Trumpy Posted November 29, 2022 Report Share Posted November 29, 2022 My speedometer seems to sit on 0 until I get to about 10 - 12 mph and then it springs into life . (Tr5) This doesn’t bother me greatly but I’m concerned it will fail its mot . I’m not sure which end to look at first, any ideas anyone? Thanks . Dave Quote Link to post Share on other sites
Lebro Posted November 29, 2022 Report Share Posted November 29, 2022 Has to be a problem in the speedo itself. Bob Quote Link to post Share on other sites
monty Posted November 29, 2022 Report Share Posted November 29, 2022 2 hours ago, Trumpy said: My speedometer seems to sit on 0 until I get to about 10 - 12 mph and then it springs into life . (Tr5) This doesn’t bother me greatly but I’m concerned it will fail its mot . I’m not sure which end to look at first, any ideas anyone? Thanks . Dave Don’t think they test it on a MOT. Quote Link to post Share on other sites
keith1948 Posted November 29, 2022 Report Share Posted November 29, 2022 Hello Dave Not part of MOT test. Sometimes had similar problems with my speedometer. Making sure the cable was aligned in a smooth curve to back of speedo helped. Make sure there are no tight bends in the cable anywhere. It could also be the unit itself as Bob has suggested. Some old grease maybe found its way inside over the years. You could unscrew the cable from the back of the units and pull out the inner part and clean that up. I then wipe it with a cloth with some thin oil just to smear the surface of the inner cable. Does it read correctly at 30 and 60mph? Keith Quote Link to post Share on other sites
Trumpy Posted November 29, 2022 Author Report Share Posted November 29, 2022 Thanks for the replies, I assumed the speedo would need to work for the mot but if not that’s good news . it seems to work correctly (or at least consistently) when registering, it has always read a little under the actual speed which is something to be wary of ! I’ll have a look at the cable connection and maybe a look inside although it’s maybe a bit specialist for the actual speedometer . Quote Link to post Share on other sites
HSM Posted November 30, 2022 Report Share Posted November 30, 2022 Speedometer working is now part of the MOT test DVSA AE Harvey Quote Link to post Share on other sites
Lebro Posted November 30, 2022 Report Share Posted November 30, 2022 I have never seen a speedo test on any of my cars ? Bob Quote Link to post Share on other sites
Harbottle Posted November 30, 2022 Report Share Posted November 30, 2022 me neither Quote Link to post Share on other sites
Trumpy Posted November 30, 2022 Author Report Share Posted November 30, 2022 Well , it’s booked in tomorrow so I'll soon find out . Quote Link to post Share on other sites
RogerH Posted November 30, 2022 Report Share Posted November 30, 2022 Hi Dave, the drive from GB to speedo is mechanical. If one end turns then so will the other. (usually) Thus I think the fault is in the speedo itself. The cable spins an Ali drum/magnet and this reacts against a fairly weak spring. If the needle is a little sticky then the drum/magnet may need to get up to speed to apply force to break the sticktion. Cleaning speedo's can be awkward. As you believe the unit needs re-calibrating then get it all done at the same time. You can use your SatNav to give a speed read-out Roger Quote Link to post Share on other sites
Ralph Whitaker Posted November 30, 2022 Report Share Posted November 30, 2022 (edited) I presume the only time they can check it is when it is on the rolling road for the brake test, but they only turn the wheels at about walking speed so I don`t know how they can tell. Some speedos have a rubber covered stop which can rot and go sticky with age and the needle sticks until there is enough force to unstick it when it will suddenly spring up, anyway I would say it is working, at 10 mph onwards. If they dispute it take the tester for a run round the block. Ralph Edited November 30, 2022 by Ralph Whitaker Quote Link to post Share on other sites
HSM Posted November 30, 2022 Report Share Posted November 30, 2022 The class 4 test manual states that a speedometer must be fitted to vehicles from 1st Oct. 1937 that have a speed over 25 mph. Section 7.8 defect (b)ii states a none functioning speedometer is a major failure, but if the vehicle is not road tested ( ie decelerometer test ) you should only reject the speedometer if it is clearly not working. Harvey Quote Link to post Share on other sites
