All of a dither

[streaky]

My speedo that is.

Took at trip to Swindon today (A33/M4), the speedo fluctuates wildly at speeds up to 30 (swings from 0 to 60), settles down at 30 - 40, then swings between 40 and 80 at speeds up to 70 .

I guess it's mechanical (I haven't had the time yet to go looking).

Is this (a) a common problem, and (b) easily fixable?

Thx in advance,
Streaky

[wedg1e]

(a) Yes!
(b) A chap from TVR told me that he used to pull the cable around and tiewrap it to a chassis rail to fix the prob...

[NHyde]

Voice of experience , as it happened to me last week .

Next thing that happens , speedo cable snaps !! The broken end is now in the ashtray , and the relacement top half ( Junction box to gauge ) sent by Peninsula has been lost by the couriers !!

[noahstvr]

Ditto NHyde, mine did the exact same thing last week. On my 3000s it is a tvr special cable, however I needed to replace tne case of the cable as well as the cable. If the inner part of the cable broke a local speedo shop should be able to replace it for you. Good Luck.

[NHyde]

OI Noah , is it you that hijacked my cable then ?

[streaky]

Now the ambient temperature has cooled somewhat and my Wedge is in the shade, I've looked at the speedo cable ... and I'm puzzled.

There is a Bowden cable from the gearbox to a metal cube that is resting (loose) on top of the OS cam cover. Another Bowden cable comes out of the cube in line with the input cable and enters the speedo. In addition, two wires also come out of the cube and disappear through a grommet on the NS bulkhead.

There is no mention in the 'bible' of this arrangement.

Ideas on its purpose are welcomed, and on the purpose and destination/origin of the two wires.

I'd post a picture, but SWMBO has 'mislaid' the charger for my camera .

[cbemoore]

That'll probably be a T-piece for a trip meter. I used to have one on my rally car.

Or maybe your car was once a taxi??

[streaky]

cbemoore said:

That'll probably be a T-piece for a trip meter. I used to have one on my rally car. Or maybe your car was once a taxi??

I never thought of the possibility that it was once a taxi. And the Haldas (sp?) I had in my rallying days were all mechanical. If I can find enough of the day when it's cool (ha, ha - I live 27 miles from Heathrow where it topped 100F yesterday!) I'll investigate the wires.

Streaky

[cbemoore]

Just looked at your profile, and noticed you've got an ex-factory test car. I suppose they might have added an extra electronic trip meter or speedo while they were testing???

[terence]

Bit of an aside but Auto Range Rovers have a junction box in the speedo cable with two wires exiting. In this guise it provides a speed signal for the transmission so that it can lock the torque converter at about 55 mph in 4th. That maybe the source of your box and as said earlier may have been used to provide an electronic speed signal to some other unit.

[streaky]

Well, I spoke to Jason (CATS in Farnham, done good work for me in the past - usual disclaimers) who fitted the alarm/immobiliser and he said that the two wires plug into a TVR style socket in the wiring loom. He has never seen one before (he used to be an auto-electician with a TVR garage apparently, and knows TVR electrics - and, BTW, he's never seen a 3X7 fusebox on a 390SE) and has no idea where it goes or what it feeds (or fed). So, the mystery remains. Maybe there is some memory in the factory - Streaky

>> Edited by streaky on Tuesday 19th August 17:22

[streaky]

terence said:

Bit of an aside but Auto Range Rovers have a junction box in the speedo cable with two wires exiting. In this guise it provides a speed signal for the transmission so that it can lock the torque converter at about 55 mph in 4th. That maybe the source of your box and as said earlier may have been used to provide an electronic speed signal to some other unit.

Well, the Range Rover Haynes manual refers to a transducer in the speedo 'train' which 'will' affect engine idle speed ... so it must be connected to the ECU (mustn't it?). Can't see how it could affect idle speed when Wedge is stationary though ... can you?

See also : www.pistonheads.com/gassing/topic.asp?h=0&f=12&t=58168

>> Edited by streaky on Friday 19th September 16:22

[terence]

Hmm interesting, I have a Range Rover as well as the wedge so if I have time this weekend, though if the sun shines I will probably be out in the wedge, I will have a look. I am supprised that the manual says the speedo signal will affect idle. I always assumed the sensor was just for the auto trans. The plot thickens.

[streaky]

terence said:

Hmm interesting, I have a Range Rover as well as the wedge so if I have time this weekend, though if the sun shines I will probably be out in the wedge, I will have a look. I am supprised that the manual says the speedo signal will affect idle. I always assumed the sensor was just for the auto trans. The plot thickens.

Haynes Manual s13.33 p 29 "A fualty speed transducer would affect the idle speed of the engine." - with acknowledgement to Haynes Publishing.

What I want to know is what the ECU does with the heated front screen input?

But what I REALLY want to know is - what is the effect of altering the value of the 'tuning resistor'?

Streaky

[NHyde]

Streaky ,
Mrs H's 400 has exactly the same arrangement . The two wires are not connected to anything , nor does it seem to have any effect on the idle speed . can take a picture of it if you wish to compare . It is a hotwire engine as well

Just had a look at the car , and it has been in to Peninsula ( and with Mark Adams ) since I last looked at the speedo cable , but it is now connected , but as far as I can tell , no difference .

>> Edited by NHyde on Friday 19th September 21:48

[danny hoffman]

Maybe the idle speed is increased to compensate for the extra current draw when the heated screen is on?