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"HELP! My manual car keeps dropping out of cruise/has intermittent cluster gear display/boost-by-gear isn't behaving"


Kirby@PCMTec
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Hey Guys,

One support query I get often is related to issues with gear detection in manual transmissions (especially FG+) where the gear detection fails and the gear indicator on the dashboard clears, cruise control keeps dropping out in 3rd/4th or CustomOS boost by gear isn't obeying the commanded boost target.

Why does it happen?

Basically, there is no gear detection sensor in any factory manual transmission (other than Reverse) and unlike the autos (BTR, 5r55 or 6hp26) where either the PCM or TCM knows the gear by the commanded clutch/brake states (along with some plausibility) the manual gearboxes are dependent 100% on a computational method.

The computational method works like this (simplified):

1) Measure Engine RPM > 2) Measure OSS (Transmission Output Shaft Speed) > 3) Calculate RPM/OSS > 4) Wait for Stable > 5) Compare to Gear Ratio Table > 6) Select Closest Gear > 7) Lock Selection > ūüėé Start Timer

In some calibrations the guys at Ford made particularly tight allowances for the gear ratio's for part 5/6 of the method:

image.thumb.png.8e6678c2c7fd4133f8a78856d2d2d57a.png

 

A number of factors (clutch slip, jitter in RPM or OSS) can cause the instantaneous ratio (e.g. RPM/OSS) to move out of range and the gear detection program will fail. When this happens, the "gear selected" will go to '0' (e.g. Neutral) and any number of routines in the PCM relying on gear will quit or 'go funny' due to the gear changing from say 3 to neutral, or in this case the cluster displaying an error.

 

How to detect?

If you think its happening you can use the PCMTec Datalogger to verify. The following logging channels are useful:

MID36381    Estimated Gear Selected - MTX
MID61215    Instant Transmission Ratio - MTX
MID37788    Current Gear Ratio - MTX
MID110730     Time In Gear - MTX

Along with Throttle, RPM, VSpeed and VSS Fail Flag.

 

image.thumb.png.3ec1007398aab899eca0136e7dc42b43.png

Here is a classic example.

You can see the orange trace (MID37788) getting "bumpy" at a few locations, at the same time you can see the estimated gear (MID36381) drop from 3rd to '0'. (Yellow Arrow, Red Arrow). Time in gear (MID110730) resets to zero (green arrow).

When the gear estimator can't 'snap' to a gear it will change to and reflect the instantaneous ratio (green trace, MID61215), NB: these are not exactly the same at my marker due to some delay -and- different logging rates and polling times.

At the indicated point you can see the instant ratio is "1.3431" (indigo box). If you look at the image above for the tuning parameters, you can see this is just outside the detection range (auF0337). So with the instantaneous ratio being out of range for the required plausibility time (few ms) of any gear, the gear selection program reverts '0' - Neutral or unknown gear.

NB: this is a valid selection because if OSS = Zero and RPM > 1 (clutch in or in neutral), this ratio will actually go infinite (there is actually a clip parameter to set the default value) and this routine must accommodate that and provide some method of detecting Netural.

 

Why?

Assuming this particular example is driving on the road or a dyno (with a decent amount of drivetrain inertia) the rapid stabs of the throttle are casing a decent torque increase (and not so much rpm increase) which is likely slipping the clutch just enough for the above check to fail momentarily. Depending on the car, its age and power targets, it might be due for a new clutch.¬†ūüėá

 

How to fix?

Once you have identified the cause and found the offending gear, usually opening up the ratio limits are sufficient (as mentioned, our opinion is they are a little tight from factory for 3rd and/or 4th). In this particular example 3.6 on the upper limit (auF0337) of the 3rd gear ratio was sufficient and the problem of gear detection abated.

 

Hope someone finds this interesting!

Happy Tuning.

 

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Great write up @Kirby@pcmtec

2 hours ago, Kirby@pcmtec said:

How to detect?

If you think its happening you can use the PCMTec Datalogger to verify. The following logging channels are useful:

MID36381    Estimated Gear Selected - MTX
MID61215    Instant Transmission Ratio - MTX
MID37788    Current Gear Ratio - MTX
MID110730     Time In Gear - MTX

This issue is one I've previously investigated with version 1.25 & found that the channels were not as clearly named. The updated naming & ID in Version 2.10 has really tidied this up.

I've just connected to a BA F6 HAAT3VC, didn't actually go for a drive & can find all channels except MID110730. I'm guessing it might not be available with the BA PCM or it's something that's still being investigated.

image.png.4073af89d43809e75b64fcfe918b6d00.png

 

All the MID channels above, are available on the BF XR6T.
MID110730 is workshop only

image.png.26a9488d4f64adff78e6211289280338.png

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12 hours ago, Bill said:

Great write up @Kirby@pcmtec

This issue is one I've previously investigated with version 1.25 & found that the channels were not as clearly named. The updated naming & ID in Version 2.10 has really tidied this up.

I've just connected to a BA F6 HAAT3VC, didn't actually go for a drive & can find all channels except MID110730. I'm guessing it might not be available with the BA PCM or it's something that's still being investigated.

image.png.4073af89d43809e75b64fcfe918b6d00.png

 

All the MID channels above, are available on the BF XR6T.
MID110730 is workshop only

image.png.26a9488d4f64adff78e6211289280338.png

 

I actually had to work through this recently and found our default names really needing some improvement, so I did this for 2.1, but generally I am trying and refer by MID numbers whenever talking about datalogging for clarity. We are putting alot of effort into names at the moment - expect continual improvement in this space.

Time in gear is definitely not needed to diagnose this, its just nice to confirm in logging. If you look it has a much higher MID number than the others, which generally (but not always) means that it was identified later - which might mean it didn't appear until later model PCM's.

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  • Kirby@PCMTec changed the title to "HELP! My manual car keeps dropping out of cruise/has intermittent cluster gear display/boost-by-gear isn't behaving"

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