Speedo calibration unit
Discussion
With what appears to be more and more issues with erratic idles due to the speedo calibration unit firing speed pulses into the ECU when the car is stationary, I think it should be possible to make a simple plug in line unit to reduce its sensitivity to stray earth voltages.
So..... anyone have a unit thats not being used (any cars being rebuilt?) I could borrow for a week or two so I can reverse engineer it properly?
So..... anyone have a unit thats not being used (any cars being rebuilt?) I could borrow for a week or two so I can reverse engineer it properly?
No Phil, I think he means the TVR speed signal box that tells the 14CUX ECU you are travelling above or below 7mph (ish).
It converts the signal from the speedo into something the ECU can understand, TVR had to fit it because they fitted a different speedo to the one on a Range Rover.
The TVR speed signal box lives under the dash, & is built to the same standards as all the other TVR electronics, so you could say there's scope for improvement
The signal is needed by the ECU so it can influence the stepper motor under certain conditions, for example I believe by restricting air when you come rapidly off the throttle gear changes are made smoother.
The problems start when you come to a rapid stop at a junction, if the TVR speed signal box is not producing the below 7mph signal, the ECU doesn't know you're approaching a junction.
So no instruction is sent to the stepper motor, the air restriction remains, and the car stalls.
The poorly made speed signal box can also give all sorts of spurious messages to the ECU, which is suspected to be the origins of many strange & unfathomable idle issues on Chims & Griffs.
It's also the reason so many people (mistakenly) set about cleaning their stepper motor.
The stepper motor itself is a dumb unit taking it's instructions from the ECU.
In turn the ECU is only doing it's best to control the idle based on the information it gets from the TVR signal box.
I discovered all this when my car started stalling at junctions, in my case I tracked it down to the a bad electrical connection at the speedo so it was easily solved.
But while investigating the way it all worked I started asking questions of my local Range Rover specialist, who's knowledge of the 14CUX helped me understand the reason TVR had to make & fit that strange speed signal box I discovered under the dash.
Steve Heath also found his way to the speed signal box, & like me started to suspect it to be the cause of many TVR idle issues.
I believe Steve was going to sell a replacement for this problematic box of tricks, but ceased trading before it was released.
If Blitzracing can manufacture a decent reliable replacement, it could really benefit those with idle issues.
It could also save a the serial stepper motor cleaners a lot of wasted time
Good luck with the project Blitz, it's a very worthy one that could help an awful lot of TVR owners still running the Lucas 14CUX system
It converts the signal from the speedo into something the ECU can understand, TVR had to fit it because they fitted a different speedo to the one on a Range Rover.
The TVR speed signal box lives under the dash, & is built to the same standards as all the other TVR electronics, so you could say there's scope for improvement
The signal is needed by the ECU so it can influence the stepper motor under certain conditions, for example I believe by restricting air when you come rapidly off the throttle gear changes are made smoother.
The problems start when you come to a rapid stop at a junction, if the TVR speed signal box is not producing the below 7mph signal, the ECU doesn't know you're approaching a junction.
So no instruction is sent to the stepper motor, the air restriction remains, and the car stalls.
The poorly made speed signal box can also give all sorts of spurious messages to the ECU, which is suspected to be the origins of many strange & unfathomable idle issues on Chims & Griffs.
It's also the reason so many people (mistakenly) set about cleaning their stepper motor.
The stepper motor itself is a dumb unit taking it's instructions from the ECU.
In turn the ECU is only doing it's best to control the idle based on the information it gets from the TVR signal box.
I discovered all this when my car started stalling at junctions, in my case I tracked it down to the a bad electrical connection at the speedo so it was easily solved.
But while investigating the way it all worked I started asking questions of my local Range Rover specialist, who's knowledge of the 14CUX helped me understand the reason TVR had to make & fit that strange speed signal box I discovered under the dash.
Steve Heath also found his way to the speed signal box, & like me started to suspect it to be the cause of many TVR idle issues.
I believe Steve was going to sell a replacement for this problematic box of tricks, but ceased trading before it was released.
If Blitzracing can manufacture a decent reliable replacement, it could really benefit those with idle issues.
It could also save a the serial stepper motor cleaners a lot of wasted time
Good luck with the project Blitz, it's a very worthy one that could help an awful lot of TVR owners still running the Lucas 14CUX system
Bassfiend229hp said:
What's it look like - mine may still be floating around under there unused... 
Mines still under there too, I asked the Lloyd boys to extract it when they fitted the Canems so I could sell it.But somehow it never got liberated.
It's a small black box about the size of a pack of fags, it's got a white label on it signed by Doris or Mabel, or some such name.
Follow the wires from the back of the speedo to find Mabel's box
ChimpOnGas said:
No Phil, I think he means the TVR speed signal box that tells the 14CUX ECU you are travelling above or below 7mph (ish).
