Road speed sensor

Road speed sensor

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Discussion

chris212

Original Poster:

102 posts

98 months

Monday 29th April
quotequote all
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..

KevtheRev

81 posts

18 months

Tuesday 30th April
quotequote all
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?

Edited by KevtheRev on Tuesday 30th April 08:24

chris212

Original Poster:

102 posts

98 months

Tuesday 30th April
quotequote all
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?

Edited by KevtheRev on Tuesday 30th April 08:24
Only just got Rovergauge and it doesn’t show any ECU input speed.
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

81 posts

18 months

Tuesday 30th April
quotequote all
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!

chris212

Original Poster:

102 posts

98 months

Tuesday 30th April
quotequote all
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
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

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blitzracing

5,639 posts

161 months

Tuesday 30th April
quotequote all
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?

Edited by blitzracing on Tuesday 30th April 18:09

chris212

Original Poster:

102 posts

98 months

Tuesday 30th April
quotequote all
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?

Edited by blitzracing on Tuesday 30th April 18:09
Yes it’s got the toothed wheel with the transducer on it.
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

12,282 posts

199 months

Tuesday 30th April
quotequote all
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

chris212

Original Poster:

102 posts

98 months

Tuesday 30th April
quotequote all
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?
It looks like the box is missing. Anyone selling one?

blitzracing

5,639 posts

161 months

Wednesday 1st May
quotequote all
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.




Steve_D

12,282 posts

199 months

Wednesday 1st May
quotequote all
Steve_D said:
............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
The resistor should be in the loom. There are 2 large connectors joining the footwell loom to the dash loom. one brown the other clear nylon. The orange/black wire goes to the clear connector and the resistor (with black connecting wires) is wired directly into the dashboard side of the connector mating half.

Steve

chris212

Original Poster:

102 posts

98 months

Wednesday 1st May
quotequote all
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)?

LeeHodges

393 posts

224 months

Wednesday 1st May
quotequote all
This is an interesting subject for me as it was something I was going to bring up. Whilst taking my car apart, there was no sensor on my '99 Griff's diff. I can't remember there being one on my '94 (T5 boxed) Griff either. I have, however, found what looks like a magnetic sensor tie-wrapped to the N/S (i think it was) drive shaft, on the diff side. Am I missing something here? Is the magnetic-looking sensor standard for a '99, or after market, or nothing to do with speed sensing?


blitzracing

5,639 posts

161 months

Wednesday 1st May
quotequote all
chris212 said:
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)?

stevesprint

965 posts

120 months

Wednesday 1st May
quotequote all
Chris
If a road speed sensor is not connected to the 14CUX the engine will stall while slowing down once hot, therefore if its not stalling I suspect the road signal is also connected direct to the ECU via the original TVR Precat (Green/Yellow) wire.

If you have a standard Griff 500 speedo transducer the speed signal isn’t strong enough for the 14CUX plus you could have too many teeth on your crown wheel for the 14CUX to read the road speed correctly. You must have an aftermarket programmable speedo.

As suggested fitting a TVR speedo interface (M0559) is your simplest option to stop the fault code, (they should be called 14CUX road speed interface).

Here’s one on ebay
https://www.ebay.co.uk/itm/TVR-SPEEDO-INTERFACE-UN...

Mark
Thanks for the wiring details of the road speed interface.

chris212

Original Poster:

102 posts

98 months

Monday 13th May
quotequote all
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)?
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.

chris212

Original Poster:

102 posts

98 months

Monday 13th May
quotequote all
Picture of speedo

blitzracing

5,639 posts

161 months

Wednesday 15th May
quotequote all
Thats not the connector- Id be looking at your speedo wires and see if you can find the two grey ones for the diff sensor input, just to see if it has them to start with. I dont recollect if its two grey plus earth or one earth plus one grey.

stevesprint

965 posts

120 months

Wednesday 15th May
quotequote all
chris212 said:
Picture of speedo
That looks like a later standard speedo, therefore I’m guessing you have a standard 500 crown wheel and sensor on the diff and therefore is safe to use the TVR M0559 14CUX interface.

Does your Griff stall at junctions?

chris212 said:
I’ve had a look behind the dash and can’t see a speed box. Is this the plug (with resistor)?
Is it just a case of unplugging it and plugging it into a box?
Sorry its not that easy, that’s the 14CUX’s main loom connector to the car for things like 12volts, ignition, road speed sensor, manual resistor (park/natural), fuel pump, A/C, heated screen.

My Precat’s road speed sensor wire is Green/Yellow and wired directly to both the 14CUX and speedo so check if there is a Green/Yellow wire on that plug. Sorry I can’t remember where is the sensor wire is split for the speedo and 14CUX.


Mark,
Do you know if the MO599 box has to be wired directly to both the speedo and 14CUX or could it be wired in after the sensor wire has been split off for the 14CUX???

blitzracing

5,639 posts

161 months

From memory the box is in parallel to the speedo input- so effectively the two grey wires from the diff spilt with one pair going to the speedo and the other the box.