Engine/Transmission (1994 - 1998) APPS or TPS Misbehaving (A Theory)

[mwilson]

TPS or APPS (whatever you want to call it) had been driving me nuts in Ole' E-Bay the last couple of weeks. Hunting, changing its mind, etc. , the whole bucket of issues that a bad one brings to mind.



Last week the headlight switch had started to crap out, got so bad by the end of this week that you could not use high beams for any more than a minute, if you did they would go out and you had to wiggle the light switch to get'em back.

I always drive with my headlights on, doesn't matter how far.



Now with all that being said, I changed out the headlight switch and harness Saturday, soldered everything up and went about my way. It occurred to me this morning that I had not encountered any hunting or lock-up confusion since driving the truck to the shop Saturday morning to change the switch. I have driven the truck 250 miles at least since.



Could the malfunctioning headlight switch have caused enough electrical interference to freak the TPS and/or the TCM out???



If so, given the amount of crappy headlight switches in these trucks, is this what may be wrong with some members trucks, they have thrown a bunch of money at the TPS with no good result when in reality it may be a headlight switch or a heater motor for example that is really bad???



Just a theory, let me know what you guys think.



Mike.

 

[dclassens]

I remember when I had my 01 CTD HO engine and 6 speed, I got the P0121 and P0122 codes after a MIL light and it died a few times on the road. I had work issues at the time so the truck was taken to a local reputable diesel/heavy truck shop. After a couple days they told me lift pump and VP were dead. They installed an in-tank pump and new VP for $2600. 4 days later same issues happened. MIL, codes P0121 and P0122, and it died a few times. Went back to shop and they installed a new APPS for $600. All was fine after this. My current job requires us to carry ASE master for cars(A1-A8) and trucks(T1-T8) to get any bonus. Long story short, being a mechanic and not being able to spend GOOD amount of time on diagnosis and also the transportation issue made it so I was about $3500 in the hole. Life was extremely busy and all the over time I was working essentially covered the bill but I would have much rather spent that money on fun stuff.



I will always believe these guys were throwing parts at known dodge problem areas. It is possible that ALL these parts broke at once but I think most shops want in and out, along with high profit on parts. As everybody says time is money and neither I or the shop could spend A LOT of time properly diagnosing.



I will agree that electrical problems can cause other strange issues but the time and techniques on diagnosing can be a big headache. I still believe from this experience that the HO's have a higher failure rate and I think codes from one area can be set by incorrect computer feedback. An example of this is the older DMAX codes being set for temp issues and the fix from GM was a Block Heater cord with a TSTAT to work only at certain temps.

 

[HFreeman]

If I remember correctly there was a TSB for the 2nd gen trucks to have the headlight switch replaced at dealer expense. It would overheat and fail in some cases if the high beams were used for long periods of time. I never heard of it messing up the TPS though. Of course anythings possible with these things. Glad you got your problem fixed though.