Trans Sensor: So what is this ???

[Intuit]

'94 1.9 LX 5-speed manual shift.

Click on thumbnail images... (and again to see full-sized photo)


Description - Looking up at the transmission from the bottom of vehicle. Centered is the transmission sensor with two green wires feeding it. (one is detached) Orientation: Left are the shift linkages. Right is the front of the vehicle. Top is the drive-side axle. Bottom is the passenger side axle.

The wire on this sensor has been broken for as long as I can remember. There used to be a tiny widdle nub of wire hanging off the sensor and had once resoldered the wire back to it. With no practical way to protect or secure it, that naturally didn't last long.

So the question is, what is this sensor called and what function does it serve ? What impact, or what are the consequences of it being disconnected ?

Best guess is that it may tell the computer whether I'm in neutral ???

Thanks for you time...

[AzTumbleweed]

I would guess it's the neutral safety switch and/or back-up lights switch. Do your back-up lights work?

[Intuit]

Reverse lights work. But what I can't figure is why it would need to sense neutral given that the engine won't crank unless the clutch pedal is depressed. (switch is in passenger compartment)

[AzTumbleweed]

You're right. My mistake. I was thinking of an automatic. So it must be the back-up lights only.

[Intuit]

Again, the reverse lights have always worked and they still work (even though one wire has been detached for most of the past 5-12 years). Very bright and in fact, still the original (assume Kelly) OEM bulbs !

Can anyone tell what this is for sure ?

If not, only other resource would be to show the picture to those down at the local dealership. If they're able to locate the part in the computer, it's name would reveal some possible details about it; and it's function.

QUESTION: For owners of other '94, '95 5-speed manual trans vehicles, if you're sitting with your clutch pedal depressed in any gear, does your shift-light activate just by revving the engine ?? Does this behavior change depending on whether you're in 1-5, N or R ???

[Selectron]

This is from the Ford service manual for the '95 (I don't have the '94) - is that the same item that you're referring to?

'95 Escort Park/Neutral Position Switch location

[AzTumbleweed]

If the ground wire is unhooked it could still work by grounding through the switch. Unhook the other wire and see if the backups still work.

[Intuit]

You're right, suppose it could be grounded via contact on the inside. Next time I'm under there will use a multimeter to check and see if it's meeting ground at both switch and wire. It's probably surrounded by fluid so gasket would likely prevent any grounding via it's casing. Doubt trans fluid has enough conductivity if any. SAE appears to charge for their chemical property sheets. :\ http://www.sae.org/technical/papers/902148

Originally Posted by Selectron

This is from the Ford service manual for the '95 (I don't have the '94) - is that the same item that you're referring to?

'95 Escort Park/Neutral Position Switch location

That's it! Mystery solved. Much appreciated Selectron.

One question remains, IF this switch isn't functioning, what impact might that have for a manual shift vehicle ?

As far as I've always known, the only requirement for the vehicle to be cranked, is to depress the clutch pedal; regardless of what gear the trans is in. As far as I've always known, it would never crank under any condition unless that pedal was depressed. But to find out for sure I'd need a '94 '95 manual shift volunteer with an intact switch to verify those conditions.

[Intuit]

From a totally different vehicle but...

PARK/NEUTRAL POSITION (PNP) SWITCH: The Park/Neutral position (PNP) switch indicates to
the PCM when the transmission is in park, neutral, or drive. This information is used for the EGR
and the IAC valve operation. (AUTOMATIC ONLY).EGR wire can be tied up out of the way for a
NON Emmission application.

Principals are generally the same amongst various vehicles. But apparently for many vehicles (if not all) it doesn't necessarily detect only N/P or R. It's possible that it may detect all gears.

[AzTumbleweed]

I'm trying to figure out why a manual tranny would have the N/P switch?

[Selectron]

There's very little information about it in the service manual for the '95. That switch only exists on the manual transaxle - not the automatic. The connecting wire colours for the '95 are brown/yellow and light green/black, and the switch is 'closed in neutral'. The only other reference to it that I can find, says:

Park/Neutral Position (PNP) Switch

The Park/Neutral Position (PNP) switch is mounted on the transaxle, near the shift linkage. The PNP switch informs the Powertrain Control Module (PCM) when the transaxle is in the NEUTRAL (N) position. The PCM uses this information to control idle speed.

I think the PCM uses it to minimise fuel consumption - there have been a few interesting discussions in the Engineering/Technical forum about whether it was ok to save gas by coasting downhill in neutral, so the input from the neutral switch would be relevant in that scenario:

Coast or engine-brake when driving downhill?

