The V8 Owners Forum -

Evening All, think I'll be having a few Q?'s over the coming weeks re' the ECU operation as this is my first 14CUX install; so tonight's Q?

I have the Road Speed Sensor cable including the 6.8k resistor [measured], but where should this be installed on my conversion?

>>TVR Chimaera 4.0[RV8 type] engine with newly rebuilt 2WD R380 gbox<<

Thank you, Andrew

Hi Andrew;
I replaced my Flapper system with a Hotwire system last winter.
The speed sensor (speedo driven) output is fed into pin 6 of the ECU. Have looked at the circuit diagram and the 6.8k resistor feeds a 'Speed trip module' which is something different to the road speed sensor.
When I made up my new wiring loom I only used the minimum sensors I could get away with.
If you would like a copy of the circuit diagram, or any other help, then PM me.
Andy.

Hi Andy, thanks for the quick reply. I have the full LR pdf.doc showing the ECU wiring and function, component test process and fault code data + the RV8R docs with I think come from Mark Thomson and his very good g33.co.uk website.

I have in fact asked the wrong Q? above. I should asking, is the the RSS really needed?

I have read that some guys with Hotwire don't use it and also spoke with a friend who completed a 944 RV8 conversion some years ago who ignored the RSS and had no issues with the car.

However, if Rover spec'd the sensor it was there for a reason. This is explained in this excerpt from the g33.co.uk website:

Road speed sensor. This provides a 12 volt switched signal dependent on the how fast the car is moving. Its trigger point is 3mph, then the ECU alters how the stepper motor controls the idle (see below) and it also provides information to the ECU to limit the top speed on some 4x4 vehicles due to the limits on off road tyres. This unit is replaced on some TVR’s by an electronic unit called a speedometer calibration unit, (found under the dashboard ) that simply gives a fixed speed signal above 3mph, but no top speed signal. This top speed limit can also be removed by reprogramming the engine fuel map within the ECU.

:.

1. Do I really need the RSS?
2. Would it be easier to get hold of a Speedo Calibration Unit? [i'm assuming this is a simple sensor that sits inline with the speedo cable and provides a T output, one to the speedo, the other to pin6 of the ECU]

If point pt.2, then I may have a problem in that I'm upgrading my speedo and tach to Smith Flight gauges whigh are electronic sender driven.

Andrew;

I have also heard that some people do not fit them and have no problems.

I fitted one as I did not want to have any problems with the idle in the future, standard Rover one bought off EBay, just requires a new speedo cable to be made up by Speedy Cables.

Could you not use the transducer that you are using for the electronic speedo?

Andy.

Thanks Andy, overnight I've thought this through. I definitely want/need the RSS...at least until such time as I get the ECU remapped at some stage in the future.

I'll need to contact CAI, the manufacturer of Smiths Gauges, and talk this through with them to understand the signal outputs available from their hall speed sensor. If this is not available I can always run a dummy output from the existing R380 gearbox speedo.

Should anyone else be using this set up in the future I post an update when done. Here's a link to the CAI website for ref'.

http://www.caigauge.com/page26.html

a better weblink

http://www.demon-tweeks.co.uk/performan ... peedometer

In case this is helpful. My TVR Griffiths 500 donor had a speed sensor on the diff and connected to the 14CUX pin 6. I also connected the same wire to my CAI speedo and it all worked. In this case, CAI (Speedy Cables) had modified the original TVR CAI speedo for me to make it adjustable whereas the TVR one was a fixed signal (because I have different wheel and gearing to standard).

Always phone Speedy cables as in my (on several occasions) experience they do not reply to e-mails. They do the job but I also find them rather expensive but then there is no other place to go.

Edited to say I've now switched to MegaSquirt and that does not use a speed sensor input but it all works just the same, including the stepper so perhaps the speed sensor is not needed.

Think the problem with the hotwire is the speed of the processor - it may not be able to process idle speed commands fast enough for a true closed loop idle system to work properly. But only a guess.

When I fitted a hotwire system to my first cobra the car would often cut out when pulling up at junctions, it had no speed sensor. A friend drew me a basic wiring diagram (which I no longer have, sorry) whereby I used a zenner diode from the standard electronic speedo output (LT77) to fool the rover ecu into thinking a speed input was there. Hope that makes sense??

Cobratone wrote:A friend drew me a basic wiring diagram (which I no longer have, sorry) whereby I used a zenner diode from the standard electronic speedo output (LT77) to fool the rover ecu into thinking a speed input was there. Hope that makes sense??

Thanks Cobratune, I too have been thinking along the same line. 12v feed into the zenner, then zenner output to the road speed sensor wire. Ultimately this is just providing a constant clipped/protected +12v to the ECU making the 'brain' think it's at constant 3-5mph 'kick-in'. The only drawback I see with this is that the ECU will then be permanently running the idle bypass, as the standard sensor set up is designed to run a switched [:. pulsed] signal. The TVR set up is different to the RR one, it uses a speedo calibration unit transducer that supplies a fixed speed signal over 3mph, and I'm certain fitting a simple zenner will do the same job. I won't be running the engine until March time so a little way to go yet...will use the zenner option first during shake down; fingers crossed.

jenand40 wrote: ↑

Thu Jan 08, 2015 8:30 am

Hi Andrew;
I replaced my Flapper system with a Hotwire system last winter.
The speed sensor (speedo driven) output is fed into pin 6 of the ECU. Have looked at the circuit diagram and the 6.8k resistor feeds a 'Speed trip module' which is something different to the road speed sensor.
When I made up my new wiring loom I only used the minimum sensors I could get away with.
If you would like a copy of the circuit diagram, or any other help, then PM me.
Andy.

G'day Andy not real sure on how to PM you I would appreciate a pic of your loom diagram if that's possible I have a page on FaceBook called modified KOMBI & VW projects if you could post it on there for me TIA ernie RoverV8bay.

Hi roverv8bay,
Happy to help, if I can. If you send me a PM with your email address I will forward the wiring diagram to you, I don’t do Facebook.
To PM me, look in my profile (jenand40) and click on “Send private message”.
Are you interested in just the speed sensor circuit or the complete wiring diagram?
Andy.