I have an intermittent problem whereby my engine will suddenly cut out on my 88 F250 (351 / C6). It happens almost as if someone has just turned the key off. She usually starts right up after the stall although it sometimes takes a little extra throttle to keep her going initially. I checked the codes and got a 29 in memory which translates to a problem with the Speed Sensor.

Can a failing speed sensor cause my stalling problem. I thought the problems with this sensor primarily affected your cruise control which, by the way, my truck does not have. My speedo also seems to be working fine.

Any help would be appreciated!

Thanks!

 

check codes.

 

In your truck the VSS is only used for the RABS system. I'm surprised the PCM even knows about it. Neither the cruise nor the speedo run off that sensor.

It's probably either dirty oil in the rear axle, a dirty ring gear, or a short somewhere in the wiring. Most "VSS problems" don't end up being the sensor itself.

 

well ya, you will think you have 200kias and you fall below Vref and kapow....

oooo wait....

wrong mode of transportation.

nevermind.

 

Darn, quicklook, did you even bother to read the guy's post before responding?

If the engine stalls as you are coming to a stop, it could be a faulty speed sensor. I haven't heard of many problems with the trucks, but I have read that mustangs are especially sensitive to having a working VSS input to the computer. The failed VSS causes them to stall if the driver clutches while rolling up to a stop. The VSS that the computer uses on the '88 is located on the tailshaft of the transmission/transfer case, in-line with the speedometer cable. This is separate from the differential speed sensor used by the ABS.

 

It is used for the speed control (but not on his) and does feed into the EEC computer but not the speedometer.
I have no idea what the EEC does with this information in an 88 with a C6.

The 88 VSS input.

/

 

clear codes and read codes again.

is what my head said to type.

my fingers did not listen.

 

Before I made my post I did a site search on the code and came up with a few folks who reported having the same problems with their trucks (i.e., stalling) and also received this error code (i.e., 29). Unfortunately, their posts never went on to say if the VSS was actually the cause of their problem. These members also are not accepting e-mail so I can't follow up with them on this matter either. Just seems strange to me that this sensor could cause you to stall. It even happens when you are just idling at times. I guess as long as it feeds the computer you never know what can happen.

Thanks for the help so far.

 

Typically the VSS in a fuel injection scheme is used to tell the computer the vehicle is still moving and the engine needs to run or that the truck is stopped and the engine can go to idle. If your truck was carbed then the VSS is only for the Cruise Control and the RABS, which then you could ignore because if you dont have cruise the RABS barely works correctly when it does work.

As a quick edit the VSS is important when the computer reads that the TPS has closed and it is trying to figure out if the vehicle is still moving and how to keep the engine running. If it gets a code that the vehicle isnt moving and the TPS is closed then it goes to idle that may be killing the engine when it is coasting to a stop.

 

The original poster did not say if this occurs when coasting to a stop. However, this problem HAS happened to me in the past where I would be slowing down for a turn (usually into the lot where I park the boat) and fire goes out (ever make a braking turn while 5000lbs is pushing you in the ***?)

anyways, I fixed it by cleaning the ISC and it was fine thereafter.

 

OK I lookup what a code 29 was for this truck and it is:
"Insufficient input from the Vehicle Speed Sensor (VSS).

So I would think you may have a broken wire, bad connection or a bad VSS generator.
Check the diagram above for the wires in this circuit.

I know the computer will limit fuel with the throttle plates closed as you coast down in speed but I do not think that it fits here.

I think the engine dieing is unrelated to the VSS.

 

Yeah, I agree with that. The stalling is probably the IAC valve and the code 29 is a short or a dirty exciter ring.

 

By the way he has an 88 so he dose not have an exciter ring. It is gear driven and the generator set on top of the cable.

/

 

Was this something that Ford only did for 1988? Because I have an '89 with an exciter ring in the rear diff.

Also, Bill, that looks like the tranny-mounted (transfer-case mounted in a 4x4) VSS sensor. We all know that all 87 to 96 trucks had a rear diff mounted VSS.

 

The excitor ring in the rear end for 87 to 91 is only for the RABS. The VSS is the sensor in the speedo cable for the 87 to 91, then in 92 the VSS was completely based on the excitor ring in the rear end.