Shifting into neutral to coast

And another one - see posts #12 & 13 discussing why it's not necessarily a smart idea to coast downhill in neutral in an automatic - not relevant to your car but the automatic drivers might want to take a look at it:

Saving gas in neutral?

Given that the switch is 'closed in neutral' and your wiring is open-circuit, then your PCM will think your vehicle is always in gear.

[Intuit]

So if it's non-operational then it's always reading open, meaning the PCM might think it's always in gear. (just noticed you said the same at the end)

Interestingly though, when coasting (pedal depressed but shifter in-gear) the engine would often idle high while moving until come to a complete stop. In the past I had assumed this was to help gaurd against user-error... failing to match RPMs with vehicle speed before releasing the clutch pedal. But it wouldn't do this all the time. Often it would idle back down regardless of movement. Haven't paid enough attention to figure out why it sometimes idles high until stop consistently, and others, idles at 800-1000RPM consistently while still moving. Lately I've noticed a low idle regardless of movement/non-movement.

I'd rather use my brakes for braking (they're designed for it) than my trans+engine so I do coast (braking intermittently) down a long steep decline near my home. Even with the trans in gear and engaged I'd still have to brake anyway; just not as often. Nothing to do with fuel economy in my case and still all original front hardware at ~236k. (except for pads+rotors of course) Only just very recently replaced rear wheel cylinders (one leaked) and still original drums. Would rather save engine/gear wear. Mistake many people make is riding the brakes and/or using "lifetime" pads. Heat is the single biggest factor. 2-3 second pulses are all that is necessary. Want to maintain below 50 then periodically slow down to 40 and allow up to 55mph. (accounting for presence/non-presence of following traffic of course) Disengaging the clutch for short or minor downgrades is excessive and wouldn't go to neutral with an automatic at all unless it had known transmission problems.

Also interesting, drove a '96 Saturn SC2 model with an automatic transmission that would automatically downshift if it detected you braking to maintain speed on a decline.

[Selectron]

I noticed on the wiring diagrams that the clutch pedal position switch (different switch to the clutch start switch) feeds into the same PCM pin as the PNP switch, so I uploaded the diagrams so you can take a look at them. The voltage source for both switches is PCM pin 46, through splice point S264 (which feeds the clutch pedal position switch) and then it disappears off the diagram bottom left at the point marked 'N', and bottom right on the lower LG/BK wire. Both of those points can be picked up on the second diagram, where point 'N' can be seen feeding into the ECT sensor, while the LG/BK wire goes to splice point S164 and then returns to diagram one where it feeds into the PNP switch. I'm not sure what voltage would be fed from PCM pin 46 to those items but it wouldn't surprise me if it was 5V instead of 12V.

So PCM pin 46 is supplying a voltage which may or may not be fed back in to pin 30, depending on the state of the two switches, and at no point is there any intention of switching that voltage to ground, so I'd be careful not to allow the broken wire to short-circuit to ground, because I don't know what the implications might be for the PCM.

If it was my car I'd be tempted to replace the switch, but it means losing and replacing transmission fluid so maybe you'll think differently since you're the one who'll actually have to do the work.

My Escort (UK '95 diesel) is a five-speed manual; I tend to stay in gear downhill, and use engine braking combined with the brakes, but I also do that pulse-braking thing - brake for a second or two and then foot off, then brake for a second or two again.

And these are the diagrams:

'95 Escort PNP switch - wiring diagram 01
'95 Escort PNP switch - wiring diagram 02
'95 Escort PNP switch - description
'95 Escort PNP switch - removal and installation

[Intuit]

As always, great and precise information Selectron.

Hadn't realized that everything was so inter-connected as opposed to just going straight back to the PCM. Luckily it's a very stiff wire and based on it's position can't ever see it grounding unless the other wire breaks free. I remember there are two switches at the clutch pedal, detecting both the up and down positions. Depending on how they're wired it's possible for the computer to know three states of clutch pedal operation. (completely released, somewhere in-between, completely depressed)

Think I'll take your advice and replace it. Based on the wiring, there are two mysterious issues that this might have some play/influence on.

Several years ago, the installer for my alarm/remote-starting system claimed he was forbidden from installing in manual transaxles when he originally had agreed to the job no-problems. He did say that I could connect the remote-starting portions myself. Briefly looked into it awhile back and saw that there was a neutral-safety position switch pin on the system control box in the diagram. Wonder if this disconnected switch (which he wouldn't have known about) may have been the actual reason he declined the install; once already well into it ?

Many thanks again Selectron.

[zzyzzx2]

Originally Posted by Selectron

There's very little information about it in the service manual for the '95. That switch only exists on the manual transaxle - not the automatic.

I agree. I have an automatic and I have never seen such a sensor or switch on the transmission like that on my 1995LX.