Speedo calibration unit or sensor?
Discussion
Rovergauge tells me I am doing 60km/h while stationary in my garage. The engine is kept idling at around 2000 rpm until eventually the speed signal drops to 0.
Typical issue for a TVR, hu?
Now, I don't think the sensor is faulty: it is a passive inductive device and will not generate a current on its own. So it can well be the infamous calibration unit.
When the unit is on its way, what's the current solution? Is there any replacement possibility?
Thanks for the advice.
Philippe
Typical issue for a TVR, hu?
Now, I don't think the sensor is faulty: it is a passive inductive device and will not generate a current on its own. So it can well be the infamous calibration unit.
When the unit is on its way, what's the current solution? Is there any replacement possibility?
Thanks for the advice.
Philippe
You sound like you know some/plenty about electronics. I only know a little but have seen issues with this box in the past.
Perhaps Blitz will be along shortly to give more detail but you could possibly be having an earth problem. The fix was to put a diode in the earth wire inside the box. This produces a small resistance so that that earth is at a different potential (or something) to another earth in the circuit.
All rocket science to me but it has worked on a couple of units I've done the mod too.
Steve
Perhaps Blitz will be along shortly to give more detail but you could possibly be having an earth problem. The fix was to put a diode in the earth wire inside the box. This produces a small resistance so that that earth is at a different potential (or something) to another earth in the circuit.
All rocket science to me but it has worked on a couple of units I've done the mod too.
Steve
The speedo calibration box you speak of is actually not a speedo box. The speedo reads a signal at approximately 40000 pulses per mile due to the number of teeth on the wheel (around 60 if I remember correctly). The Lucas 14CUX however needs something like 6000 pulses per mile to operate, so rather than adapt the signal, TVR chose to implement the little black box. Basically it takes in the speedo signal and fires it out unfettled to the speedo, it also uses this signal to activate a circuit which generates a fixed pulse rate which goes to the ECU to tell it that the car is moving (nothing more than this) - this is why your Rovergauge says you're doing 60 - it actually should only do this when you're moving so theres something wrong there.
On my car I've removed the box and built myself a little circuit which divides the signal so that the ECU can see how fast the car is going, though I've halved the speed as my GEMS ECU has an inbuilt speed limiter which I never want to hit.
You could do something similar, or you could fix the box you have, the 14CUX doesn't really do anything with the speed value other than knowing the car is moving though so you wouldn't gain much.
On my car I've removed the box and built myself a little circuit which divides the signal so that the ECU can see how fast the car is going, though I've halved the speed as my GEMS ECU has an inbuilt speed limiter which I never want to hit.
You could do something similar, or you could fix the box you have, the 14CUX doesn't really do anything with the speed value other than knowing the car is moving though so you wouldn't gain much.
The issue of the false signal is down to the tiny signals the transducer produces combined with electrical noise on the earth plane- basically one side of the input of the speedo unit is grounded, and the other takes the signal from the transducer. I think the problem arises when there is electrical noise or bad connections on the input of the box that gets picked up as a transducer signal, so it generates a false output. There is a fix that involves putting a couple of resistors on the input to clamp the voltages to an extent, but the first thing to remove the speedo box from under the dash, and check the plug connectors and solder connections to the plug on the board, as an open circuit here will generate a false signal. Like wise a broken wire to the sensor would do the same thing, but you would loose the speedo reading as well if this was the case.
Edited by blitzracing on Monday 3rd July 13:31
See the big solder joints in the centre of the board that connect to the plug- re solder these and make sure the joint flows properly as Ive seen dry joints under the solder that are not visible from the outside. It wont be that easy to test unless you have an oscilloscope and circuit diagram- but the basic operation is there is a differential op amp on the input, and the output remains low as long as there is no traducer signal, but once there is a signal the output goes high and triggers a circuit to produce a string of pulses that looks like a speed signal. I got a second hand unit to experiment with from the TVR breakers "Bell", and it cost me about £40.
e635815 said:
Ok, I did resolder the 6 legs but without any success....
Still showing-up 60 km/h on Rovergauge.
And I have called around my area and just can't find any shop willing to repair electronic circuits.
Grrrrr......
Re your high idle problem and fixed speedo reading:Still showing-up 60 km/h on Rovergauge.
And I have called around my area and just can't find any shop willing to repair electronic circuits.
Grrrrr......
To be in Idle Mode the road speed must be 3-4 MPH or lower and the throttle pot must be below a threshold of typically 6%. Your symptoms would seem to confirm that the TVR road speed signal module is either permanently stuck at 37.2 mph (60 kph) and not sending the correct road speed signal to the ECU on pin 6; or the ECU's road speed sensing process has failed and you are always getting the last sampled reading but I doubt this.
A 'normal' road speed signal from the TVR module looks like this in RoverGauge (note the 37.2 mph ceiling and the sharp drops to zero):
The signal is simply a Car Moving/Car Stopped indicator, if the ECU fails to receive the part of the signal where it passes 3-4 MPH going towards Car Stopped it will not enter Idle Mode, hence your high idle.
If the output is permanently fixed at 60 kph I would suspect the Op Amp and /or associated Pump control circuit has gone U/S and the Pump is always On. From your PCB pic it looks like you have a Powers Performance version, contact http://www.powersperformance.co.uk/ to verify and perhaps source a replacement.
Ok, I am on to two different fronts here... And maybe a third one.
Called Bell-Hill garage and they might have a replacement unit.
Caller TVR Powers Performance and they might also have a replacement
In touch with 450Nick in order to create a speed divider box.
One of them will hopefully prove to be successful.
Called Bell-Hill garage and they might have a replacement unit.
Caller TVR Powers Performance and they might also have a replacement
In touch with 450Nick in order to create a speed divider box.
One of them will hopefully prove to be successful.
And another solution is opening-up to me. There is a broken Chimaera somewhere in France and the brakeyard is selling the parts away.
Engine and gearbox are gone already. Remain the seats and some electronics.
Expecting a price for the Tacho Interface Unit.
And all of the electonics are ordered on ebay for the speed divider circuit....
Working hard on the resolution!!!
Engine and gearbox are gone already. Remain the seats and some electronics.
Expecting a price for the Tacho Interface Unit.
And all of the electonics are ordered on ebay for the speed divider circuit....
Working hard on the resolution!!!
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