Jim Warman Posted August 28, 2011 Share Posted August 28, 2011 Here is another "one of those" trucks. Previously, it was setting APP codes and got a new APP sensor and a new ETC assembly before I married it. It would lose throttle response when the problem occurred but this is an effect of FMEM and I could get the concern to happen when I drove it. A retail job, I finally (and against my better judgement) overlayed a circuit from the APP to the PCM -sorry, can't remember which particular one - and the truck was sent on it's way. Several months later... it got another new ETC assembly as well as a PCM. Then, they gave it back to me. Driving down the road, it will set a P2112 (throttle plate stuck closed) along with a P2104 and go into FMEM... but never for me. So, I load test the TAC motor circuits to about 4 amps. And my light looks good.... After fruitless exploration of other avenues, I come back to the TAC motor circuits. With my lamp substituting for the TAC motor and B+ and ground applied to pins 51 and 34 of C175E I checked my source = 12.5 volts. Then I check my drop across the lamp = 11.1 volts. (v3) Hmmmm, this is 18 gauge wire and 4 amps should be a walk in the park. Checking volt drop across the conductors (CE426 and CE412) we see, in no particular order, .55ish volts and .75ish volts (v1 and v2). Loading the wires with a lamp didn't show anything obvious.... measuring the resistance of the circuits with an ohmmeter gave a reading of 000.0 ohms. Measuring volt drops revealed a concern. Now it is time to pull out the harness and inspect it to determine the next step. Sorry Kieth... this should have gone into the electrical forum... stercus does indeed accidit. Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted August 28, 2011 Share Posted August 28, 2011 Excellent find All of us are going to be thinking alike pretty soon. It's a slow process, but I have quite a few guys in the shop starting to think outside the "box". It would definitely seem that the "burn your fingers and blind your eyes" test was a good progression from the ohm meter. But now and more than ever the voltage drop test can be a time and money saver. Just got to get every body on board. Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted August 28, 2011 Author Share Posted August 28, 2011 Oh God, no!!!! As long as 'they' keep missing these, I'm not lifting cabs Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted September 6, 2011 Author Share Posted September 6, 2011 Epilog..... With the harness stretched out across the engine compartment, no obvious damage was noted. "Wasted" voltage continued to be on the order of 1.5 volts and wiggle testing the harness made no change. Now, the original wiring was 18 Ga. and the only replacement pigtail was 14 Ga. I settled on using 16 Ga... for no particular reason other than increased current flow capabilities. Time to digress... The AWG charts that I can access show 18 Ga. wire as having a current flow capacity of 2.1 amps. I think that this is for solid conductor wire (a single 18 Ga copper strand) with no insulation in free air. Obviously, improving the cooling of a conductor will improve its currrent carrying capability. So... is my 4 amp load going to overburden that circuit making the readings I got acceptable or at least "expectable"? With 16 Ga wire substituted (except for about 3 inches (75mm) at the PCM connector), "wasted" voltage is now .4 volts - that is to say that the volt drop across the 4 amp load is now .4 volts less than source voltage. Other considerations... I did check for excessive system ripple. But, the concern is intermittent - could I have a temperature sensitive diode in the alternator? I can't find out if this circuit works on voltage divider technology or not. It is duty cycled but there is no indication of designed voltage capabilities nor what voltage would be read at 100% duty cycle.... If it sounds like I'm worried that it will come back again, I am... and, considering everything has been replaced except for two 3 inch pieces of wire and two female connectors....... and I do mean EVERYTHING - some of it TWICE. And an Expedition that doesn't set the expected code when you trip the IFS is still fresh in my mind.... Off to school on Tues... and the weather is nice enough to take the scooter.... Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted October 15, 2011 Author Share Posted October 15, 2011 So...... I'll bet nobody knows that "the cat came back the very next day, the cat came back, we thought he was a goner....". This time, it brought with it codes indicating the ETC plate is stuck closed.... and stuck open... and the TPS is low... and the APP is low..... and the TAC motor is a concern... and that this truck loves to be in FMEM. Every problem with this truck revolves around the electronic throttle control system. And every timne it comes back, it is giving codes indicating a concern with different parts of the system. So.... I'm sitting there doing some "navel gazing" - you've seen pictures of me... navel gazing is easy for people shaped like me... and I had an epiphany. Contacting hotline, I asked if this truck had multiple V_REF power supplies in the PCM. Hotline replied that this was the case... indeed, the APP and TPS share a V_REF power supply that no other sensors share. The plot thickens..... Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted October 30, 2011 Author Share Posted October 30, 2011 I figured I owed it to you guys to finish this one out. As I was preparing to overlay some more circuits in an attempt to verify the need for a harness, I noticed a spot under the battery that I had somehow previously missed. Why the harness from under the dash should go that far forward is a mystery to me... but there were three chaffed wires. One of them being one of the two ETCREF wires going to the APP sensor. I found out that the APP sensor and the TPS sensor on the throttle body share a V_REF power supply inside the PCM. No other sensors share this power supply. If ckt LE136 or LE137 (both of these are V_REF supplied to the APP) shorts to ground, it can set a code for the TPS sensor. Anyway, wiring repaired - truck gone for a decent amount of time and no reports of additional problems. Which is a good thing because this same customer has a 2011 F350 that lasted a day before it set a "yaw rate sensor compare failure" code. And to think I used to fix trucks with a point file and a feeler blade. Quote Link to comment Share on other sites More sharing options...
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