Speedo

[blues3]

Possible I guess, but if I recall aright (and from a long way back mind) it looks like classic diode breakdown.
The 'magnetic' feeling notchy-ness when turning the old one compared to the new has me hopefull of a fix.
Might check resistance to earth through the loom to see if its anything substantial.

[Jed-S]

blues3 said:

Quite right. Got the new replacement yesterday. Turns nice and smooth. Just got to sort out the plug as the old one has to go back on.
The original has Brown, Yellow, Black and the new has Brown, Black/Brown and Yellow/Brown.
Any idea what goes to which?

I think the circuit diagram for the original has :-

Brown - +12v
Black - Earth
Yellow - Signal

Logically I'd wire it up as:

black : brown/black
brown : brown
yellow : yellow/brown

If it wasn't that logical I'd have wired it up wrong and would have mentioned it on my site.

Just a note, when quoting wire colours that have two colours you mention the base colour first and the stripe second.

Edit: Just been out to the car and removed the SA tape from the connector and I did wire it up as I thought. I've now added that not to my website should anyone else need it.

Edited by Jed-S on Friday 24th September 18:48

[Barkychoc]

I guess we could be getting confused here - as I have a sender unit and a speedo but no car loom in between - I've wired it as per this

http://www.tvrsseries.com/drawings/tvrswiringdiagr...

N - Black
B - Brown
Y - Yellow

Regardless I have it wired correctly as I have Voltage on the correct pins of the IC's and the speedo signal going through the first amplifier.

[blues3]

Job done. Change the plug over, chucked it it, reset the ECU, ran up the engine and wow, what a difference!

Here's the shots of the old and new sensors.....







New one on the left.

[Loach1]

I have a speedo problem, but I'm not sure if it is the gauge or the sender. It barely goes over 20mph and fluctuates quite a bit. So far I have cleaned and checked the connector behind the dash, and opened up the speedo for a better look at the circuitry, which looked to be in great shape considering the age.

The next logical step is to check the sender, but I can't see an easy way to get it from below - is it possible to get to it from above? Is the connector above or below? I'd like to clean it up if possible.

[phillpot]

from memory the connector is accessible from above once centre console, gaiter etc. out of the way?

[Loach1]

Thanks for the pic Mike. I was looking on the wrong side after seeing a pic in the bible.

After wrestling the exhaust off I found the connector was very oily, which was promising news. I pulled the sensor out, removed the gear and spun it with a drill. It managed 60mph on full speed, which was good because I've never seen it go over 20 before. After putting it together I took it for a drive and speedo did precisely nothing. Very frustrated, I parked it and went for a cup of tea. Then I remembered that I hadn't put the ;:/-))&@ gear back on the sender. So, wrestle with the exhaust again, put it together, and it's the same as it always was!

Now I'm stumped. I don't know if it's the sender or gauge, but one of them has issues. I've checked the ground back to the battery and that had very low resistance, and the 12v power is there when the engine is running. There are clever boxes that can modify the signal frequency to adapt problematic signals, but that is a last resort. Does anyone know what the potentiometer on the back of the speedo is for? Is it to zero, or is it gain? I'll have a fiddle tomorrow but when I messed with it before it didn't seem to do anything much.

[Loach1]

Sorry to ramble, but there isn't much info out there on this subject, so I want to add my notes....

The potentiometer appears to do absolutely nothing to the reading.

My drill spins at 1300rpm, and the rotor in the VSS has 8 dogs, so that should have given a 173Hz signal to the speedo, which gave roughly 60mph. Looking around the web, the Scorpio VSS produces 4000 pulses per mile, and a guy testing his Scorpio speedo used an AC adapter at 60Hz (USA mains) to show 27mph. Assuming the TVR VSS is roughly the same pulses per mile (give or take), I'm off by about 50%, which makes me suspect the VSS is the problem.

I'm going to see if I have an old 9-12VAC adapter that I can test the speedo with, but I'm not sure if 60Hz=27mph on our speedos.

My needle does go to zero with ignition on, so I know both coils of the motor are working. The odometer is way off - it registered 0.4 miles on a 1.3 mile drive. Since both the speedo and odometer are under reading, it makes me think it must be the VSS.

[Loach1]

I didn't have any luck with the AC adapter test - it made the ECU make a buzzing noise so I quickly unplugged it.. No harm done though, everything is just as broken as before.

I broke down and ordered a new sensor for £40, which seems reasonable. It's the same as the Transit part, so I bought it from http://www.ebay.co.uk/itm/BRAND-NEW-SPEED-SPEEDO-S...

It'll be a couple of weeks before I try it because it's being hand delivered by my father when he visits.

[glenrobbo]

Good luck with it David.
Fingers crossed it works for you.

[Loach1]

glenrobbo said:

Good luck with it David.
Fingers crossed it works for you.

Glen, your crossed fingers did the trick! I put the new sender in today and the speedo works perfectly. I managed to replace it by just loosening the front exhaust clamps and lowering the rear of the exhaust, which saved a bunch of wrestling. No more excuses if I get nabbed by NYPD.