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DonL · 11-12-2016, 11:09 AM

I am posting here because I need a fresh perspective on a recurring problem and this forum gets more traffic of RV knowledge than any other.
Last February we bought a 2003 Tiffin Allegro Bay on a 2002 Workhorse chassis and 8.1 liter Chevrolet engine with 32,000 miles on the odometer. On the first cold morning start it displayed a check engine light and a reduced power (limp mode) light. After letting it idle for about five minutes I shut it down and restarted it without the reduced power light. The coach ran normally, but of course the check engine light stayed on. Trouble codes were:
· P1516 Throttle Actuator Control (TAC) Module Throttle Actuator Position Performance,
· P1517 Throttle Actuator Control Module Performance, and
· P1518 Throttle Actuator Control Module Serial Data Circuit.
I cleared the codes using a hand held code reader. Later cold weather starts (below 50 degrees) reset the P1518 code and set the reduced power condition. A five minute warm up followed by shutdown and restart cleared the reduced power light and the coach ran well. With weather above about 50 degrees it started and ran normally. I replaced the Throttle Actuator Control Module, but by then we were into warmer weather and I put about three thousand miles on the coach without that issue. I did replace the ignition switch when I found a melted horn ground wire that is routed through the ignition switch.
With cooler fall weather the old gremlin returned with a P1518 code and reduced power at first cold start below 50 degrees. After checking voltages and grounds, even adding a new engine to chassis ground strap, I took the coach to a shop that is better equipped than I to analyze the trouble code. So far the shop has replaced spark plugs (very badly burned), the mass air flow sensor, and the throttle body, all to no avail. They have a replacement wiring harness on order.
What are we missing? I would really welcome any insight into other things that might cause the symptoms we are experiencing. Thanks in advance for your ideas.

WillsDP · 11-12-2016, 11:51 AM

Not an expert in the 8.1, is the throttle or has pedal electronic? I have 3 cars where the throttle actually was defective and caused it to go to limp mode.

DonL · 11-12-2016, 03:33 PM

Yes, this is a throttle by wire engine. The throttle pedal position sensor is about the only part we haven't thrown at the problem, but I if that sensor was bad, or the connector damaged I would expect a P1125 code, Accelerator Pedal Position System. Thanks for your response.

wa8yxm · 11-12-2016, 05:34 PM

Yes th 8.1 (Well modern ones) are "Drive by wire" No mechanical linkage between pedal and engine, all electronic.

Sounds like the module is defective but it might be just a dirty connector or rodent damage to the wires. Wish that was all that was wrong with my 8.1 (Big hole in oil pan, connecting rod cap lying in pan).

partskenn · 11-12-2016, 05:51 PM

Quote:

Originally Posted by WillsDP

Not an expert in the 8.1, is the throttle or has pedal electronic? I have 3 cars where the throttle actually was defective and caused it to go to limp mode.

I am not qualified to diagnose your problem, but as a former GM dealer parts department employee, all I can say is that we did sell a pretty fair number of pedal assemblies because of defective sensors.

Arch Hoagland · 11-12-2016, 11:50 PM

Did they replace just the plugs or did they also replace the wires?

Keep us updated on what you find with this problem.

ScottyBye · 11-13-2016, 12:43 AM

Yes please...Update us when you find the problem and fix for it! I have the same engine and chassis (2004) and similar milage but should be the same.

I had issues with Throttle Body Position Sensor on my 2009 Chevy Silverado, just 6 months ago. Local dealer fixed at no cost because they said even though the warranty had expired a few years ago, it was a known issue and there was a TSB issued for it.
Maybe worth checking for TSBs

Clay L · 11-13-2016, 12:10 PM

I replaced the throttle module and the pedal assembly at the same time (P1518 code). On the pedal assembly it was easier to remove the three bolts holding it to the bracket and then remove the keeper and connector. Don't know which one was the culprit.
Years later I had to replace the the throttle plate motor assembly that is on the top of the throttle body P1516 code)

bluepill · 11-13-2016, 12:34 PM

FWIW:

Be aware that although OBDII codes can be very specific, they do not always point you in the exact direction you need to go.

Jim Morton, a multi-decade Auto Tech trainer who also owned a "Last Chance" auto diagnostics center in Philly (for cars that had been "Impossible to Diagnose" by other shops and dealerships) offers this advice for any diagnosis:

First: Do a complete check/test of the basic electrical system (alternator and battery). This only takes about 15 minutes. If there is a problem here, it can effect any of the electrical/computer systems downstream. "Garbage In, Garbage Out" as they say in the computer world.

Second: Check all connections in the problem circuit. Power, grounds, plug-in connectors.

His shop charged a hefty flat fee for doing a complete diagnostic, and he says they often found problems with these basic tests. Those jobs were his biggest money makers.

Good luck.

DonL · 11-14-2016, 05:29 PM

Quote:

Originally Posted by Arch Hoagland

Did they replace just the plugs or did they also replace the wires?

Keep us updated on what you find with this problem.

So far as I know, just the plugs. I don't understand how plugs or plug wires could cause the P1518 code. Can somebody educate me? (or am I beyond that).

ScottyBye · 11-14-2016, 06:55 PM

You are correct in my book. Spark Plugs and or wires are not going to cause this problem. My guess is the service centre you used replaced the plugs because as you stated. They were in very bad condition. My question is why would they even remove the plugs while diagnosing this code. Unless you asked? As for the wires. I personally only change the wires if I need to. I do a visual check and test each wires resistance and compare to manufacturers specs. Cracked/Deteriorated insulation could cause engine idle fluctuations but not cause this code IMO.

Arch Hoagland · 11-14-2016, 07:19 PM

I don't think the plugs or wires were causing the problem.

However, based on my own experience and others on here, replacing the wires on an 8.1 should be done every now and then. Especially at the age of your coach.