Road speed sensor
Discussion
Rovergauge flagged up a road speed fault. No speed showing on the Rovergauge speedo when driving . Car Speedo works fine.
It’s a 4.3 pre cat but with a T5 box. It’s always had shunting and sometimes high tickover but I have improved it over the years with a new stepper, improved ignition parts ( plugs and leads, etc) but it still doesn’t feel as good as it could.
I’ve cleaned and gapped the speedo transducer but can only think it can be the splitter box behind the dash?
Am I right in thinking the precats got the ECU signal from the gearbox, and didn’t have a splitter box? maybe my precat ECU isn’t compatible with the speed box? Could it be a faulty speed box ( but the speedo is functioning fine). Not 100% sure it has a speed box!
Ultimately I need to find a way of getting a signal to the ECU .
Any help appreciated..
It’s a 4.3 pre cat but with a T5 box. It’s always had shunting and sometimes high tickover but I have improved it over the years with a new stepper, improved ignition parts ( plugs and leads, etc) but it still doesn’t feel as good as it could.
I’ve cleaned and gapped the speedo transducer but can only think it can be the splitter box behind the dash?
Am I right in thinking the precats got the ECU signal from the gearbox, and didn’t have a splitter box? maybe my precat ECU isn’t compatible with the speed box? Could it be a faulty speed box ( but the speedo is functioning fine). Not 100% sure it has a speed box!
Ultimately I need to find a way of getting a signal to the ECU .
Any help appreciated..
KevtheRev said:
Yes, that's how I understand it.
LT77 gearbox cars ECU speed input was direct from a gearbox sensor as per original Rover installation.
T5 gearbox cars ECU speed input is from the diff transducer via the under-dash emulator.
Speedo receives the raw signal from the diff transducer in both applications.
Won't your car have been fitted with an LT77 gearbox originally? If so maybe an the person who fitted the T5 box never considered or installed an emulator box. From viewing on Rovergauge, Has your ECU speed input ever worked?
Only just got Rovergauge and it doesn’t show any ECU input speed.LT77 gearbox cars ECU speed input was direct from a gearbox sensor as per original Rover installation.
T5 gearbox cars ECU speed input is from the diff transducer via the under-dash emulator.
Speedo receives the raw signal from the diff transducer in both applications.
Won't your car have been fitted with an LT77 gearbox originally? If so maybe an the person who fitted the T5 box never considered or installed an emulator box. From viewing on Rovergauge, Has your ECU speed input ever worked?
Edited by KevtheRev on Tuesday 30th April 08:24
Even if I fit/ get the existing box working will my precat ECU accept it , or will it be looking for a signal from the LT77 which isn’t there?
I also have fault 69 Neutral (gear selector) switch. Is that connected? Looking for the signal from the gearbox?
KevtheRev said:
Gear selection input is not used in the TVR.
Yes your 14CUX ECU will accept the black box speed input. The black box was designed to emulate the LT77 speed sensor output, however, it is not linear, only has two outputs, zero speed and ~37mph when the car is moving.
The reason is on our cars, the road speed input is only used for engine idle control via the stepper therefore the ECU only needs to know if the car is stationary of moving. Lack of that input could contribute to your shunting.
As I said before, I suspect your speed sensor side has not been upgraded with the new gearbox, but that really is armchair guessing. You could go under the car and see what's been done with the old LT77 sensor connector and take it from there.
Also consider this- if someone who retro fitted a T5 box also understood how to retro fit the black box, and your car does actually have one, would they bother fit it behind the dash or would they graft it in somewhere else that's easier!
Thanks for that Yes your 14CUX ECU will accept the black box speed input. The black box was designed to emulate the LT77 speed sensor output, however, it is not linear, only has two outputs, zero speed and ~37mph when the car is moving.
The reason is on our cars, the road speed input is only used for engine idle control via the stepper therefore the ECU only needs to know if the car is stationary of moving. Lack of that input could contribute to your shunting.
As I said before, I suspect your speed sensor side has not been upgraded with the new gearbox, but that really is armchair guessing. You could go under the car and see what's been done with the old LT77 sensor connector and take it from there.
Also consider this- if someone who retro fitted a T5 box also understood how to retro fit the black box, and your car does actually have one, would they bother fit it behind the dash or would they graft it in somewhere else that's easier!
I’ll take out the stereo and see if I can see the box . I assume that’s where it should be if it’s there?
I haven’t seen it anywhere else.
Has anyone got a picture of where it is, and what it looks like? Thanks
blitzracing said:
If its fitted its a black box about 5 x 8 x 3 cm, with a 6 plug connector. Normally sits near the center of the dash amidst the loom, may be marked speedo calibration unit. The unit takes the very small input from the tooth wheel diff sensor and makes it into something the ECU can understand- so where is your car getting the speedo signal from- does it have the diff' toothed wheel or not?
Yes it’s got the toothed wheel with the transducer on it.Edited by blitzracing on Tuesday 30th April 18:09
I’ve taken the stereo it and had a look around but can only see Gemini (alarm? ) black boxes.
Maybe there’s no box, and the feed just goes to a he speedo?
Do you know the colour of the wires and plug in case it’s there for the box to be connected?
Steve_D said:
The box will likely have the following wire colours.
Black...............Earth
Green............+12V
Green/yellow..signal to ECU
Grey...............Signal from sender
Blue/yellow.....Signal to speedo
The ECU is expecting a signal from a Range Rover gearbox so on a TVR there should be a 510 Ohm resistor between ECU pin 34 and earth. the wire colour is orange/black.
Steve
Should that be standard or do I need to add the resistor?Black...............Earth
Green............+12V
Green/yellow..signal to ECU
Grey...............Signal from sender
Blue/yellow.....Signal to speedo
The ECU is expecting a signal from a Range Rover gearbox so on a TVR there should be a 510 Ohm resistor between ECU pin 34 and earth. the wire colour is orange/black.
Steve
It looks like the box is missing. Anyone selling one?
blitzracing said:
You would get the symptoms you describe without the box- the diff signal can feed the speedo directly as it will take the very low input. The speedo calibration box input is in parallel to the speedo input, but turns it into both frequency and voltage the ECU can understand.
Thanks.Could you show me a picture of the connector plug showing the pins so I can find the corresponding plug behind the dash ( if it’s there)?
Steve_D said:
The box will likely have the following wire colours.
Black...............Earth
Green............+12V
Green/yellow..signal to ECU
Grey...............Signal from sender
Blue/yellow.....Signal to speedo
The ECU is expecting a signal from a Range Rover gearbox so on a TVR there should be a 510 Ohm resistor between ECU pin 34 and earth. the wire colour is orange/black.
Steve
I’ve had a look behind the dash and can’t see a speed box. Is this the plug (with resistor)?Black...............Earth
Green............+12V
Green/yellow..signal to ECU
Grey...............Signal from sender
Blue/yellow.....Signal to speedo
The ECU is expecting a signal from a Range Rover gearbox so on a TVR there should be a 510 Ohm resistor between ECU pin 34 and earth. the wire colour is orange/black.
Steve
Is it just a case of unplugging it and plugging it into a box?
The speedo is Caerbont ( I think!) and has always worked ok.
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