Freaky speedo and running rich - advice please!!
Discussion
Afternoon all. I wonder if anyone can offer some advice on a speedo issue I'm trying to sort. My 98 450 was left under a storm cover for a few months over winter and not driven. When I eventually took it out I noticed, among other minor gremlins, that the speedo wasn't working - not intermittently or erratically - it was completely dead. I thought maybe dodgy earth or damp connector somewhere and then weirdly the next time I drove it it seemed to have completely fixed itself and was behaving completely normally. Sadly not - ever since then it's been completely erratic and getting worse and worse. The general pattern is nothing below 20mph (although I seem to recall it never read anything at low speeds), then swings between not working at all (needle stuck at zero even when barrelling along at 60) to working fine. It tends to work for a few seconds then drops off again, or the needle swings about wildly. Other odd symptoms:
- when I lift off or change gear the needle drops to zero and when I engage new gear/floor it it pops back up again
- when the needle is at zero and not moving I've found thumping the top of the dash brings it back to life, if only for a short while
Some additional info: since the speedo has been playing up I've noticed that it has gradually started to run rich with lots of pops from the exhaust and tell-tale sooty marks up the rear above the exhausts. In fact, it failed its MOT on emissions. The MOT chap reckoned it was a failed/failing lambda sensor. I took it to a local TVR specialist who diagnosed the same issue, changed the failing sensor and got it through its MOT. Whilst he had it I asked him to look at the speedo. He adjusted the transducer and reckoned it had helped but not completely solved it. To be honest it seemed to be worse when I picked it up so ordered a new transducer which I've fitted this morning. Just taken it out for a spin and doesn't really seem to have helped - it's slightly more stable but the speedo still moving about erratically and banging the top of the dash still seems to be having an effect when it drops off completely. It also still seems to be running pretty rich.
I'm going to try and adjust the transducer and will get the dash off again (only just put the dash back together after replacing alarm and immobiliser! grrrrrr) to see if there's a dodgy connection back there somewhere.
Anyway, it's all very odd and just wondered if anyone had any ideas what the issue might be??
Cheers
- when I lift off or change gear the needle drops to zero and when I engage new gear/floor it it pops back up again
- when the needle is at zero and not moving I've found thumping the top of the dash brings it back to life, if only for a short while
Some additional info: since the speedo has been playing up I've noticed that it has gradually started to run rich with lots of pops from the exhaust and tell-tale sooty marks up the rear above the exhausts. In fact, it failed its MOT on emissions. The MOT chap reckoned it was a failed/failing lambda sensor. I took it to a local TVR specialist who diagnosed the same issue, changed the failing sensor and got it through its MOT. Whilst he had it I asked him to look at the speedo. He adjusted the transducer and reckoned it had helped but not completely solved it. To be honest it seemed to be worse when I picked it up so ordered a new transducer which I've fitted this morning. Just taken it out for a spin and doesn't really seem to have helped - it's slightly more stable but the speedo still moving about erratically and banging the top of the dash still seems to be having an effect when it drops off completely. It also still seems to be running pretty rich.
I'm going to try and adjust the transducer and will get the dash off again (only just put the dash back together after replacing alarm and immobiliser! grrrrrr) to see if there's a dodgy connection back there somewhere.
Anyway, it's all very odd and just wondered if anyone had any ideas what the issue might be??
Cheers
May be worth checking speed calibration box. I believe it receives a speed signal and splits it to the Speedo and the ECU.
It's apparently known to cause problems, worth a search before you spend money on other bits.
https://www.pistonheads.com/gassing/topic.asp?h=0&...
It's apparently known to cause problems, worth a search before you spend money on other bits.
https://www.pistonheads.com/gassing/topic.asp?h=0&...
Running rich and an erratic speedo are not likely to be connected- the signal from the transducer is tiny, and the signal is feed into the speedo calibration unit, and also the speedo its self- but as parallel inputs- ie both units share the transducer signal, there is no conversion of the signal for a speed reading, just for the ECU speed signal. Its quite possible you have an earthing issue somewhere as its easy to swamp the signal with electrical noise if the earthing is poor.
As to running rich- chicken and egg really- its possible that if it was over fuelling, its sooted the lambda sensors up and they have then failed so they look like the issue. You need to either at the lambda probe outputs directly, or better the long and short term fuel trim values shown in Rovergauge or ECUmate (short term only) to see what the ECU is doing to try to correct the fuelling. Mind you jet black plugs will tell you a lot as well. Most likely culprit is AFM as it gets cooked by the TVR exhaust, or possible a temp sensor, but Id trust by now that the TVR dealer would have the diagnostics for these sensors and ECU by now its no longer black art reading the 14CUX,
As to running rich- chicken and egg really- its possible that if it was over fuelling, its sooted the lambda sensors up and they have then failed so they look like the issue. You need to either at the lambda probe outputs directly, or better the long and short term fuel trim values shown in Rovergauge or ECUmate (short term only) to see what the ECU is doing to try to correct the fuelling. Mind you jet black plugs will tell you a lot as well. Most likely culprit is AFM as it gets cooked by the TVR exhaust, or possible a temp sensor, but Id trust by now that the TVR dealer would have the diagnostics for these sensors and ECU by now its no longer black art reading the 14CUX,
Hi, thanks for this really helpful info. I hadn't appreciated the transducer feeds the signal converter and speedo separately. Yes, does sound like an earth or connection issue, possibly behind the instrument cluster somewhere.
Do you know where the transducer circuit is earthed and how the signal is fed into the back of the speedo? Just trying to map out possible trouble points before I take top of dash off again...
Cheers
Do you know where the transducer circuit is earthed and how the signal is fed into the back of the speedo? Just trying to map out possible trouble points before I take top of dash off again...
Cheers
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