p1518

[tomloans]

2003 Tahoe 5.3 with about 230k miles...All right folks. I have a p1518 code and after I erase it I just turned the key without starting and it'll hit me again it comes with the reduced engine mode. This is a new one for me after 22 years of owning this car. Any feedback would be great. Thank you

 

[Fless]

Tell us what you're working on -- year, make, model, engine, etc. Consider putting that in your signature so it's always there.

Sounds like a TAC module/wiring issue, but I'll let others chime in once we know the vehicle.

 

[tomloans]

Tell us what you're working on -- year, make, model, engine, etc. Consider putting that in your signature so it's always there.

Sounds like a TAC module/wiring issue, but I'll let others chime in once we know the vehicle.

2003 Tahoe 5.3 with about 230k miles... thank you. Bought new

 

[Fless]

See if @rockola1971 has any more to add than what is in this post:

Weird misfire in 2004 chevy tahoe

I have bought a 2004 chevy tahoe and it has missfire. I have changed all spark plugs, spark plug wires, O2 sensors, and knock sensors with their wire. I also cleaned the MAF sensor, throttle, intake manifold, and intake manifold sensor. All of my Injectors were tested and they work. What could...

www.tahoeyukonforum.com

 

[tomloans]

I have already changed out the harness many years ago. This is an odd code to get all by itself. And it seems severe. I literally just turn the key to the on position without starting the engine and the code and reduced engine power came back.

 

[Fless]

Are you talking about the connection at the TAC, or at the throttle body?

 

[tomloans]

Are you talking about the connection at the TAC, or at the throttle body?

Throttle body only. I had a new spare TBS and I just changed out the throttle sensor ( only ) and it did nothing. I sprayed some cleaner into the throttle body and it is clean and smoothly operating. It was already relatively clean.

Here are the updated symptoms. Very weird. I erase codes. Then I turn the key to the on position with no cranking, the p1518 code comes up with reduced engine mode before I even start the engine. If I continue to start from that position, then all hell breaks loose. (LOL) I get the P1516, P1518, and P2108 codes along with the reduced engine as well. I really think this more to do with the TAC or some short thereof. I love this truck with this one exception. Sure wish we can go back to fly by wire.

 

[tomloans]

Are you talking about the connection at the TAC, or at the throttle body?

One other thing, my long term trims are rather high. 5.5 at idle and 8.0 in drive and stopped. Short term fluctuates by hovers around zero to negative 1 to positive one.

 

[rockola1971]

Get your step stool out and prepare to have a sore back. Your engine to firewall braided ground is broken/loose/corroded. Clean both ends at the block and at the firewall stud with a wire brush. Be sure to give the ring terminals on the end of the braided ground a good cleaning too. Dont be surprised if the braided ground is corroded in half and needs replaced.

 

[Fless]

FWIW drive by wire is the electronic throttle system. Drive by cable is the physical throttle cable system.

 

[tomloans]

FWIW drive by wire is the electronic throttle system. Drive by cable is the physical throttle cable system.

Sorry yes that is what I meant. What a pain this is. Not the first time i have dealt with this either. I have two 2003 tahoes ( his and hers ) and both have had issues. These codes are different this time and it is very pervasive. Not too intermittent.

 

[tomloans]

Get your step stool out and prepare to have a sore back. Your engine to firewall braided ground is broken/loose/corroded. Clean both ends at the block and at the firewall stud with a wire brush. Be sure to give the ring terminals on the end of the braided ground a good cleaning too. Dont be surprised if the braided ground is corroded in half and needs replaced.

I actually took a solid ground from the fire wall and grounded it directly to the engine a few years back. I will give it a tug though and see if that is an issue. Also 30K miles ago, I change the 706 cylinder heads. One had cracked. When I did this, I made sure both grounds back there were solid with the small one on the passenger side replaced, cleaned and reattached. I will figure this out and let you guys know. Start up is the issue. This time it doesn't do it while running ever.

 

[Fless]

You can bet there's a circuit or module issue because when you turn the key to "run" without starting it, it's already sensed that something's wrong. It's tested the circuits in preparation for startup and found something lacking.

In addition to confirming that the grounds are clean and solid, check the connection at the TAC.

Here's the troubleshooter for P1518:

P1518 — 2003 Chevrolet C Tahoe 2WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2003 Chevrolet C Tahoe 2WD V8-5.3L VIN T.

charm.li

 

[rockola1971]

Another failure point is the wiring harness that plugs into the TAC module. It plugs in at the bottom so the wiring harness just dangles off the plug under its own weight. The common breaking point there is one or more of the wires right where they enter the plug body of the harness to the TAC module.

 

[tomloans]

I swapped out the harness some 30 or 40k ago and all has been well since. I just cleaned all the grounds again and checked under the fuse box as well to see if there were mouses chewing up wiring. Took TAC off and sprayed the harness and TAC connection points with electronic cleaner. Put it all back together and still same problem. By the way I appreciate all your feedback! Thanks again. P1518 the minute I turn the key on. Now when I erase it and start it up and all is well, it wont do it for awhile. But it always comes back after a bit. Much quicker than other codes I have had. I ordered a used TAC on ebay. We'll see if that might be the issue. The thing about my other codes is that it would grow gradually and get worse over time. This one was immediately bad and has never changed. So it should be easier to locate you would think since it is easily repeatable. So hopefully the TAC is the issue. For $30.00 it doesn't hurt to try. If you guys think of anything else, please let me know. Thanks again.

 

[tomloans]

You can bet there's a circuit or module issue because when you turn the key to "run" without starting it, it's already sensed that something's wrong. It's tested the circuits in preparation for startup and found something lacking.

