Early Chim, LT77 speed transducer and ecu issue
Discussion
Hoping someone with in depth early chimaera knowledge can help me here.
I've got a chimaera in, early car, LT77 gearbox.
It has a speed transducer driven from the gearbox output shaft which sends a speed signal to the ecu at pin 6. (note I am NOT talking about speedometer and the diff sensor here)
Using the 14cux manual the test proceedure is to unplug the ecu and put a voltmeter across from pin 6 in the ecu plug to earth and turn a road wheel, the voltage at the voltmeter should vary from 12volts to zero.
Mine reads constantly 12 volts. It did this on the previous sensor, and on the brand new sensor. No varying of volts as you turn the wheel in the slightest.
Testing the wiring shows a correct 12 volts on the feed wire, and continuity back to pin 6 on the signal wire.
However .. on testing with the transducer out of the circuit I'm getting about 0.14volts at pin6, and if I apply a load eg a test bulb where the transducer plugs in, then remove it, instead of going instantly to zero volts as I think it should , the voltage floats down very very slowly over a period of many seconds, so there's something feeding pin6 other than the signal wire from the transducer.
Trouble is, I don't know if there's supposed to be anything else connected to ecu pin 6. The speedometer shouldn't be connected because it's signal is from the diff sensor and a completely separate circuit.
Does anyone know what connects to ecu pin 6 other than the LT77 transducer?
The simplest way to get rid of the floating voltage at pin 6 is to cut the wire and then wire pin 6 direct to the signal wire from the transducer, but if there's a splice somewhere in the loom to feed pin 6 with another signal then obviously cutting the wire stops that signal reaching the ecu.
anyone got any ideas?
I've got a chimaera in, early car, LT77 gearbox.
It has a speed transducer driven from the gearbox output shaft which sends a speed signal to the ecu at pin 6. (note I am NOT talking about speedometer and the diff sensor here)
Using the 14cux manual the test proceedure is to unplug the ecu and put a voltmeter across from pin 6 in the ecu plug to earth and turn a road wheel, the voltage at the voltmeter should vary from 12volts to zero.
Mine reads constantly 12 volts. It did this on the previous sensor, and on the brand new sensor. No varying of volts as you turn the wheel in the slightest.
Testing the wiring shows a correct 12 volts on the feed wire, and continuity back to pin 6 on the signal wire.
However .. on testing with the transducer out of the circuit I'm getting about 0.14volts at pin6, and if I apply a load eg a test bulb where the transducer plugs in, then remove it, instead of going instantly to zero volts as I think it should , the voltage floats down very very slowly over a period of many seconds, so there's something feeding pin6 other than the signal wire from the transducer.
Trouble is, I don't know if there's supposed to be anything else connected to ecu pin 6. The speedometer shouldn't be connected because it's signal is from the diff sensor and a completely separate circuit.
Does anyone know what connects to ecu pin 6 other than the LT77 transducer?
The simplest way to get rid of the floating voltage at pin 6 is to cut the wire and then wire pin 6 direct to the signal wire from the transducer, but if there's a splice somewhere in the loom to feed pin 6 with another signal then obviously cutting the wire stops that signal reaching the ecu.
anyone got any ideas?
Anything’s possible with Tvr wiring!
I can’t help much other than if it were me id try to check the loom to see if anyone had tampered with it. If it runs under the centre console it’s quite easy to access.
If it were my car I’d probably cut the pin 6 wire close to ecu and do some tests again, maybe running a temp signal back from sensor.
I can’t help much other than if it were me id try to check the loom to see if anyone had tampered with it. If it runs under the centre console it’s quite easy to access.
If it were my car I’d probably cut the pin 6 wire close to ecu and do some tests again, maybe running a temp signal back from sensor.
Hi thanks, and that's exactly what I did in the end and guess what, still no bl**dy speed signal at the ecu!
I put it back to standard and put the car back together and did a few test drives to adjust the base idle and fitted a stepper motor pipe restrictor so I could at least give the car back to the customer in a not-stalling-at-junctions state.
Guess what .. on the third test drive the speed signal back to the ecu mysteriously started working again for no apparent reason.
Had to admit defeat (I hate it when that happens!) and gave the car back to the customer with no charge on a "see how it goes" basis. But it looks once again like a dodgy transducer .. maybe a bad batch ?
I'm still not 100 percent convinced with what i found on the tvr wiring, but even with the transducer off the car and tested on the bench there was absolutely not signal at all so that takes any unusual wiring issues out of the equation anyway.
I put it back to standard and put the car back together and did a few test drives to adjust the base idle and fitted a stepper motor pipe restrictor so I could at least give the car back to the customer in a not-stalling-at-junctions state.
Guess what .. on the third test drive the speed signal back to the ecu mysteriously started working again for no apparent reason.
Had to admit defeat (I hate it when that happens!) and gave the car back to the customer with no charge on a "see how it goes" basis. But it looks once again like a dodgy transducer .. maybe a bad batch ?
I'm still not 100 percent convinced with what i found on the tvr wiring, but even with the transducer off the car and tested on the bench there was absolutely not signal at all so that takes any unusual wiring issues out of the equation anyway.
spitfire4v8 said:
......However .. on testing with the transducer out of the circuit I'm getting about 0.14volts at pin6, and if I apply a load eg a test bulb where the transducer plugs in, then remove it, instead of going instantly to zero volts as I think it should , the voltage floats down very very slowly over a period of many seconds, so there's something feeding pin6 other than the signal wire from the transducer......
Who was it the other day who had a capacitor on a signal wire. That would give you the slow drop in volts.Steve
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