Trumpy Posted November 30, 2022 Author Report Share Posted November 30, 2022 I just gave the mot station a call to check. I told him the issue and he said ‘don’t worry we won’t check the speedometer anyway ‘ He didn’t specify if that was because it’s a classic and the mot is optional tho’ Quote Link to post Share on other sites
Trumpy Posted November 30, 2022 Author Report Share Posted November 30, 2022 2 hours ago, RogerH said: If the needle is a little sticky then the drum/magnet may need to get up to speed to apply force to break the sticktion. Now that’s a great new technical term ! And sounds a likely cause as Ralph diagnosed as well . I think I’ll leave it be , unless it gets any worse . Thanks folks . Quote Link to post Share on other sites
ianc Posted November 30, 2022 Report Share Posted November 30, 2022 No letter K in stiction, a term which has been around for ever. If one has a limited slip differential, a test of the rear brakes can only be made by driving on the road and using a decelerometer. The test cannot on a rolling road where each wheel is tested separately - this could damage the LSD. I carry a note to this effect, but John, who has tested my car every year for nigh on 20 years, doesn't bother because he can see that all is operating underneath and that the handbrake holds the car securely on the steep slope at the garage. And he takes the odometer reading from the speedometer, so it must be working (even though the speed reading wobbles!). Ian Cornish Quote Link to post Share on other sites
HSM Posted November 30, 2022 Report Share Posted November 30, 2022 Quite correct Ian a cars fitted with L.S.Ds. are always checked using a decelerometer & the results entered onto the system, if not the test result will show brake effort not recorded & a failure will be issued. Harvey Quote Link to post Share on other sites
ianc Posted December 1, 2022 Report Share Posted December 1, 2022 I can't say what John enters on the MOT form for 4VC, but he never tests the rear brakes (neither on rolling road nor using decelerometer on the highway), and the car has never failed the test. I feel sure that had he any doubts, we would go out to the road and give it a go! Ian Cornish Quote Link to post Share on other sites
Trumpy Posted December 2, 2022 Author Report Share Posted December 2, 2022 Update - Car passed with a clean sheet with no advisories so very happy with that . . Quote Link to post Share on other sites
Andy Moltu Posted December 2, 2022 Report Share Posted December 2, 2022 Does your speedo read accurately beyond this? if it under reads you could take the glass off, push the stop pin back and see where the pin actually stops. It should be over the white dot. If not replace the indicator off carefully lever it off(protect the dial wth card or tissue paper) and put back over the dot or slightly after. Move past the stop pin and rod test against gps. If it doesn’t work you can always get it rebuilt. Quote Link to post Share on other sites
Trumpy Posted December 3, 2022 Author Report Share Posted December 3, 2022 Thanks, if it gets any worse I’ll have to have a look . It reads about 2 mph under consistently , (always has while I’ve owned it) . Just have to remember where there’s a speed camera! D Quote Link to post Share on other sites
Andy Moltu Posted December 4, 2022 Report Share Posted December 4, 2022 That’s interesting. Speedometers are supposed to overread slightly and the TR ones tend not to move off the pin until 5mph so that might fit with your speedo needle being 5 mph or so out. Quote Link to post Share on other sites
Charlie D Posted December 4, 2022 Report Share Posted December 4, 2022 The first Triumph Herald based kit car I built had to be taken out to be tested with a Tapley decelerometer for some reason. I remember that the tester was very short and his feet could not reach the pedals, so I had to do the driving When we had done it he told me that the brake test had failed but he then said that Triumph Herald brakes were never very good anyway, so he passed it! Different times. Charlie. Quote Link to post Share on other sites
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