Is There A Way To Imitate A Governor? (remove Something To Limit Rpm/speed?)

98Mustang2017 · Nov 7, 2017

some time ago i remember my grandfather talking to his son about something he did to limit HIS sons speed. i think it involved the emission system or something?
i'm wondering because maybe there is something wrong with my car that is limiting me and that may be it.

wmburns · Nov 7, 2017

It might be real helpful to mention things like:

the make, model, and year of your ride.
Which engine
Which transmission (manual/automatic)
Rear end gear ratio
Tire size
What speed do you think you are being limited to
What happens at the limit?
Any other modifications done to the car.

98Mustang2017 · Nov 7, 2017

wmburns said:
It might be real helpful to mention things like:

the make, model, and year of your ride.

Which engine

Which transmission (manual/automatic)

Rear end gear ratio

Tire size

What speed do you think you are being limited to

What happens at the limit?

Any other modifications done to the car.

modular motor. (coil pack) v6 3.8 5speed
4.56gears.
RPM limited to 3000 rpm in 5th gear. 25inch tall tires. 68mph going down a slope. i believe i should get 90mph at least.
it sounds like it revs a bit higher when i try to accelerate at 3k but i think its a fuel cutoff because rpm doesnt change at all, not even a hair. Sounds like valve float.

wmburns · Nov 8, 2017

Let me ask. Can you rev above 3k RPM's in 1st through 4th gear? If so, the problem is not valve float.

What is the model year? This matters as the 1996-1998 uses a VSS speed sensor where as the 1999+ uses an OSS speed sensor. The procedure to adjust for a new rear end is different for the VSS vs the OSS sensor.

Is the speedometer accurate?

What does the PCM "think" the car's speed is? An ODB2 scanner is needed for this. I really suspect that the PCM does not have the car's correct speed (either by re-programming or VSS gear).

Here's some information on an affordable Windows based ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

The 3.8 OHV V6 is not a modular motor.

Hoytster · Nov 8, 2017

I agree with @wmburns, sounds like the PCM is not getting the correct speed input. Just for sheetz and giggles, I crunched a few numbers.

IIRC (and do correct me if I'm wrong as I'm not a V6 expert),
1998 V6 car has a 110mph speed limiter from the factory
2.73 factory rear end ratio, .68 final drive ratio in 5th gear, stock 25.5" tire.
@110mph, the engine should be turning around 2690rpm

You have a 4.56 rear end, .68 final drive ratio in 5th, an let's say the stock 25.5" tall tire
@66mph, the engine should be turning around 2690rpm

What this means is the output shaft speed on the transmission is the same @110mph with 2.73 gears as @66mph with 4.56 gears. 110mph should be your factory speed limiter. Isn't the speedometer pickup on the output shaft of these v6 cars? Seems like a strange coincidence.....

98Mustang2017 · Nov 8, 2017

Thanks for the help and great knowledge wmburns and hoytster. i've been trying to pinpoint this problem for almost 5 months now.

It is a 1995 model 3.8 v6 with the VSS at the output shaft which i need to replace (no speedometer reading) as well as the speedometer gears. The wire itself did register on the OHM meter (295) . Would a working VSS fix the problem? The way you talk about the PCM it sounds like it may.
I would like to know how the VSS goes bad when it appears to be in good shape, unless the speedometer gear crumbles easily like the odometer gear.

"the output shaft speed on the transmission is the same @110mph with 2.73 gears as @66mph with 4.56 gears. 110mph should be your factory speed limiter." < That makes good sense. The PCM could think that i'm at the speed governor.

-
Sorry about the confusing username. when i first registered a couple years ago i was looking into a 98 mustang which didn't happen.

wmburns · Nov 13, 2017

One of the problems here is I don't believe that you understand WHY the V6 Mustang has a speed limiter in the first place. It has to do with the "critical speed" of the drive line (U-joints, drive shaft, pinion, and slip yoke). The Ford engineers set the max speed to keep the drive shaft itself from spinning faster than it's "critical speed". So when going to a lower rear end gear and keeping the same drive line, the car's top speed will need to be lowered in order to stay below the "critical" speed of the drive line.

Regarding trouble shooting the VSS sensor. The VSS sensor generates a saw tooth wave form when turned. If this were my car I would start by CONFIRMING a good ground on the VSS PK/O wire.