It converts the signal from the speedo into something the ECU can understand, TVR had to fit it because they fitted a different speedo to the one on a Range Rover.
The TVR speed signal box lives under the dash, & is built to the same standards as all the other TVR electronics, so you could say there's scope for improvement
The signal is needed by the ECU so it can influence the stepper motor under certain conditions, for example I believe by restricting air when you come rapidly off the throttle gear changes are made smoother.
The problems start when you come to a rapid stop at a junction, if the TVR speed signal box is not producing the below 7mph signal, the ECU doesn't know you're approaching a junction.
So no instruction is sent to the stepper motor, the air restriction remains, and the car stalls.
The poorly made speed signal box can also give all sorts of spurious messages to the ECU, which is suspected to be the origins of many strange & unfathomable idle issues on Chims & Griffs.
It's also the reason so many people (mistakenly) set about cleaning their stepper motor.
The stepper motor itself is a dumb unit taking it's instructions from the ECU.
In turn the ECU is only doing it's best to control the idle based on the information it gets from the TVR signal box.
I discovered all this when my car started stalling at junctions, in my case I tracked it down to the a bad electrical connection at the speedo so it was easily solved.
But while investigating the way it all worked I started asking questions of my local Range Rover specialist, who's knowledge of the 14CUX helped me understand the reason TVR had to make & fit that strange speed signal box I discovered under the dash.
Steve Heath also found his way to the speed signal box, & like me started to suspect it to be the cause of many TVR idle issues.
I believe Steve was going to sell a replacement for this problematic box of tricks, but ceased trading before it was released.
If Blitzracing can manufacture a decent reliable replacement, it could really benefit those with idle issues.
It could also save a the serial stepper motor cleaners a lot of wasted time
Good luck with the project Blitz, it's a very worthy one that could help an awful lot of TVR owners still running the Lucas 14CUX system
Dave Range Rover never used the same diff mounted Transducer that TVR fitted to our cars It converts the signal from the speedo into something the ECU can understand, TVR had to fit it because they fitted a different speedo to the one on a Range Rover.
The TVR speed signal box lives under the dash, & is built to the same standards as all the other TVR electronics, so you could say there's scope for improvement
The signal is needed by the ECU so it can influence the stepper motor under certain conditions, for example I believe by restricting air when you come rapidly off the throttle gear changes are made smoother.
The problems start when you come to a rapid stop at a junction, if the TVR speed signal box is not producing the below 7mph signal, the ECU doesn't know you're approaching a junction.
So no instruction is sent to the stepper motor, the air restriction remains, and the car stalls.
The poorly made speed signal box can also give all sorts of spurious messages to the ECU, which is suspected to be the origins of many strange & unfathomable idle issues on Chims & Griffs.
It's also the reason so many people (mistakenly) set about cleaning their stepper motor.
The stepper motor itself is a dumb unit taking it's instructions from the ECU.
In turn the ECU is only doing it's best to control the idle based on the information it gets from the TVR signal box.
I discovered all this when my car started stalling at junctions, in my case I tracked it down to the a bad electrical connection at the speedo so it was easily solved.
But while investigating the way it all worked I started asking questions of my local Range Rover specialist, who's knowledge of the 14CUX helped me understand the reason TVR had to make & fit that strange speed signal box I discovered under the dash.
Steve Heath also found his way to the speed signal box, & like me started to suspect it to be the cause of many TVR idle issues.
I believe Steve was going to sell a replacement for this problematic box of tricks, but ceased trading before it was released.
If Blitzracing can manufacture a decent reliable replacement, it could really benefit those with idle issues.
It could also save a the serial stepper motor cleaners a lot of wasted time
Good luck with the project Blitz, it's a very worthy one that could help an awful lot of TVR owners still running the Lucas 14CUX system
Sardonicus said:
ave Range Rover never used the same diff mounted Transducer that TVR fitted to our cars
Well it seems pretty conclusive to me that the 14CUX ECU definitely needs a speed signal.Disconnect the speed signal box or the speedo on a Chim with the 14CUX, & watch it stall at junctions.
Reconnect and see the fault disappear, well that's what happened with mine.
How a Range Rover delivers this speed signal to the ECU is a mystery to me, but I'd be surprised if it doesn't do it somehow.
I'm pretty sure TVR didn't create the speed signal box for fun, if anyone can explain the system better then I'm all ears
Actually on a personal level all this is of little consequence, my Canems system seems to do everything way better that the Lucas 14CUX ever did.
But I'm sure others would benefit from the collective wisdom on this one
ChimpOnGas said:
Well it seems pretty conclusive to me that the 14CUX ECU definitely needs a speed signal.
Disconnect the speed signal box or the speedo on a Chim with the 14CUX, & watch it stall at junctions.
Reconnect and see the fault disappear, well that's what happened with mine.
