Random P1518 and a few other questions on a new build.

[wlwarnke]

Brand new 5.3 build is giving me random P1518 (TAC to PCM Com error), and VE looks a little high to me but I am also fairly new.

Engine is an LM7, with .030 over flattops, about 10.5:1 CR
Heads were milled .005, ported, polished, new valves and valve job.
Cam is a summit big truck torkinator
Stock DBW with a 90s diesel pedal.
Flex Fuel injectors, running at non FF return style rails.

The tune you see is currently setup for VE tuning. MAF curve has been started, and smoothed out. Hasnt been out on the highway yet as i don't trust the TAC not to fault out on me.

Initially, when troubleshooting I found the ground for the TAC and PCm (on the fenderwell) to be 6.6 ohms to battery so i added a ground to the battery and was able to finish up MAF (still, just on my backroads, so not totally done). Since swapping to VE, it still does it at random times.

I can tell you when in the log the fault occured, but it did.

Normally, I can clear on the fly and keep driving.

Also, does the VE appear to be higher than it should? I haven't checked the fuel pressure while driving, but idling and rev in the driveway, and in gear its holding 56 or so psi.






K5_LS_7_VE_1.hptMAF with TAC Error.hpl

[5FDP]

The throttle body, pedal and TAC need to match. So this calibration file is from a 2003 Suburban, you need to use a pedal/tac that works with that calibration. Usually 2003-2004 stuff works because 2005+ things changed.

Update the cylinder volume for the new engine size, even though it's not a lot bigger that stuff does matter.

I'm not sure what flex injectors are 35lb/hr, all of the GEN 3 ones are 33lb/hr as far as I know.

[blindsquirrel]

Assuming 'Flex Fuel injectors' means 12580426, you should take a look at an '05-up 58psi returnless L59 file. The 0kPa VAC column in the Offset table is quite different from the same column in the '04-down 51psi return-type L59.

[blindsquirrel]

all of the GEN 3 ones are 33lb/hr as far as I know.

33 with the early 51psi systems, 35.4 with the later 58psi returnless ones. Same injector p/n.

[wlwarnke]

The TAC, and PCM match. The pedal part number came from PSI conversions and several other forums. It?s a 90s diesel pedal (APS130). I got the PCM, injectors, and TAC from the same donor.

Based on my calcs the FF 32pph Injectors (25326903) work out to 35pph at the 58psi pressure. My regulator isn?t exactly 58 psi but changing the calc to match fuel pressure didn?t change the flow rate enough so I set it at 35pph.
I thought I did update the cylinder volume. Maybe I forgot. I?ll check it out.

Thanks peeps and keep the ideas coming.

The P1518 is so random. I can?t make it happen in the driveway and only every few test drives. Usually after a write cal. I was hoping the less than perfect ground was the culprit.

I?m also seeing what appears to be the normal 130% pedal position when I send it to the carpet.

Also in my cal is the pedal vs throttle table is slightly tweaked to desensitize the pedal but the P1518 happened a few times before that was changed.

[blindsquirrel]

You still had the stock Offset vs Volts vs VAC table from when it was 51psi. It's not that anymore. Here's your file with an Offset table taken from the 0kPa column of an '05-up L59.

K5_LS_7_VE_1 - ECM 12310 Offset table for 58psi referenced.hpt

screenshot.28-04-2023 20.03.58.png

[wlwarnke]

Updated the tune with the correct cylinder displacement and the injector offset table. VE still around 90% in some cells around 4000rpm. Is this abnormal?

Also, pretty sure I found the random P1518 issue. New TB connector on order. One of the TB pins was damaged and not making a solid connection. Was able to wiggle the TB connector while idling and cause the fault.

[5FDP]

Yes, that is fine. Just look at stock 4.8, 5.3 and 6.0 tunes, they all have values around 94-98 around peak torque.

[wlwarnke]

Awesome. Thanks. Now I?m just waiting on ICT to ship the new connector kit.

I really like that you can order kits with pins, seals, tpa and all relatively cheap. Just wish Amazon stocked them!

[wlwarnke]

Installed the new connector, everything seemed great....for a while.

I still get the P1518 within a mile or so of writing a new tune, sometimes.

Drove across town, 30 minutes each way getting AC Hoses made and got the P1518 codes probably 10 times over the course of the trip. Seems like after some time at speed, then slowing down or stopping for traffic or intersections, it'll pop up and run like crap.

Sometimes a quick click to clear codes and its gone for a while, some time it will comeback and take 3-4 times to get it to stay clear for a while.

Other than that, Tuning is basically done and it runs great. Really great. Have a little bit of occasional knock retard coming in so i need to refine my timing some more.

I'm thinking of heading to the junk yard to grab a TAC and Throttle Body.

When i found the TB connector jacked, i was able to jiggle it at idle and cause the error. I can't make the error happen at this time and it's really annoying.

Attached is a copy of my current tune and the log from the ride home.
Also, I noticed, the help see when the fault occurs, you can see when it goes into Open Loop - ACCEL/DECEL.

The last part of the log is about 1.5 miles of neighborhood driving, the fault occurred once or twice during that time frame.

RoadTrip_1.hplK5_LS_12_Timing_3.hpt

[blindsquirrel]

DTC P1518
Circuit Description
The throttle actuator control (TAC) module and the powertrain control module (PCM) communicate via a dedicated serial data circuit. This serial data circuit is separate from any other serial data circuit on the vehicle. Accurate transmitting and receiving of serial data requires not only good circuit integrity but also adequate system voltage. This diagnostic test monitors the accuracy of the serial data transmitted between the TAC module and the PCM. If the PCM detects a loss of data or invalid data, this DTC sets.