Next disconnect the VSS connector and put a Volt-Ohm meter (VOM) on the VSS sensor. Put the car on axle stands and disable traction control (if equipped). Spin the wheels and MEASURE the output of the VSS senor using the VOM set to the AC scale. What the reading is will depend upon the VOM meter. What we are just trying to do is a basic functional test to confirm that the VSS is generating ANY signal. You should also see a change when the wheels spin faster.

IF the VSS is not generating any voltage then first suspect that either the plastic gear on the VSS has worn down or the worm gear on the transmission output shaft has worn down. You should be able to see the transmission output shaft worm gear once the VSS sensor has been removed.

Note for cars with a VSS sensor, the sensor output goes to both the cluster AND the PCM. A bad VSS sensor would cause the speedometer and PCM to not have a good speed signal.

IMO it's far easier to use a scan tool to "ask" the PCM what the car's speed is.

98Mustang2017 · Nov 13, 2017

wmburns said:
One of the problems here is I don't believe that you understand WHY the V6 Mustang has a speed limiter in the first place. It has to do with the "critical speed" of the drive line (U-joints, drive shaft, pinion, and slip yoke). The Ford engineers set the max speed to keep the drive shaft itself from spinning faster than it's "critical speed". So when going to a lower rear end gear and keeping the same drive line, the car's top speed will need to be lowered in order to stay below the "critical" speed of the drive line.

Regarding trouble shooting the VSS sensor. The VSS sensor generates a saw tooth wave form when turned. If this were my car I would start by CONFIRMING a good ground on the VSS PK/O wire.

Next disconnect the VSS connector and put a Volt-Ohm meter (VOM) on the VSS sensor. Put the car on axle stands and disable traction control (if equipped). Spin the wheels and MEASURE the output of the VSS senor using the VOM set to the AC scale. What the reading is will depend upon the VOM meter. What we are just trying to do is a basic functional test to confirm that the VSS is generating ANY signal. You should also see a change when the wheels spin faster.

IF the VSS is not generating any voltage then first suspect that either the plastic gear on the VSS has worn down or the worm gear on the transmission output shaft has worn down. You should be able to see the transmission output shaft worm gear once the VSS sensor has been removed.

Note for cars with a VSS sensor, the sensor output goes to both the cluster AND the PCM. A bad VSS sensor would cause the speedometer and PCM to not have a good speed signal.

IMO it's far easier to use a scan tool to "ask" the PCM what the car's speed is.

hmm..critical speed. Now that you bring it up i may remember topping out in different vehicles (the usual 110-120mph limit) and hearing vibrations. Now that is a really good safety procedure for limiting speed. That would be a terrible scenario if the drivetrain shredded at very high speed.Sounds like an aluminum driveshaft may be a good investment.

The VSS wire did show 295ohms so i know it is good. The VSS gear is good and still sharp. And the scanner does pick up the speed without the speedometer actually showing it. The cluster gears are in good shape but i dont think they have anything to do with the speedometer(?) I'm hoping i dont have to replace the cluster just because of a speedometer issue.

((sorry i jumped the gun before reading your post and made one on this specific issue))

wmburns · Nov 14, 2017

98Mustang2017 said:
And the scanner does pick up the speed without the speedometer actually showing it. The cluster gears are in good shape but i dont think they have anything to do with the speedometer(?) I'm hoping i dont have to replace the cluster just because of a speedometer issue.

Bad Odometer gears will not "cause" a speedometer issue.

But..... A speedometer issue "might" cause an Odometer issue.

I take it that neither the speedometer or odometer works. Both the speedometer and the odometer require a good VSS signal to work. So it would seem to me that it would be handy to "know" if the cluster is actually receiving a good VSS signal. This should be cluster pin #5(GY/BK) and pin #6(PK/O).

So.....If there's a VSS signal at the cluster THEN the problem is in the cluster.

Since the PCM does know the speed this proves the VSS is sending a signal. IF there's no VSS signal at the cluster THEN there has to be a wiring fault in the car's body wiring harness.

Here's a link on how to repair broken printed circuit foil traces. It's not for you model year but may give you some ideas of what to look for. There's also some information on how the test the Speedometer stepper motors. It's under the link about how to calibrate after removing the needles.

Cluster PC foil trace repair.
http://www.allfordmustangs.com/forums/v6-tech/304608-my-odometer-goes-off.html#2657361

Is There A Way To Imitate A Governor? (remove Something To Limit Rpm/speed?)

98Mustang2017

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98Mustang2017

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Hoytster

I don't dare do that to my Knob

98Mustang2017

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98Mustang2017

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Is There A Way To Imitate A Governor? (remove Something To Limit Rpm/speed?)

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