ECU road speed sensor?
Discussion
I took my '95 400 for a spin earlier with Rovergauge connected up for a bit of data logging.
The speedo on the car works, although typically over reads by 10 MPH. The speed on Rovergauge remains a consistent 0 MPH, regardless of actual road speed.
It is obviously information that the ECU would benefit from receiving, as otherwise the input wouldn't be allocated, although the car does drive perfectly.
So, does the ECU share the same speed sensor as the cars speedo via some type of splitter or is there a separate sender located elsewhere (I've done a bit of searching and found some info relating to an LT77 gearbox/early Chim/Griff, but mine is a late '95, with a T5)
The speedo on the car works, although typically over reads by 10 MPH. The speed on Rovergauge remains a consistent 0 MPH, regardless of actual road speed.
It is obviously information that the ECU would benefit from receiving, as otherwise the input wouldn't be allocated, although the car does drive perfectly.
So, does the ECU share the same speed sensor as the cars speedo via some type of splitter or is there a separate sender located elsewhere (I've done a bit of searching and found some info relating to an LT77 gearbox/early Chim/Griff, but mine is a late '95, with a T5)
Edited by peaktorque on Sunday 7th February 20:02
The box basically intercepts the very small signal generated by the speed transducer. The speedo itself can read the signal directly, but the ECU cant, so the box turns the small variable signal from the transducer into a fixed speed signal the ECU can read. This is typically between 30 and 50 mph at anything above 3 mph. The boxes electronics is pretty basic, but Ive certainly seen dry joins where the main connector meets the PCB that causes it to fail.
blitzracing said:
The box basically intercepts the very small signal generated by the speed transducer. The speedo itself can read the signal directly, but the ECU cant, so the box turns the small variable signal from the transducer into a fixed speed signal the ECU can read. This is typically between 30 and 50 mph at anything above 3 mph. The boxes electronics is pretty basic, but Ive certainly seen dry joins where the main connector meets the PCB that causes it to fail.
Some years back when I was still on the 14CUX system I had a mysterious stalling problem that revealingly coincided with a failed speedo, typically the engine idled in 90% of all situations, it only stalled when I came to a fairly rapid stop after being at speed, an example of this being coming off a fast moving motorway and stopping at a junction after fairly rapid deceleration. I traced the lack of speedo to a bad connection so an easy fix, interestingly as soon as the speedo came back to life the stalling issue was resolved too. A bit of research revealed the 14CUX needs a speed signal to help with idle management. It's quite a crude system by modern standards, basically the the ECU just needs to know if the car is moving or not.
I believe the figure the ECU uses is quite low, something like this....
- Above 5mph = car moving
- Below 5mph = car stopped = reference min RPM idle target (say 800rpm?)
My 5mph figure and the 800rpm min engine speed target are just examples to explain the process, I'm not 100% sure of the exact figures so stopped could easily be below 3mph or 7mph and the mim idle target could be as low as 700rpm which is just enough to catch the stall?
While my issue was a simple loose connection at the back of the speed and easy to spot because the speedo had stopped working at the same time as the stalling fault appeared, I could see the same issue occurring if TVRs very basic speed signal box was faulty.
I can also see if the base idle isn't correctly set you could confuse the system or even mask the stalling issue, for these reasons alone I would say following the correct base idle setting procedure is very important to the overall correct running of the car.
Another point of interest is that on my Canems systems Lloyd Specialist Developments recently gave me what they call "Closed Loop Throttle" (CLT), this system does not use or need a road speed signal like the 14CUX but references upper & lower engine speed targets. It also has a "Degrade Rate" feature which appears to be a critical element, when you sharply lift off the throttle the engine speed isn't allowed to simply plummet which could easily see the revs falling well below the lower engine speed target. During sharp throttle lifts the system still bleeds some air passed the throttle butterfly and hangs the throttle just long enough for you to change gear and get back on the throttle, the result is super smooth and very un-TVR like gliding through town progress
The results of this closed loop throttle strategy are also a significantly more stable idle, but what you immediately notice is the massive improvement in those low speed gear changes, the degrade rate feature is hanging onto just enough revs as I come off the throttle to change gear that when my foot returns to the throttle engine speed is a little higher than it would e without the CLT feature activated, its a sort of rev matching function that promotes a far smoother transition between the on/off throttle event during gear changes.