Conditions for Running the DTC
? The ignition switch is in the crank or run position.
? The ignition voltage is more than 5.23 volts.

Conditions for Setting the DTC
? Invalid or missing serial data messages are detected for a predetermined amount of time.
? The above condition is met for more than 1 second.

[...]

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.
? For an intermittent condition, refer to Intermittent Conditions.

Also, there is a #PI that may be of interest:
PI00411C Document ID 1418033.pdf

[wlwarnke]

I?ll confirm grounds again tomorrow. My harness has a ground for the relays, PCM, and TAC separate from the sensors and engine stuff (O2, sensors, etc) so I added a jumper from this ground terminal (on the fender) directly to the battery as I found it to have 6ohms from that bolt to the battery. I did this before I found the bad connector at the TB.

While checking grounds, I ohmed between the TAC and PCM as well as TAC to TB pin for pin. No issues found.

Today at the yard I picked up a TB, TAC, and TAC to pedal harness to have on hand.

So far nothing has jumped out at me.

How sensitive is this thing to power and ground? As it seems to get worse with heat, and alternators do drop some voltage as they heat up, but it?s always more than like 13.3v according to the data.

Tomorrow I will unplug connectors while powered up and see which ones can bring in this code only if I can replicate by jiggling and tugging.

Since I have a TAC on hand. Maybe quick and easy swap for testing.

[SiriusC1024]

Listen. You should follow those service bulletin instructions exactly. There's a difference between continuity and a good ground. You don't know which of the circuit paths to ground you're probing. Also, a ground can become saturated. If there's loose/corroded contact your meter's low current is one thing. When everything is powered up a weak ground might not be able to sustain the demand.

Hell, I've had it happen with a headlight. Turn on brights and it killed the ignition. Saturated ground. Ignition and headlights evidently were using the same ground path. Added a separate ground for the headlights and problem solved.

That's why the TSB mentions tightening to ensure good contact. GM doesn't issue TSB's on a whim. "Please follow this diagnosis process thoroughly and complete each step."

Go a step further and wire brush the ground points. Put in honest work and you'll be rewarded.

[blindsquirrel]

Isn't that 'dedicated serial data circuit' they go on and on about supposed to be a twisted pair, and also shielded? They really do not want any noise on those lines. Always a suspect in swaps where the harness has been modified.

[wlwarnke]

Listen. You should follow those service bulletin instructions exactly. There's a difference between continuity and a good ground. You don't know which of the circuit paths to ground you're probing. Also, a ground can become saturated. If there's loose/corroded contact your meter's low current is one thing. When everything is powered up a weak ground might not be able to sustain the demand.

Hell, I've had it happen with a headlight. Turn on brights and it killed the ignition. Saturated ground. Ignition and headlights evidently were using the same ground path. Added a separate ground for the headlights and problem solved.

That's why the TSB mentions tightening to ensure good contact. GM doesn't issue TSB's on a whim. "Please follow this diagnosis process thoroughly and complete each step."

Go a step further and wire brush the ground points. Put in honest work and you'll be rewarded.

Thanks for the reply!

If the weather holds up, I?m going to clean and inspect all my power and ground connections today.

[blindsquirrel]

If I had a Gen3, it would be cable-throttle only. I don't trust the stuff from before they wised up and put everything inside the same box. With everything completely stock it's fine but...

[SiriusC1024]

Isn't that 'dedicated serial data circuit' they go on and on about supposed to be a twisted pair, and also shielded? They really do not want any noise on those lines. Always a suspect in swaps where the harness has been modified.

Good catch. If that has been modified to where the cable has been cut the correct thing to do would be to splice in a serial cable the whole length. Remember, the shield is to be grounded at only one end. If the swap was done with the original harness intact between the PCM and TAC it should be fine.

dbw_2.gif

If I had a Gen3, it would be cable-throttle only. I don't trust the stuff from before they wised up and put everything inside the same box. With everything completely stock it's fine but...

Absolutely.

[wlwarnke]

Cleaned and torqued all my grounds.
Checked power feed to TAC.

Unplugged TB with ignition on and got like 5 codes.

Unplugged TAC while powered up, got like 3 codes.

Finished up AC evac and charge. Engine idling for a long time while charging and testing AC. Used bidirectional controls to idle at 1000rpm. AC on, headlights on, stereo system playing. Jiggled connections and fuses. Couldn?t get it to fault.

On the bright side, my PCM controls the AC compressor and cycles fans as planned, and my custom Math reflects AC pressure pretty darn accurately.

Swapped out the TAC for shits and grins. Will test drive later.

[wlwarnke]

Isn't that 'dedicated serial data circuit' they go on and on about supposed to be a twisted pair, and also shielded? They really do not want any noise on those lines. Always a suspect in swaps where the harness has been modified.

Good idea on the twisted pair. This aftermarket harness does not have twisted pair for speed sensors or com links. I thought about that briefly, but figured there would be way more posts on people having this issue.

I have never seen a shield on any factory harness, usually just a really tight twist, as if they used UTP (Unshielded Twisted Pair). I wonder if the big name, high dollar aftermarket harness manufacturers use UTP in their harnesses.

At work (Power Generation), we shield every signal wire but they are also usually routed in cable trays along side 480v and 4160v cables.

It is nice bouncing stuff off guys that know stuff and not just regurgitate stuff they have read.

[SiriusC1024]

At work (Power Generation), we shield every signal wire but they are also usually routed in cable trays along side 480v and 4160v cables.

It's all fun and games until someone does an AC upgrade wrong and runs the motor leads coming off a VFD through the same location as the original DC motor leads. I work with clowns.