How a Range Rover delivers this speed signal to the ECU is a mystery to me, but I'd be surprised if it doesn't do it somehow.
I'm pretty sure TVR didn't create the speed signal box for fun, if anyone can explain the system better then I'm all ears
Actually on a personal level all this is of little consequence, my Canems system seems to do everything way better that the Lucas 14CUX ever did.
But I'm sure others would benefit from the collective wisdom on this one
My speedo would fail randomly for 2 years of summer Dave sometimes days (faulty transducer, since rectified) I used the Sat-Nav speedo but I never experienced stalling hanging idle or any strange behavior whatsoever in fact I cleaned the stepper & housing on purchase of my Chim as preventive maintenance only once Disconnect the speed signal box or the speedo on a Chim with the 14CUX, & watch it stall at junctions.
Reconnect and see the fault disappear, well that's what happened with mine.
How a Range Rover delivers this speed signal to the ECU is a mystery to me, but I'd be surprised if it doesn't do it somehow.
I'm pretty sure TVR didn't create the speed signal box for fun, if anyone can explain the system better then I'm all ears
Actually on a personal level all this is of little consequence, my Canems system seems to do everything way better that the Lucas 14CUX ever did.
But I'm sure others would benefit from the collective wisdom on this one
I have said many times before I had no beef with the Lucas apart from I could not dial the low speed fueling (dull the lurch) and I could not map it myself along with no mapped ign 
it's all about correct base idle settings IMO with the Lucas, if I was to be totally honest in was the MS2 that i struggled with initially to get the same smooth cold start behavior/control of the stepper that the Lucas gave me 
the speedo vs stepper behavior is blown out of proportion if you ask me .... all things being equal 
ChimpOnGas said:
How a Range Rover delivers this speed signal to the ECU is a mystery to me, but I'd be surprised if it doesn't do it somehow.
I'm pretty sure TVR didn't create the speed signal box for fun, if anyone can explain the system better then I'm all ears
Range Rover / LR & TVR all had LT77 boxes that have the integral speedo transducer that the ECU was designed for... TVR in addition to that circuit used a diff transducer to drive their own (ETB) dash speedo. NONE of the early cars have or need that box (except pre-cat Griffs which is a different scenario again).I'm pretty sure TVR didn't create the speed signal box for fun, if anyone can explain the system better then I'm all ears
Then along came the 'improved' T5 and it all went tits up..... so a bodge box to split the diff signal from the speedo to the ecu simulating the LT77 transducer was built. At this time the decision was made that the calibration wasn't deemed to be critical as they didn't use the speed limiting function of the ecu, only the stop/start for idle AIUI.
I don't think that could be classed as describing the system better, it simply has a ring of truth rather than complete deluded fiction.
spend said:
Range Rover / LR & TVR all had LT77 boxes that have the integral speedo transducer that the ECU was designed for... TVR in addition to that circuit used a diff transducer to drive their own (ETB) dash speedo. NONE of the early cars have or need that box (except pre-cat Griffs which is a different scenario again).
Then along came the 'improved' T5 and it all went tits up..... so a bodge box to split the diff signal from the speedo to the ecu simulating the LT77 transducer was built. At this time the decision was made that the calibration wasn't deemed to be critical as they didn't use the speed limiting function of the ecu, only the stop/start for idle AIUI.
I don't think that could be classed as describing the system better, it simply has a ring of truth rather than complete deluded fiction.
This post has scarcity value - it's on the mysteries of speed sensors, and even I understand it! Ta. Then along came the 'improved' T5 and it all went tits up..... so a bodge box to split the diff signal from the speedo to the ecu simulating the LT77 transducer was built. At this time the decision was made that the calibration wasn't deemed to be critical as they didn't use the speed limiting function of the ecu, only the stop/start for idle AIUI.
I don't think that could be classed as describing the system better, it simply has a ring of truth rather than complete deluded fiction.
spend said:
Range Rover / LR & TVR all had LT77 boxes that have the integral speedo transducer that the ECU was designed for... TVR in addition to that circuit used a diff transducer to drive their own (ETB) dash speedo. NONE of the early cars have or need that box (except pre-cat Griffs which is a different scenario again).
Then along came the 'improved' T5 and it all went tits up..... so a bodge box to split the diff signal from the speedo to the ecu simulating the LT77 transducer was built. At this time the decision was made that the calibration wasn't deemed to be critical as they didn't use the speed limiting function of the ecu, only the stop/start for idle AIUI.
I don't think that could be classed as describing the system better, it simply has a ring of truth rather than complete deluded fiction.
Great, so it definitely needs that speed signal just like I thought.Then along came the 'improved' T5 and it all went tits up..... so a bodge box to split the diff signal from the speedo to the ecu simulating the LT77 transducer was built. At this time the decision was made that the calibration wasn't deemed to be critical as they didn't use the speed limiting function of the ecu, only the stop/start for idle AIUI.
