Tuscan Mk1 speedometer problem - advice please!

[RMS5]

Hi, my Mk1 Tuscan (2002) has developed a problem (or possibly always had it and I've never noticed it before) with the speedometer dial. When I switch on the ignition, the needle resets to a position about 4 mph below zero (though this can vary between -2 and -8 mph). The reading on the dial remains 4 mph below the car's actual speed, at least up to 70 mph, compared to the digital read-out (which I believe is accurate). I've tried reaching round the back of the pod and turning the speedo motor's worm gear - with the ignition off! - so that the needle points to zero or to different positions above and below zero, but on turning the ignition back on the needle usually returns to -4 mph.

I assume this is a problem with the mechanism that detects the position of the needle (I gather that the needle is supposed to drop below 0 mph and reset to zero when the ignition is switched on). The question is: should I be intrepid (aka foolhardy) and attempt to take the PCB assembly out and look for loose connections/damaged microswitches/missing screws (with all the attendant risks of causing considerably more damage), or should I be 'sensible' and let the local TVR garage do it?

- Mike

[RMS5]

stevieeg said:

The fact that the digital speedo still seems sensible and is divergent from the analog suggests it's in the pod.

The fact that the divergence varies doesn't rule out a problem with the worm drive or stepper motor. Have you opened the pod to take a look at them?

If it's any consolation it's a problem I would have rather had than the speedo swaying between 180 and 200... and adding 1000 miles to my odometer on a 1 hour trip earlier this year!

Many thanks! Apart from taking off the instrument cover to move the worm gear with a fingertip, I haven't yet dismantled the contents of the pod. I presume I would have to take out the whole PCB assembly to get a closer look at the stepper motor and its connections. I shall have to be in intrepid mode to do that. My experience so far is that every time I disconnect one of the car's multipin connectors, one of the wires breaks off - fixing the connectors for a front indicator and the heater/blower ECU was awkward enough, but rewiring the connector to the PCB assembly would be a nightmare. (I don't think the breakages are a result of me being ham-fisted; I suspect that the wire that the factory used gets brittle with long-term use.)

I agree that the fault in my speedo is preferable to the one you had. My car's mileage is rising fast enough without that sort of help (impressive though it might be to tell your friends that you'd just clocked up a thousand miles in the past hour... )

[RMS5]

nrick said:

There is a microswitch on the speedo to auto zero everytime ( think from memory) I thought it came up with a fault code if it was in the wrong place, may be the lever on the switch is in the wrong place?

The speedo needle does 'kick' when I turn on the ignition, as if it's trying to autozero but is ending up at the wrong position. You may be right about the lever on the switch being in the wrong place - or perhaps the microswitch has come loose and has moved out of alignment. Unless I can get a dentist's mirror into the middle of the pod assembly, I think the only way to find out will be to take the PCB out. Do you know if anyone has posted a diagram of the internal structure of the PCB assembly on the web? I'm sure I saw an 'exploded diagram' some time ago but I can't find it. It would be useful to have a map of the area before starting out, so to speak...

With regard to fault codes, I've been expecting to see either code A (speedo sensor not activating zero switch) or code B (speedo sensor not releasing zero switch), but what usually appears is code L (erratic road speed sensor input), though this is only an occasional appearance.

[RMS5]

330p4 said:

Has the car had a new lcd ? If the speedo needle has been remove it has probably been put back in the wrong position. When I did a friends I turned on the ignition then carefully refitted the needle to read zero with ign on
Ian

Hi Ian, I believe the car had a new (or at least refurbished) LCD just before I bought it, so that may be the root of the problem. I can see that the position of the needle relative to the stepper motor worm gear can be changed by loosening the fixings until the worm gear disengages from the gear on the needle's spindle and realigning them. Is that what you did, or did you have to re-position the needle on its shaft? If the trip for the microswitch is on the same spindle as the needle, presumably it would have to be the latter remedy, otherwise the needle would simply drop back to the same position as before as soon as the ignition was turned off and on again! If you did have to rotate the needle on its shaft, how easy was it to do? From the outside the mechanism looks rather delicate and I don't want to risk breaking the spindle.

[RMS5]

I eventually got the courage to take the electronics/speedometer block out of the pod and found the root of the problem. The grub screw which holds the speedometer needle onto its shaft is supposed to trip a microswitch when the needle is at 0 mph (or possibly just below, triggering the stepper motor to move it up one step). In my speedometer the grub screw was slightly short and wasn't triggering the switch until the needle was 4 mph below where it should have been. This left me with various options: (1) Put up with the speedometer being 4 mph out (not good for my OCD); (2) Turn the grub screw anticlockwise until it triggered the microswitch at the correct point (not good as the needle was no longer held onto the shaft); (3) Find a replacement grub screw of the correct length (not impossible, I suppose, with the help of eBay, but some trial-and-error foreseen); (4) Use brute force to bend the arm on the microswitch until it triggered when the needle went below zero. Guess which I went for...and it worked (at least after the second application of force - the first time I put everything back together, the needle was still 2 mph below the correct reading!)

Many thanks to everyone for their help and suggestions, and a special mention to Paul Smith of PS Electronics for knowing it would be the grub screw just from my telephone description of the problem!