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Discussion Starter · #1 ·
I need help!! I have a 96 F250 Heavy Duty Powerstroke. I’m having trouble getting overdrive to engage. It’s shifts great 1-4. But past 60mph you have to push the pedal down to get it engage overdrive, and once overdrive engages, it starts ya know, goin faster, 60, 65, 70, and it just keeps going up, as SOON as I let off the throttle pedal even if it’s just a millimeter, it comes out of overdrive. And there’s a sweet spot on the pedal, if it moves at all it will kick in and out of overdrive and I can’t maintain a constant speed. It’s either; go into overdrive and keep accelerating, or stay in fourth gear and run 2400RPM at 70mph.
It’s confusing and extremely irritating. I’d like to either a.) Somehow add in a manual overdrive switch which I don’t know how to do or if it’s even possible on these fords, or b.) Figure our if it’s a crap torque converter or whatever

the TPS has been replaced by the previous owner. It seems like I only have the first 25% of the pedal too. It doesn’t downshift when you give it throttle at 60mph+ it just goes into overdrive and won’t stay at a constant speed. Always accelerating.
Anyway if anyone else has experienced this, they’ll know 😂 I’m a trained Toyota mechanic so I know how to fix stuff, I just don’t know JACK about fords. I got this thing in a trade a while back. Hoping for answers or directions. Can’t seem to find anything about a manual overdrive switch for these power strokes. Or anyone describing the same issue.
Note: * THE OVERDRIVE BUTTON WORKS, and the OD light works, and does not flash at me ever. 256000 miles
Please help this poor ford. My code reader doesn’t pick up any codes. And it has a tune in it that is correct for the computer I have, I’m certain of that. I bought the chip hoping it would help the problem. It didn’t. It’s lifted on 35’s.
 

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Welcome!

I would remove the tune and make sure it is all back to stock. You need to get a scanner on it that will read live data and check your TPS range and %.

You should be able to see if overdrive is commanded by the PCM too.

A bluetooth adapter like OBDLinkLX and an android device with Car Gauge Pro app will do everything you need.
 

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None of the OBS trucks are OBDII compliant. The all have the port, it's just whether it is active or not. All 96's and 97's will be active. The 94's were not and I've seen where some say 95's are not, but the 95's did not have the OBDI port in the engine compartment as the 94's did so I would think most of them should have had the port active as well. Cheers!.
 

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So, the lockup clutch is failing -- you gonna need a new torque converter, and at those miles be prepared to rebuild the tranny
 
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1st off 4th is overdrive. You should have converter lockup that almost feels like another shift in 2nd, 3rd and 4th/overdrive. I have seen on some, mine included, the tunes mess with converter lockup under light or no load. A simple way to overcome this issue, if that is what the problem is, you can install a manual converter lock switch. A wire would get connected to the yellow/red wire going to the transmission, a on/off switch installed in-line with the other end connected to ground. Just never leave the switch on when coming to a stop or it will stall your engine. It does come in very handy for using the engine to help slow the truck down since normally the converter will only stay locked for about 30 seconds when in coast mode like when coming down a hill
 
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Discussion Starter · #7 ·
Hey guys, after some diagnosis, I found, with a scan tool, my brake PID is staying in the “on” position, I also found that my brake switch is not getting a ground on the LG wire. (LG standing for Light Green) However the brake switch works as my brake lights come on/off. I believe it’s shorted out somewhere. Checked most the grounds, they’re okay.
The LG wire going into IPM has 12v. If you apply a ground (via power probe) to the LG wire at the switch, and depress the brake pedal, the stop lamp fuse blows. Even with the wire going to the IPM disconnected, the RABS module, the tail lamps, AND the trailer harness disconnected, the fuse (fuse #13 in the cab) will blow with ground applied. I know this wire is supposed to be grounded, because I tested it on a like-for-like-vehicle. I still see voltage on the LG wire (where I’m supposed to see 0.0V) about .5V.
I believe this is why the truck will not engage overdrive.
Ps. No codes, and the TC is getting commanded on, but the TCC(%) stays at zero (0% ≠ TC not locked 90%~100% ≠ TC locked) (because it thinks the brake is being applied constantly)
Anyway, I will post again when I solve the problem.
 

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Hey guys, after some diagnosis, I found, with a scan tool, my brake PID is staying in the “on” position, I also found that my brake switch is not getting a ground on the LG wire. (LG standing for Light Green) However the brake switch works as my brake lights come on/off. I believe it’s shorted out somewhere. Checked most the grounds, they’re okay.
The LG wire going into IPM has 12v. If you apply a ground (via power probe) to the LG wire at the switch, and depress the brake pedal, the stop lamp fuse blows. Even with the wire going to the IPM disconnected, the RABS module, the tail lamps, AND the trailer harness disconnected, the fuse (fuse #13 in the cab) will blow with ground applied. I know this wire is supposed to be grounded, because I tested it on a like-for-like-vehicle. I still see voltage on the LG wire (where I’m supposed to see 0.0V) about .5V.
I believe this is why the truck will not engage overdrive.
Ps. No codes, and the TC is getting commanded on, but the TCC(%) stays at zero (0% ≠ TC not locked 90%~100% ≠ TC locked) (because it thinks the brake is being applied constantly)
Anyway, I will post again when I solve the problem.
Thanks for the follow up and good troubleshooting! Look forward to more findings.
 

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Discussion Starter · #9 ·
Also, does anyone have recommendations for factory wiring diagrams. I have AllData, but I’m having trouble following the harness because the information is not exact. Factory information would be much appreciated.
 

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The factory Electrical and Vacuum Troubleshooting Manual (EVTM) is the way to go for wiring diagrams. Helm Inc. used to sell them for about $20-$30, but they don't seem to have them. The paper verson for a 1996 is FCS1225496. You can also sometimes find electronic versions on DVD (usually it will cover numerous Ford vehicles for 1996-2001 with general Service Manuals, EVTMs, and Powertrain Control and Emissions Diagnosis (PCED) manuals on one disk). Cheers!
 
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Discussion Starter · #11 ·
Well boys and girls, ITS FIXED. With the brake now reading in the off position, the torque converter locks up, and does its thing. Instead of doing 65mph @ 2400RPM, I can now go 65mph @ 1800rpm OR LESS. Good mpg, HERE I COME. So, the LG wire as mentioned before was broken, not sending signal to the IPM. However, I don't have any idea where the wire goes or where it was broken, adding a wire in straight the IPM would not have fixed it, due to the fact that we simply don't know where the wire goes without factory information, ergo, I do not know where it's SUPPOSED to be getting the ground from, if I did, I would have repaired it more "correctly". So I "phoned a friend" of mine to help. What we did was added a relay into the circuit, the relay makes it so that the LG wire has a ground at all times, WITHOUT popping the fuse EVERY SINGLE TIME YOU HIT THE BRAKE. So, with the relay wired installed, the brake on/off switch is reading as designed. My friend saved my *** on this one, I didn't want to go chasing wires anyway. Attached is a picture of how the relay is wired in. For anyone in the future with this issue, make sure the brake on/off is working, and for the love of god before you throw parts at it, MAKE SURE YOU HAVE A GROUND ON THE LIGHT GREEN WIRE @ the brake switch before you let someone tell you need a transmission or a torque converter.

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Just so you have them, here are the diagrams I could find in the 1996 EVTM for the BOO switch and LG wire. Cheers!
 

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