TAC Module? Ignition Switch? PCM Fried?
Thread Starter
Heel & Toe
Joined: Dec 2018
Posts: 16
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From: Fort Walton Beach, FL
TAC Module? Ignition Switch? PCM Fried?
I’m at a loss on what I can do. My vette been intermittently dying while driving and having starting issues for the week and today it finally kicked the rock. I know that the alternator, battery, and starter along with all the fuses and relays. After trouble shooting the best I can with some help I came up with it being the TAC module. I’m throwing P1518 along with getting low voltage, reduced power, and service traction says on my DIC. I don’t really wanna start throwing money at expensive parts that won’t fix my problem. I haven't taken apart the ground connectors (car has never been up north and only has 50k on it) but i cleaned the lugs and added dielectric grease. Hopefully my insurance will pull through for me so I can get the car towed back on base for me. For the past 3 hours Ive had my head buried in the forums associated with P1518 and it seems the most common culprits are either the ignition switch or the TAC module, Ive heard a bad TAC module can fry other sensors and the PCM though, I'm really trying to avoid going to a shop, and would rather do the work myself, if i can. Tomorrow I am going to try and find a ride to the dealership tomorrow to get my hands on some wiring diagrams. What do you guys think my next course of action should be?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
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From: Cape Coral, Florida
It’s most likely your ignition switch (99.9%) especially when it throws a P1518...the DTC is an “effect” of your problem and not the “cause” !! (Low voltage going to your TAC module) !!
Last edited by C5 Diag; May 16, 2019 at 11:07 AM.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
100% DIT DOT CORRECT! 
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired
Last edited by Bill Curlee; May 16, 2019 at 12:04 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
Likes: 4,057
From: Cape Coral, Florida
100% DIT DOT CORRECT!
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired
Tech Contributor
Joined: Dec 1999
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
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From: Cape Coral, Florida
Last edited by C5 Diag; May 16, 2019 at 01:16 PM.
Thread Starter
Heel & Toe
Joined: Dec 2018
Posts: 16
Likes: 0
From: Fort Walton Beach, FL
Luckily I got the car home, as C5 Diag suggested i cycled the key 20 or so times.. I got under the floor board and poked around with the two main ignition fuses and they are good. I cycled the key a few more times and replicated the no start 'low voltage' condition again, and just as Rob called it my DIC displayed 4.9 volts. Looks like ill be ordering a new switch :P
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
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From: Cape Coral, Florida
GREAT !!...at least you saved yourself a towing fee !!...4.9 volts ??...that's a 7 volt "voltage drop" !!...that circuit that feeds the IPC (and also the TAC Module) where you read that "voltage" comes from the ignition switch in ON or START...that PINK wire which GM has been using like....forever !!
Last edited by C5 Diag; May 16, 2019 at 01:25 PM.
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Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
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From: Cape Coral, Florida
Thread Starter
Heel & Toe
Joined: Dec 2018
Posts: 16
Likes: 0
From: Fort Walton Beach, FL
I appreciate both of you !! Saved me lots of money on labor time couldn't ask for better help. I need to get my hands on some wiring diagrams because I'm blind without them! lol
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
https://www.bbbind.com/tsb-wiring-diagrams-database/
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
Likes: 4,057
From: Cape Coral, Florida
Check out this website. It gives Technical Service Bulletins (TSBs) and wire diagrams for almost any vehicle. Just make sure you select wiring diagrams before you search. It defaults to TSBs:
https://www.bbbind.com/tsb-wiring-diagrams-database/
https://www.bbbind.com/tsb-wiring-diagrams-database/
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,598
Likes: 4,057
From: Cape Coral, Florida
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Along time ago Army Knowledge On Line AKO use to have a database that provided Automobile wiring diagrams that you could use. Don't know if that still exists I use the Navy On line data base for mando training a lot and have not pulled up Army in a while.
Thread Starter
Heel & Toe
Joined: Dec 2018
Posts: 16
Likes: 0
From: Fort Walton Beach, FL
100% DIT DOT CORRECT!
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired
The ignition switch contacts get corroded/burnt and the output voltage on the "HOT in RUN/START" buss is low. Modules do not function under low voltage so, you get what you are seeing. Very common problem on C5s. Well documented!
Start there. Measure the voltage on the TAC Module FUSE. There are TWO small test points on top of each fuse. Measure both test points (one at a time) to chassis ground. Measure with the key ON and when you attempt to crank the engine. See what the voltage reads. It should be exactly what the voltage on the battery terminals are. If it reads LOW, your ignition switch need to be replaced. You can service the contacts inside but, its not for the faint of heart.
See my post below:
- C5 ignition Switch repair - http://forums.corvetteforum.com/c5-t...ch-repair.html
Bill
Bill FTCS (SS) E8 Retired