You only really notice it below 40mph but during gear changes between 20-30mph it's an absolute revelation, the dramatic improvement in smoothness and drivability when moving through town is very pronounced and extremely welcome.
The other thing I noticed after the Lloyd team gave me closed loop throttle was how I could just stay in the same gear through town, indeed regulating my speed in town became as simple as driving an automatic. Where before I might find myself shifting to a lower gear or even de-clutching and popping the car in neutral to glide up to a junction on a light trailing brake. With CLT I found I can just waft through town coming on and off the throttle with complete smoothness while still leaving the car in third gear, this makes for a much more relaxed drive which is far more important that I ever realised.
I can honestly say the CLT strategy given to me by Lloyd Specialist Developments (without me even asking BTW) delivered the single biggest improvement to my TVR's low speed drivability, and by a huge margin. Closed loop throttle really is that good, indeed I would strongly recommend anyone with a Canems ECU who doesn't currently have CLT to get in touch with Lloyd Specialist Developments to ask them to add this excellent feature.
Back to the original topic, I have no idea if the 14CUX operates a similar CLT strategy? The 14CUX certainly has closed loop throttle to some degree as revealed by the stalling issue I had and fixed, and the need for the TVR made road speed signal box. Indeed if the 14CUX does have CLT management up to say 40mph and it's not working correctly, I can easily see such a fault causing what people often refer to as "shunting".
This would explain why some Chims and Griffs shunt horribly while others exhibit none of this nasty trait, perhaps shunting has more to do with what the stepper motor is being told to do when you sharply come off and then back on the throttle during low speed gear changes than we fully understand?
Interesting
Edited by ChimpOnGas on Tuesday 16th February 10:03
Just to add to this I put up with an intermittent speedo for over 3 summers (used sat nav)it would drop out for hours at times due to a failing diff sensor but never suffered any downsides whatsoever with idle wind down abnormalities however my base idle was set correctly and I always favour a slightly higher idle speed 900/1000 yet I know other cars have not been so tolerant 
and I am no more patient with stalling problems than unreliable cars 
however my base idle was set correctly and I always favour a slightly higher idle speed 900/1000 yet I know other cars have not been so tolerant and I am no more patient with stalling problems than unreliable cars Edited by Sardonicus on Tuesday 16th February 10:12
There are also possible issues caused by earth loops in the box and we have had success fitting a diode in the earth wire.
Google 'pistonheads TVR speed signal' or somesuch and find earlier threads on how and where to fit the diode.
Blitz may also come top your rescue as i think it was his mod in the first place.
Steve
Google 'pistonheads TVR speed signal' or somesuch and find earlier threads on how and where to fit the diode.
Blitz may also come top your rescue as i think it was his mod in the first place.
Steve
For anyone interested there are five idle management features in the Canems software, although here are the three key ones directly or indirectly associated to the closed loop throttle/idle feature I've been raving about.
The bottom table is "Idle Valve Warm Up" which is self explanatory but is shown to demonstrate the fact closed loop throttle "Idle Control Setup" actually does nothing until the engine coolant has reached 60c.
After that the idle management steps away from the percentage the "Idle Valve Warm Up" feature adds to the fixed base idle setting and looks to dynamically adjust the idle air valve to achieve my chosen 1,000rpm idle target.
Even if the coolant is at 60c or above none of this takes place unless:
1. The throttle is 4% open or less
2. The engine speed is below 1,500rpm
You can also see the system has a configurable fast target idle speed which is set at 1,100rpm, this combined with the decay rate (0.35 seconds in my case) is the really clever bit.
Sudden engine deceleration to idle speed can cause the closed loop control to over compensate and reduce the amount of air available to the engine. To combat this, a "Fast target idle speed" is set higher than my 1,000rpm main target idle speed. When the engine returns to idle speed, the ECU will detect this, and aim for the fast target.
The "Idle target decay rate" specifies the amount of time it takes to change from the fast target to the normal target idle speeds. Once a decay time has elapsed, the fast target will be reduced by 50 rpm. This process continues until the fast target speed is equal to the normal target speed.
As already stated, what I've found is all this not only translates to much better overall idle quality but also far smoother low speed drivability. Lets say you're driving slowly through town, as you sharply come off the throttle either to slow or change gear the ECU will look to achieve the higher 1,100rpm fast target. Hanging onto this slightly higher idle speed for the 0.70 seconds (2 x 50rpm increments at 0.35 seconds each) is just enough time for you to select the next gear and return to the throttle, the engine speed never falls in a 850rpm hole that you must then break out of as you come back on the throttle, this is where you'd ordinarily feel a shunt.
