Reverse causing false 338 Degree Temp Reading !! Fake P0117 & P1700

[shanonaz]

Driving me crazy !!!!! .. I have a 1998 5.0 Limited, AWD, 81K miles.

I have a MOTOPOWER MP69035 OBD2 Scanner. The truck kept always giving me a P0117 code ( Engine Coolant Temperature (ECT) Sensor 1 Circuit Low), no matter what I did …. 3 new sensors, 2 new dash sensors, new thermostat, all air purged from cooling system. When the truck was running, the reader said the temp was, 190, then 240, then the max at 338 degrees, over and over again.

Then I decided pay close attention to the readings while running. Long story short … after several hours of troubleshooting, the reader shows the temp to be maxed at 338 degrees when you put the truck in reverse, not in P, N, D, 1 or 2. Foot on break or not, every time you move the lever into R the temp reading on the Scanner read 338 degrees, then the code of P117 is given.

OK, now I used a new scanner using my iPhone ( Vgate iCar Pro Bluetooth 4.0 OBD2 Fault Code Reader). Truck runs great and the temp show a nice 195 degrees when warmed up. BUT NOW when the truck was running, the reader ONLY said the temp was 195 degrees, and did not jump allover and read 190, then 240, then the max at 338, over and over again. THEN when you put the truck in R, not P,N,D,1, or 2, the reader shows a temp of max 338 degrees, and the truck throws a P1700 code.

Ignoring the codes for a min, the main issue here is how in the heck can a trans shift position of R cause the engine temperature reading on any OBDII scanner to trigger a quick max out of 338 degrees and throw any code, and then go back to 195ish degrees when removed from R??

Clear the code, and the issue is easy to repeat, every time, just put truck in R and you get insta code of P0117 on the MP69035 scanner, and a P1700 code on the Vgate scanner.

The trans had the fluid changed a few weeks ago hoping to fix this code issue, but all that did is cause the truth to run and shift like a New Explorer. The trans is in perfect shifting condition, smooth as butter. The engine runs perfect like a 81K mile 5.0 should.

And Ideas? I have hit a dead end with this one.

 

[C420sailor]

Shifting to R is causing a dead short in the ECT circuit. As temperature of that sensor increases, resistance decreases. Looking at a few Ford ECT calibration charts, 338°F correlates to just about 0 ohms.

Gotta dive into the schematic for that circuit and figure out where the common link is. If it isn’t obvious in the schematic, you need to go through the harness.

I’d first verify the shifter and linkage aren’t chafing anything directly to rule out mechanical causes.

Then I’d start thinking about things that receive power when R is selected. DTRS, reverse lights, etc. Inspect those circuits.

I mean it could even be a failure inside the PCM, but I wouldn’t jump straight to that.

 

[shanonaz]

Shifting to R is causing a dead short in the ECT circuit. As temperature of that sensor increases, resistance decreases. Looking at a few Ford ECT calibration charts, 338°F correlates to just about 0 ohms.

Gotta dive into the schematic for that circuit and figure out where the common link is. If it isn’t obvious in the schematic, you need to go through the harness.

I’d first verify the shifter and linkage aren’t chafing anything directly to rule out mechanical causes.

Then I’d start thinking about things that receive power when R is selected. DTRS, reverse lights, etc. Inspect those circuits.

I mean it could even be a failure inside the PCM, but I wouldn’t jump straight to that.

After pulling 7 Fues's, and following the linkage for anything rubbing against it, the issue is still happening. I have decided that it has to be the Back-Up & Neutral Safety Switch on the side of the trans. It is the only item being moved at the time. Unless there is something that the transmission communicates with after you put in in R, like the Shift, Pressure, or TCC control solenoids. Nothing is rubbing on any moving part, and there is no exposed wires hitting the linkage. But every time you put it in R, you will get that 338 reading.