Vehicle Speed Sensor effect on tuning

-
dartslantsix

dartslantsix

Well-Known Member
Joined
Mar 3, 2011
Messages
202
Reaction score
78
Location
Klamath Falls, OR
In this month's Hot Rod magazine, there is a "rescue" of a 97ish Dodge Ram where they say the vehicle speed sensor was making the engine go lean. Reason being, the truck had larger tires so the sensor was reading incorrectly.

Does this really change the fuel mixture? I've got mine hooked up but have not looked into whether the gear is correct for my tire size. I'm also running a manual trans, so not sure if that also effects it.
 
Did not read the article but yes speed density fuel injection must have a correct reading from the speed sensor.
 
Just read that last night. The sensor reading vs actual mph was off 10 mph. He had many issues going on, and I'm sure most they didn't write about, on purpose or not. I worked at a shop years back where we put out quite a few cars that were in national magazines. Boy what a wake-up call to the world of car magazine stories that was !!!! Either the people that owned, worked on, or presented the cars had amazing imaginations, or the magazine writers did !!! (I believe the magazines were duped most of the time, and weren't sharp enough to check/catch it, or didn't really want the truth). These were cars that won a lot of events, one even was Hot Rod's car of the
year. I do not believe much of what I read in them anymore. Anyway, that guys truck was going into OD lock-up at 35 mph, and he would just lug the thing around town like that. This compounded his other issues. You would think if you put together something like that, you would notice the early lock-up and MAYBE shift the thing out of OD during around town driving !!!!! No way that system leans the engine out to the point of melting valve guides strictly on an erroneous vss signal. Map, O2, ETC, IAC sensors might have something to about that. On that system VSS signal, or lack there of, is used mainly for trans operation and idle conditions, I believe.
 
I did a magnum swap in a friends 1974 D100 short bed truck. We never installed a VSS. And he has been driving it around for 4+ years without issues. Runs great and it's a completely stock harness, ECU, tune, etc. We ended up later finding a Dakota at a salvage yard that had a VSS with a speedo cable hookup to keep the ECU happy and the truck speedo functioning. We never took the time to install it.
 
ya, i put an 01 5.2 in my dart 5 years ago. running a 904 and just cut out the transmission and vss wires. never give me any problems yet. idles fine too.
 
AJ/FormS said:
If the computer doesn't see the VSS, wouldn't it go into limp-in mode and turn on the check-engine light?
Click to expand...

We wired up a light on the D100 and it worked when the O2 sensor failed. But it stays off and doesn't have any codes stored.
 
Speed density has nothing to do with the vehicle speed. Speed density is calculating fuel requirements based on cylinder displacement and engine speed. The program is set to deliver a given basic amount of fuel at a given engine rpm, and is then adjusted further based on engine load from the MAP sensor, O2 sensor, etc. This a very basic description, and hopefully I did not butcher it.
For Magnum trucks, vehicle speed is used for speedometer, trans controls, ABS, etc. There will be no limp mode on a truck the does not have an electronically controlled trans. Further, the PCM will run the engine just fine without VSS input, as others have mentioned, regardless if it's OBD1, OBD2, electronic trans, or non-electronic trans. Think of it as revving the engine in park or neutral. Same idea. That's why Magnum swaps are easy.
 
so for a 89 318 Magnum TBI retrofit onto a 65 273/904, whats the minimum sensors needed to get it to work?
sbec-circuit.jpg
 
First, the control module needs powers and grounds. Next, it will not run or will run very poorly without distributor, MAP sensor, throttle position sensor, idle speed control motor, ignition coil, and fuel injectors. Finally, it will run properly in closed loop if it has heated oxygen sensor, engine coolant temperature sensor, and TBI temp sensor. Just about the same applies to full fuel injection, although most, including Magnum engines, use a crankshaft position sensor separate from the distributor, and TBI temp sensor is changed to a intake air temp sensor.
 
Do I require a Crank position sensor on an LA? What does it do as I got a TBI distributor.

Disregart\d, looked at schematic and I dont even see CPS on an 89, maybe for MPFI.
 
Last edited:
pishta said:
Do I require a Crank position sensor on an LA? What does it do as I got a TBI distributor.

Disregart\d, looked at schematic and I dont even see CPS on an 89, maybe for MPFI.
Click to expand...
I know you didn't ask, but in my opinion the Chrysler TBI system is a downgrade from a well tuned carb. They had a plethora of reliability and drive ability issues... I'd spend my time on the magnum system any day
 
It was free, the TBI works with a MS2 and its a challenge. Im up for all 3
 
My B Body, a '72 Satellite Sebring Plus 318 auto, has a factory speed sensor on the speedo cable to manipulate ignition timing based on speed (mph) (not hooked up at the moment).
 
dibbons said:
My B Body, a '72 Satellite Sebring Plus 318 auto, has a factory speed sensor on the speedo cable to manipulate ignition timing based on speed (mph) (not hooked up at the moment).
Click to expand...
This is not at all what this thread was about, but it is pretty darn interesting.
 
Thanks for everyone's input. It doesn't appear that the VSS effects much with the running of the engine. The only thing I've heard is coast-down may be effected.
A friend of mine swapped into a Jeep and the engine would die on coast-down until he hooked up the VSS.
 
What were the specifics of that swap that was stalling; year of donor vehicle, manual or auto trans used, electronic trans or non-electronic trans originally in donor?
Also as a side note, I had 4x4 Ramcharger with factory equipped throttle body injected 318. The vehicle speed sensor fell apart somewhere around 175,000 miles and I drove it about five more years without a VSS input. The 318 never failed to start, never stalled out, and always had the same power and fuel economy the entire time I drove it.
 
Bobzilla said:
What were the specifics of that swap that was stalling; year of donor vehicle, manual or auto trans used, electronic trans or non-electronic trans originally in donor?
Also as a side note, I had 4x4 Ramcharger with factory equipped throttle body injected 318. The vehicle speed sensor fell apart somewhere around 175,000 miles and I drove it about five more years without a VSS input. The 318 never failed to start, never stalled out, and always had the same power and fuel economy the entire time I drove it.
Click to expand...

I don't have a lot of specifics, but he said he used the entire drivetrain out of a 5.2L Grand Cherokee. So I'd assume it was the electronically controlled transmission.
It's possible the VSS supplied info to unlock the torque converter and without that, it stalled the engine?
 
You must log in or register to reply here.
-

Back
Top