I don't think that could be classed as describing the system better, it simply has a ring of truth rather than complete deluded fiction.
Where it comes from, (LT77 gearbox or speedo transducer/speedo in later T5 equipped cars) is somewhat irrelevant.
The ECU still needs that signal.
Which is what Blitzracing's post was all about in the first place.
I still say creating a speed signal box for T5 equipped TVRs is a project with merit.
I'm pretty sure it'll work out cheaper than replacing a perfectly good T5 with an old LT77 box.
Not that there's anything especially wrong with the LT77, but I believe TVR dropped it in favour of the T5 when supply dried up.
That was over 16 years ago now, so I suspect it means all these years later LT77 parts aren't all that readily available?
A T5 on the other hand is a completely different proposition in this respect, and will continue to be well supported with parts for many many years to come given the number produced and the cars it went in.
That's where a T5 wins every time in my book, but I guess if you've got one of those old LT77 jobs you're bound to defend it.
Good luck with your speed signal box project Blitz, 14CUX users with the far more common T5 box will thank you I'm sure
All I want to do is put a small amount of filtering or voltage bias on the input of the unit so it needs the correct level signal from the diff to trigger it, not simply some passing earth noise on the TVR chassis. It should not take more than a couple of resistors / diodes, and I should be able to just make up a small plug in cable that runs inline with the unit- I dont think it nessasary to re design the unit just to stop the false triggering- anyway Steve Heath has already been on the quest to produce a unit that produces a better speed signal in the first place, and thats his baby :-)
So..... I just need a unit to check what the input connections are and plug types.
So..... I just need a unit to check what the input connections are and plug types.
Edited by blitzracing on Wednesday 8th May 13:13
On a related subject, my '97 Chimaera also has speedo problems which started recently (T5 box):-
When it's cold, the speedo works fine but after 10-15 minutes of driving, i.e. warmed up, the speedo will become erratic, bouncing between actual speed & zero. It then stops working completely except when i decelerate, when it will again briefly bounce to actual speed & back to zero. Engine is lumpy when the speedo stops completely of course as I assume the ecu thinks it's stationary. It also stalls occasionally when stopping at a junction.
SH bible says the proximity sensor on the diff is heat sensitive but I can't believe it heats up that quickly as it's external. I haven't got around to testing the sensor pulses at the ecu yet, on my todo list
Where under the dash is the calibration unit?
Thanks.
When it's cold, the speedo works fine but after 10-15 minutes of driving, i.e. warmed up, the speedo will become erratic, bouncing between actual speed & zero. It then stops working completely except when i decelerate, when it will again briefly bounce to actual speed & back to zero. Engine is lumpy when the speedo stops completely of course as I assume the ecu thinks it's stationary. It also stalls occasionally when stopping at a junction.
SH bible says the proximity sensor on the diff is heat sensitive but I can't believe it heats up that quickly as it's external. I haven't got around to testing the sensor pulses at the ecu yet, on my todo list
Where under the dash is the calibration unit?
Thanks.
Its hung off the loom for the dash panel. There are 2 main connectors that allow the dash panel to be extracted. The speedo with ring terms normally has its own connector so that you don't need to unscrew the ring terms and connected to this (clamped along with the rest of the dash loom) is the connector with a black box hung off it that is the splitter unit.
Another thing that alt ecu fitters don't bother removing, just clamp the U - G/something wire to pass the speed signal direct to the speedo, and ignore the ecu feed IIRC.
Another thing that alt ecu fitters don't bother removing, just clamp the U - G/something wire to pass the speed signal direct to the speedo, and ignore the ecu feed IIRC.
Oldred_V8S said:
Blitz
How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
It's only a problem for Griff owners with the Stewart Warner Gauges AFIK. The speed sensors are pretty generic, and the ETB speedo is easily calibrated to use any coloured 'daisy wheel' that fits the LT77 or R380 LR box. It's the original daisy wheel that is hard to get ~ other ratios (different colours) are readily available I believe.How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
Oldred_V8S said:
Blitz
How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
I've got one these speedo correction kits:How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
http://www.jaycar.com.au/productView.asp?ID=KC5435
Sits between the speed transducer output and speedo and allows for different ratio transducers by adjusting pulse frequency to suit.
But it looks like they're out of stock.
Oldred_V8S said:
Blitz
How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
I think the Ginetta with the LT77 box is different to the TVR- I have a chopper disk in a metal unit thats inline with the mechanical speedo cable- is this the same as the TVR?How about putting a kit together to replace the speed sensor in the LT77 box? There are a number of these gearboxes around but the speed sensors are like hens teeth. I for one would purchase a kit to keep in my "spares dept" as I am sure will other Lt77/14cux users.
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