nor does it compensate if I turn the AC on. My logger shows the ISC steps moving, but no change in idle speed. It eventually pegs at 0 or 120 steps.

I have no prior knowledge of the condition of the ECU. The board is all original with no repairs. Capacitors were original and showed obvious signs of leakage, so I replaced those before testing it. I cleaned up the corrosion as best I could and nothing seemed obviously damaged. I do still see some corrosion up underneath the heatsink, in the general area of the four ISC pins. I still need to remove the heatsink to gain access to that area to clean/trace the circuit and inspect for broken traces.

I've heard of two ways an ECU can lose the idle circuit - either a malfunctioning ISC or corrosion. Either one could be a possibility. If it were caused by the ISC, what components would I be looking to replace? Any other tips on fixing this ECU?

I'm pretty competent when it comes to soldering and electronics repair, but not so much at diagnosing without just throwing parts at it. I'd like to avoid sending for repair, but always an option if this goes above my head.
 
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92AWDHX40

Supporting Member
13,022
906
May 22, 2007
Kalamazoo, Michigan
I never seen anyone get that involved withe the ecu . A few links online that's it . I tried to fix it once and fried the board .
 

dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
Bring me a bottle of Tamdhu Batch Strength Scotch and I'll walk you through the ISC circuits.
How do I know you're over 21? :p ROFL

I found some good stuff on ECMTuning's Wiki that shows a blown ISC driver from a faulty ISC, and mine has no damage like that: http://www.ecmtuning.com/wiki/driver01#g_parts_90-_94_ecus. I'll start by pulling the heatsink and give it an inspection/cleaning and double check all the ISC traces. Even if it's not under the heatsink, I need to clean it really good to make sure the cancer doesn't spread anymore.

Here's a pic of the ECU before I cleaned the board and replaced caps:
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steve

DSM Wiseman
15,001
1,802
Feb 3, 2002
Boulder, Colorado
Given what I'm seeing around the voltage regulator and two of the injector drivers I'd say you have a good chance of finding some issues under the heatsink and maybe under that front capacitor.

It was a nice 93 EPROM ECU but they didn't change the caps soon enough.
 

dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
I pulled the heatsink last night but didn't immediately find any broken traces. Some more corrosion, though. Here's a few pics of what I found under the heatsink. The two blue marked pins and the two directly underneath are the ISC circuit. I need to look at these pics more closely from the computer; my phone doesn't do it justice.

When I replaced the CAPs, I noticed my solder didn't wick to the top of the board. So I need to touch up the topside and test it again. I'm really hoping it's something as simple as that.

I also need to buy some more electronics PCB cleaner. Any recommendations for a good product to use in this situation?


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dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
I've hopefully found the problem under the rear capacitor. It looks like the trace (via?) is broken and missing the solder pad. I should get continuity to pin 68 from this via and I do not. Now I just need to figure out how to fix it. :hmm:
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dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
Pics say more than words.
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dwb

Proven Member
359
191
Sep 9, 2021
Broomfield, Colorado
Just tested my repair and no bueno. Still no idle control. Back to the drawing board. I might send into ECMTuning if I can't find anything else.
 
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