LT77 Speedometer, 1993 Chimaera 400, CAT

[LLantrisant]

my speedometer shows around 11Miles more than my GPS

i have an early Chimaera 400, built 1993, its a pre-serpentine with catalytic converter and LT77 box.

on propshaft-side of the diff is a toothed-ring... so i have 2 speed sensors...one in the box, one at the diff.


wheels / tyres are original spec, the important rear ones 225/50 R16

now my questions:

1. which sensor drvies the speedo
2. can the speedo be calibrated (e.g. with dip-switches)?
3. why such a big difference between speedo and GPS?

Edited by LLantrisant on Tuesday 5th March 11:54

[Penelope Stopit]

If you take a look at this http://continuouswave.com/ubb/Forum6/HTML/002179.h... and also take into account that your cars speedo could be something like + or - 5%? (this is a percentage I've worked to in the past) I guess this accounts for much of the discrepancy but I could be wrong

[Rowley-Birkin]

Hi,

This is the same as my 1993 Chim. Sensor on the nose of the diff does the speedo, the sensor on the side of the gearbox tells the ecu you arr moving above 4mph or something like that.

Jase.

[Jon100p]

My chim is an early 93

The original rear tyres on the early Chims should be 225/55/16, as the original handbook shows, so that could be part of your problem


Also as far as I know there is no sensor on the GKN diff and the rover gearbox sensor is used for both speedo and ECU on early cars. Has the diff been changed at some point? The original diff ratio is also shown in the handbook if you want to check yours.

[LLantrisant]

to be honest...i just have seen the toothed disc on propshaft side of diff and thought "than there must be also a sensor"....but physically i have not checked if there is a sensor.

whats new to me is the tyre size...i thought 50ies is the original height...searchign around the www it seems that there is no clear answer for that..lots of owners report 50, others 55.....but my 50ies would make the speedo show 4% more....which still would not explain the big difference.

[wuckfitracing]

Nope there is a sensor on the early cars on the GKN diffs. Well there was last time I replaced my sensor. 93 430. Yes mine also reads a different speed to my sat nav.

[wuckfitracing]

From memory about 10 mph faster than Im actually doing.

[blitzracing]

Drop the speedo out and have a look at the back, there is normally a small hole (it may be covered with a calibration void if removed sticker) that give access to a variable resistor you can tweak to get the reading correct against the GPS.

[FoxTVR430]

Agree with above, there is a sensor on the prop-shaft and the gear box.
Thanks for the information blitzracing. :thumb:

Photos attached during my chassis strip and rebuild .

[Loubaruch]

wuckfitracing said:

From memory about 10 mph faster than Im actually doing.

I thought all TVRs did that.


My 1996 Griffith 500 has always read 10 mph fast at 70!

[Loubaruch]

wuckfitracing said:

From memory about 10 mph faster than Im actually doing.

I thought all TVRs did that.


My 1996 Griffith 500 has always read 10 mph fast at 70!

[QBee]

My later Chimaera (99) is absolutely spot on.
All German cars I have owned have been 3 mph fast at 70.
All Scandanavian cars have been 1 mph fast

[Jon100p]

Well all I can say is my chim definitely does not have a sensor at the diff/propshaft and the speedo is within 2 mph of a gps at 70. TVR must have changed them after March 93 to add the extra sensor and make accuracy worse!

[Rowley-Birkin]

Early cars had a sensor on the diff for the speedo and a sensor in the LT77 Gearbox to tell the ECU that you were actually moving.

Cars with the T5 Gearbox had a sensor on the diff only and a box which split the signal, one to the speedo and another to the ECU.

It was sometime in/after 1994 whenthey switched over to the T5 I believe.

Jase.