Topic: PCM and speed question

[neale_rs]

Was reading the other day about how some 4x4s (Chevy) have a low range sensor that tells the PCM when low range is in use so that the PCM can interpret the speed correctly in relation to the transmission gears.

Why don't YJs (probably TJs neither) have this sensor?

Do YJs with manual transmissions use the speed information for much, or just to suggest to the driver when to shift up?

[Jeffy]

Was reading the other day about how some 4x4s (Chevy) have a low range sensor that tells the PCM when low range is in use so that the PCM can interpret the speed correctly in relation to the transmission gears.

Why don't YJs (probably TJs neither) have this sensor?

Do YJs with manual transmissions use the speed information for much, or just to suggest to the driver when to shift up?

Chevy's have the VSS on the transmission unlike Jeeps.  The VSS on Jeeps reads correct no matter what gear it's in since the VSS is on the transfer case.

[neale_rs]

That's an important point about the speed sensor location on the Chevys.

But in the case of a Jeep, when in low range the PCM would be seeing a low speed and a "too high" engine RPM.  Wouldn't this be seen as a problem by the PCM?   With a PCM controlled automatic transmission, it would tend to want to delay shifting up. 
 

[neale_rs]

From the '95 FSM:

"The speed sensor generates 8 pulses per sensor
revolution. These signals, in conjunction with a
closed throttle signal from the throttle position sensor,
indicate a closed throttle deceleration to the
PCM. When the vehicle is stopped at idle, a closed
throttle signal is received by the PCM (but a speed
sensor signal is not received).
Under deceleration conditions, the PCM adjusts the
idle air control (IAC) motor to maintain a desired
MAP value. Under idle conditions, the PCM adjusts
the IAC motor to maintain a desired engine speed."

So that's all it's doing with the speed information.

It looks like all that could happen is that it might try to lower the idle engine speed too much (and cause stalling) when slowing down in low range.  But since it has a minimum idle speed, it doesn't cause stalling problems.