I have a 94 f250 XLT custom with a 5.8l and e4od trans. It has 115k on it with 265-75-16 tires. It has a true dual exhaust full length headers, H pipe and flow masters

Today i was on the hwy driving home from work, when i got up to 80 mph on the speedo, the truck would act like it was hitting a speed cut off like on a newer truck. The rpm will not go past 3k and speed stays at 80. I then took itout of OD and ran it up to 80 mph in 3rd. Again the truck cut off at 80mph but this time the rpm was around 3800. So I don’t think it is starving for fuel. My GPS says the actual speed is 77 mph. I would think that the oversize tire would not make the speedo read slower.

The truck will shift a little hard at times. I think it may be a protection mode from the Trans, like it is using trans pressure to shift rather than electrical signal from the ecm. Recently I lost the selection control for my heat/air blower direction. Its stuck on defrost. So this has me thinking I havea vacuum line off somewhere? When the speed cut off issue started the Trans started shifting very smooth and seams to go into OD and lock the torque converter at lower speeds.

I pulled codes. 656= converter slip, and the other was a lean code. I have had both these codes since I bought the truck. and had no problems going 80+ in the past

I have replaced the fuel filter along with the plugs, wires, cap,rotor.

Thank for any ideas you may have.

 

I am thinking it could be a bad VSS ?

 

These trucks also have rev limiters from what I've been told.

 

Rev limiter is around ~4500 on the bigblocks and 300-6, and around ~5000 for the small blocks.

The OBS trucks do have speed limiters, two separate ones to be exact. One for 4x2 and another for 4x4. In 4x2, it's set ludicrously high, like 300 MPH. In 4x4, it's set much lower, around 112-114 MPH. It's possible the speedometer is feeding garbage information to your computer, but it's pretty unlikely if the speedometer is working fine.

Since these trucks do not have electronic throttle bodies, the speed and rev limiters are spark-cut type, which is very abrupt and harsh. You'd know it if you hit them. This makes me think you have a fuel delivery issue, or an intake/exhaust problem. Do you still have catalytic converters?

I'd start by checking for vacuum leaks, and doing a fuel pressure test. I'd also verify that the throttle blade is opening all the way up with the pedal to the floor.

 

I did check the throttle and all loold good.

I also ran across this as a way to test the vss and psom.

"The PSOM converts the analog vehicle speed sensor signal into a digital signal and sends it on to the PCM for a vehicle speed input. When the PCM receives this erratic indication from the PSOM, it generally calculates vehicle speed above 100 MPH and cuts out an injector bank and locks the converter. This causes the stall or surge type condition to avoid vehicle overspeed. If the vehicle speed sensor (VSS) in the rear axle were causing an erratic input, the vehicle operator would notice the ABS or RABS warning light would also illuminate as the PSOM and the ABS module are fed the VSS signal in parallel with the PCM.

The above points to the psom as the problem. Also explains thenlean code and the lockup of converter at low speed. As the truck thinksnit is going faster than it is.

 

almost all computerized vehicles have some sort of speed limiter on them. most are set at 80 mph, some "high performance" cars will do 90 or 100

 

Ridiculously high? My 94 F-250 RCLB 460/E4OD/4x2 would do maybe 95 before it cut out. Speedo was pegged against the high limit stop so I had to estimate it.

My current truck, I ain't ever tried it. Not worth the ticket. Well actually it would be ticket>court>jail>probation, the law here is mandatory court for 30+ over the speed limit.

The PCM does NOT get the raw signal from the VSS, it gets a conditioned signal from the PSOM. If I remember right, the RABS module also gets the same conditioned signal. The only thing to get the raw data from the VSS is the PSOM.

The trans shifting hard at times tells me that it's worth looking at the PSOM a little closer.

 

the first part of your statement is true,the second part not so much. Not sure what the cut off is on my truck,but it is over 85 mph. and ummmm according to google,the cut off for my stock daily driver 2005 ford focus is 108.

 

My buddies OBS 460 truck would do 114 MPH before it plan ran out of power. Later confirmed when we turbo'd it and tuned it. The 4x2 speed limiter is set very high, over 10,000 driveshaft RPM.

 

Dixie460,
I understand that the VSS does not feed the PCM. The PSOM converts the analog from the VSS to a digital signal for the PCM. As i have allready stated above. Am I corect in thinking that the VSS is probably not the issue because it is not kicking on the ABS or RABS light?

I found a place in Florida That will fix my PSOM if I send it in. So i think i will pull it out this weekend and get it sent out to be checked out.

 

Sometimes the cruise control module goes bad, and can mess with the VSS/PSOM signal. It's located on the drivers side fender, might be worth unplugging it.

Like I said though, the actually speed limiter in these trucks are quite abrupt, and apply in two stages. It first disables half the cylinders, so it feels (and sounds) like a bad misfire, than it goes to a full fuel cut, in which the engine will cut out completely for a fraction of a second over and over again. If it comes up "smoothly" to 80MPH, I'd lean more towards a fuel delivery issue, and the lean code may signify that.

 

Can someone tell us what PSOM stands for?

 

I will check the fuel pressuer to make sure the pressure is good.

the only thing that makes me think it is not a fule problem is the fact that the truck will pull hard to 80 with out missing a beat. Then at 80 it just shuts down then comes on strong when it slows to about 75 then as soon as it hits 80 she dies off again. In OD the max rpm is around 3K and in drive or "3rd" gear the speed stops at 80 but the rpm is about 3800 when it hits the speed shut off. so the motor is capable of more.

I will start with fuel test then work my way up the list to the stuff that cost more $
Thanks for all the info and help so far. I have a long weekend I will let you know what i find out

 

My RABS module didn't show a light, but it was screwing up the PSOM output so the EEC had the trans in limp mode. I unplugged it and everything was fine.

My 97 Thunderbird 4.6/4R70W would cut off at 105 til I re-flashed the EEC. I think 99+ Mustangs (GT anyway) cut off at 155.

Huh... strange. I swear I remember hitting a fuel cutoff, but perhaps she was just out of power. It was a few years ago. And yeah maybe it was faster than 95, I don't remember lol. I didn't stay there for long since I doubted that my Load Range E tires were rated for that speed.

PSOM = Programmable Speedometer/Odometer Module (the circuit board behind the speedo dial)

 

Dat2109 it stands for Programmable Speedometer/Odometer Module