The end result of all this is a kind of idle speed bridging effect between gear changes and hence much smoother progress. I'm sure other stand alone ECUs can play the same closed loop throttle/idle trick but as my Canems ECU is the first and only configurable engine management system I've explored it's very much a new feature to me. While closed loop throttle/idle seems very clever in theory (which it is), I have to say it's the end result that really impresses.
Having gone from my Canems system running open loop where it simply gives a PWM output based on temperature, to the more sophisticated closed loop, I have become a total convert simply for the way it transforms the car's low speed drivability manners.
Its just one more in a long list of benefits associated with moving to a more modern and user configurable engine management system, and without any of the reliance on road speed information the 14CUX needs. As such my Canems system has absolutely no need for the rather crude and unreliable TVR made speed signal box, no great loss there then
In summary, all of this may also be telling us something about why Chims & Griffs on the 14CUX can often suffer from violent shunting
Here's closed loop throttle/idle in action on a graph, the low speed gear change or on/off throttle speed regulation through town being represented by the circle.
Progress through urban areas where you tend to need to regulate your speed between 20 & 30 mph is now smooth as a baby's bum
The bottom table is "Idle Valve Warm Up" which is self explanatory but is shown to demonstrate the fact closed loop throttle "Idle Control Setup" actually does nothing until the engine coolant has reached 60c.
After that the idle management steps away from the percentage the "Idle Valve Warm Up" feature adds to the fixed base idle setting and looks to dynamically adjust the idle air valve to achieve my chosen 1,000rpm idle target.
Even if the coolant is at 60c or above none of this takes place unless:
1. The throttle is 4% open or less
2. The engine speed is below 1,500rpm
You can also see the system has a configurable fast target idle speed which is set at 1,100rpm, this combined with the decay rate (0.35 seconds in my case) is the really clever bit.
Sudden engine deceleration to idle speed can cause the closed loop control to over compensate and reduce the amount of air available to the engine. To combat this, a "Fast target idle speed" is set higher than my 1,000rpm main target idle speed. When the engine returns to idle speed, the ECU will detect this, and aim for the fast target.
The "Idle target decay rate" specifies the amount of time it takes to change from the fast target to the normal target idle speeds. Once a decay time has elapsed, the fast target will be reduced by 50 rpm. This process continues until the fast target speed is equal to the normal target speed.
As already stated, what I've found is all this not only translates to much better overall idle quality but also far smoother low speed drivability. Lets say you're driving slowly through town, as you sharply come off the throttle either to slow or change gear the ECU will look to achieve the higher 1,100rpm fast target. Hanging onto this slightly higher idle speed for the 0.70 seconds (2 x 50rpm increments at 0.35 seconds each) is just enough time for you to select the next gear and return to the throttle, the engine speed never falls in a 850rpm hole that you must then break out of as you come back on the throttle, this is where you'd ordinarily feel a shunt.
The end result of all this is a kind of idle speed bridging effect between gear changes and hence much smoother progress. I'm sure other stand alone ECUs can play the same closed loop throttle/idle trick but as my Canems ECU is the first and only configurable engine management system I've explored it's very much a new feature to me. While closed loop throttle/idle seems very clever in theory (which it is), I have to say it's the end result that really impresses.
Having gone from my Canems system running open loop where it simply gives a PWM output based on temperature, to the more sophisticated closed loop, I have become a total convert simply for the way it transforms the car's low speed drivability manners.
Its just one more in a long list of benefits associated with moving to a more modern and user configurable engine management system, and without any of the reliance on road speed information the 14CUX needs. As such my Canems system has absolutely no need for the rather crude and unreliable TVR made speed signal box, no great loss there then
In summary, all of this may also be telling us something about why Chims & Griffs on the 14CUX can often suffer from violent shunting
Here's closed loop throttle/idle in action on a graph, the low speed gear change or on/off throttle speed regulation through town being represented by the circle.
Progress through urban areas where you tend to need to regulate your speed between 20 & 30 mph is now smooth as a baby's bum
Edited by ChimpOnGas on Friday 19th February 09:04
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