In addition to confirming that the grounds are clean and solid, check the connection at the TAC.

Here's the troubleshooter for P1518:

P1518 — 2003 Chevrolet C Tahoe 2WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2003 Chevrolet C Tahoe 2WD V8-5.3L VIN T.

charm.li

Thanks for the link. It may be a bad connection between the PCM and TAC. Maybe I will pull of the connector at the PCM and see if there is corrosion in there. We will figure this out.

 

[tomloans]

SIGH!!!.....


DIAGNOSTIC AIDS
- DTC P1518 sets if the battery voltage is low. If the customer's concern is slow cranking or no crank because battery voltage is low, ignore DTC P1518. Clear any DTCs from memory that may have set from the low battery voltage condition.
- DTC P1518 sets when there is a short to B+ on the TAC module ground circuit. Inspect the fuses for the circuits that are in the TAC module harness-i.e. cruise, brake. An inspection of the fuses may lead you to the circuit that is shorted to the TAC module ground circuit.
- DTC P1518 sets if the TAC module ignition feed circuit is shorted to a B+ supply circuit. The TAC module stays powered-up when the ignition switch is turned OFF. When the ignition switch is turned ON, the TAC module is powered-up before the PCM. DTC P1518 sets because no communication is detected by the TAC module from the PCM. Inspect related circuits for being shorted to a B+ supply circuit.
- Inspect the TAC module power and ground circuits and the TAC module/PCM serial data circuits for intermittent connections.
- Inspect the TAC module connectors for signs of water intrusion. If water intrusion occurs, multiple DTCs may set without any circuit or component conditions found during diagnostic testing.
- When the TAC module detects a problem within the TAC system, more than 1 TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing an individual condition may correct more than 1 DTC. Remember this if you review the stored information in Capture Info.

INTERMITTENT

IMPORTANT: Inspect for improper installation of electrical components if an intermittent condition exists. Inspect for aftermarket theft deterrent devices, lights, and cellular phones. Verify that no aftermarket equipment is connected to the class 2 circuit. If you can not locate an intermittent condition, a cellular phone communication signal may cause the condition.
The problem may or may not turn ON the malfunction indicator lamp (MIL) or store a DTC.

Faulty electrical connections or wiring cause most intermittent problems. Perform a careful visual and physical inspection of the suspect connectors for the following conditions:
- Improperly mated connector halves
- Terminals that are not seated
- Terminals that are damaged or improperly formed

Repair or replace connector terminals in the problem circuit in order to ensure proper contact tension. Remove the terminal from the connector body in order to inspect for poor terminal wire connection.

Road test the vehicle with the DMM connected to the suspected circuit. An abnormal reading that occurs when the malfunction occurs is a good indication that there is a malfunction in the circuit being monitored.

Use a scan tool in order to help detect intermittent conditions. Useful features of the GM Techline scan tool include the following:
- Trigger the Snapshot feature in order to capture and store engine parameters when the malfunction occurs. Review this stored information in order to see the specific running conditions that caused the malfunction.
- Freeze Frame/Failure Records can also aid in locating an intermittent condition. Review and capture the information in the Freeze Frame/Failure Record associated with the intermittent DTC being diagnosed. Drive the vehicle within the conditions that were present when the DTC originally set.
- Use the Plot Function on the scan tool in order to plot selected data parameters. Review this stored information to aid in locating an intermittent problem. Refer to the scan tool Users Guide for more information.

IMPORTANT: If the intermittent condition exists as a start and then stall, test for DTCs relating to the vehicle theft deterrent system. Test for improper installation of electrical options such as lights, cellular phones, etc.

Any of the following may cause an intermittent malfunction indicator lamp (MIL) with no stored DTC:
- The ignition coils are shorted to a ground or arcing at the ignition wires or the spark plugs.
- The PCM grounds are loose or dirty. Refer to Computers and Control Systems Schematics.
- The ignition control (IC) wires are routed too close to the secondary ignition wires, coils, or the generator. Ensure that all of the circuits from the PCM to the ignition coils have good connections.
- There is an open diode across the A/C compressor clutch or any other open diodes.

 

[tomloans]

If it is not the TAC, it likely this issue here below???

- DTC P1518 sets when there is a short to B+ on the TAC module ground circuit. Inspect the fuses for the circuits that are in the TAC module harness-i.e. cruise, brake. An inspection of the fuses may lead you to the circuit that is shorted to the TAC module ground circuit.
- DTC P1518 sets if the TAC module ignition feed circuit is shorted to a B+ supply circuit. The TAC module stays powered-up when the ignition switch is turned OFF. When the ignition switch is turned ON, the TAC module is powered-up before the PCM. DTC P1518 sets because no communication is detected by the TAC module from the PCM. Inspect related circuits for being shorted to a B+ supply circuit.

 

[Fless]

The only way to know for sure is to go through the diag steps. The helpful thing here is that it's fairly consistent and you can replicate it easily. The intermittent ones can be a bugger to find.

 

[tomloans]

So I disconnected both harnesses from the ECU. I disconnected the battery first of course and touched both ends together fora few minutes with the key in the on position for good measure. I cleaned with electronic spray, dried them to evaporating with my new TEMU hand air blower and reconnected them both. All looked super clean. i have since started it up about 7 or 8 times and so far so good. Yesterday it would happen at every turn. Today nothing so far after the cleaning mentioned here. Not sure if ECU had corrosion that cant be seen with the naked eye or there is a broken wire in that mass that just got jostled straight as I was cleaning.