Please Help
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1st Gear
Joined: Mar 2026
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Please Help
Hi all I'm new to this. My husband recently did an engine and transmission swap on a 99 c5. After the swap the fuel pump was not triggering or turning on. He tracked it down to pins 85 and 86 both having 12v with the key on. Any ideas on what might cause this? He checked all 9 body grounds on the corvette , all were good but still cleaned pulled original ecu out, bench tested it , fuel pump control wire does send ground on power up for 2 seconds on the bench , with ecu disconnected and key on there is only 12v on 1 of the two pins on the relay. Connected new ecu and same issue. Grounds for 02 sensors show no continuity on 1 ground wire, the second ground wire from the 02 sensor shows very high resistance. Have also added 2 extra grounds from c105 and back of heads straight to battery.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,319
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From: Cape Coral, Florida
After reading your testing sounds like you need an auto electric shop...how did he "bench test" the PCM ??...no idea of how you tested these circuits...we don't check these circuits using resistance testing...cleaning is not testing grounds !!
Last edited by C5 Diag; Mar 24, 2026 at 10:21 PM.
Tech Contributor
Joined: Jan 2007
Posts: 19,246
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From: Dyer, IN
Hi all I'm new to this. My husband recently did an engine and transmission swap on a 99 c5. After the swap the fuel pump was not triggering or turning on. He tracked it down to pins 85 and 86 both having 12v with the key on. Any ideas on what might cause this? He checked all 9 body grounds on the corvette , all were good but still cleaned pulled original ecu out, bench tested it , fuel pump control wire does send ground on power up for 2 seconds on the bench , with ecu disconnected and key on there is only 12v on 1 of the two pins on the relay. Connected new ecu and same issue. Grounds for 02 sensors show no continuity on 1 ground wire, the second ground wire from the 02 sensor shows very high resistance. Have also added 2 extra grounds from c105 and back of heads straight to battery.
