ecm/ power delivery issue
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
ecm/ power delivery issue MAJOR UPDATE
bought a 85 vette and the next morning it wouldnt start i believe it may have been caused by me disconnecting the battery over night. however now im not getting any signal from my ecm to the green/ white wire on my fp relay. i recently replaced the ecm thinking it might be that, however still no signal no start. the wire going to the fp relay is fully intact and the ecm is reading just over 12.4 volts of power. if anyone has an idea of what else this could be or needs more info please let me know Update: pulled the distributor cap off and the icm out, tested my ground to the icm and foudn that it has 1640 ohms of resistance believe this may be the issue also replacing original icm while its out
Last edited by n0tdads86vette; Sep 11, 2025 at 05:11 PM.
Team Owner
Joined: Jan 2003
Posts: 20,290
Likes: 238
From: Summerland B.C. Canada
You have started a few threads, why not continue with the ones you already started ?
The ecm will send 12v to green/ white wire at relay for 2 seconds when ignition turned on.
Is that there ?
If the ecm doesn’t see a reference pulse after that, it will stop powering relay.
Is there 12v at orange wire of relay ?
Do you have spark to plugs during cranking ?
The ecm will send 12v to green/ white wire at relay for 2 seconds when ignition turned on.
Is that there ?
If the ecm doesn’t see a reference pulse after that, it will stop powering relay.
Is there 12v at orange wire of relay ?
Do you have spark to plugs during cranking ?
Racer
Joined: Mar 2025
Posts: 485
Likes: 229
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
Team Owner
Joined: Jan 2003
Posts: 20,290
Likes: 238
From: Summerland B.C. Canada
The reference signal only comes into play after the initial 2 seconds of power, and your cranking the engine.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
I don’t have an electrical diagram of your year, but a search should tell you the pinout from ecm to relay so you can ohm test that circuit and rule out wiring.
The reference signal only comes into play after the initial 2 seconds of power, and your cranking the engine.
The reference signal only comes into play after the initial 2 seconds of power, and your cranking the engine.
Racer
Joined: Mar 2025
Posts: 485
Likes: 229
Your responding in two different threads, which makes this difficult to follow. In the other thread you said you weren't getting 12V from the green/white wire correct? This is the rudimentary question, but how are you measuring the voltage?
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
measuring it from the relay location green to black with multi meter having another person cycle the key
Team Owner
Joined: Jan 2003
Posts: 20,290
Likes: 238
From: Summerland B.C. Canada
I looked up 85 ecm pinouts and see two 12v power wires.
Do both those wires have 12v+ ?
B1 and C16
Just thinking the ecm uses one to power itself and the other to control relays ?
Do both those wires have 12v+ ?
B1 and C16
Just thinking the ecm uses one to power itself and the other to control relays ?
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
Last edited by n0tdads86vette; Sep 5, 2025 at 03:37 PM.
Thread Starter
Intermediate
Joined: Jul 2025
Posts: 40
Likes: 1
From: ohio
pulled the old icm due to hearing they go bad on these cars and tested the ground and foundout its got super high resistance, additionally found there was sero thermal compound on the back of the icm to protect it from heat so ill be hopefully fixing both issues this weekend will update post if i find the fix