Jul 052016

The joy of finally having the hood completed was short lived. I had hoped for at least a reasonable period of trouble free motoring. Alas, as the speedo had packed up on the way to the trimmers, I now found myself having to take things apart and rather urgently. Its 2nd MOT is overdue.

There are two issues to address; firstly to investigate why the wretched thing has stopped working and secondly to have it re-calibrated. It was reading a mere 56mph whilst following a lead car travelling at 70mph – 20% too low. The suggestion from the E-Type forum was to send it to Speedograph Richfield for the recalibration. So that left just tracing why it had stopped working.

The cable was detached from the back of the speedo in order to see whether the inner cable was rotating whilst driving. It was not. So this pointed to issues at the gearbox end. Most likely a failed angle drive, which is a known weak spot if everything isn’t operating smoothly. I didn’t want to contemplate if that wasn’t the cause. The only other option would be the speedo driven gear, which would require the engine & gearbox to be removed due to the lack of clearance.

Location of angle drive. Unfortunately the trim
had been put back in place since this was taken
A faulty speedo driven gear needs engine &
gearbox removal, as it’s removed sideways

Even so, accessing the angle drive is not easy, let alone getting it off once you have! The transmission tunnel is so close, the gearbox needs to be levered to the left side in order to withdraw the angle drive. The two options are from above, removing the seats, radio & centre consoles and gearbox cover or attack it from underneath.

It was almost possible to get at the angle drive from below with a very small pair of mole grips when the rear was raised on ramps. However the grip’s handle impacted the gearbox mount. At least it confirmed that it should be possible from below.

Exhaust was lowered to gain
access to the angle drive

The plan hatched was to lower the exhaust by undoing the mountings rear of the front downpipes rather than removing this whole section of the exhaust. The gearbox would be supported in order to remove the gearbox mounting.

In my haste I’d completely taken leave of my senses. Even though I had supported the gearbox, I’d forgotten that the supporting spring in the gearbox mounting was still under considerable compression.

I merrily set about undoing all the rear mounting bolts a bit at a time. As soon as the third of the five bolts was removed, the spring suddenly ‘let go’ with an almighty bang, pressing the gearbox mounting against the bodywork on one side and the dropped exhaust on the other.

Fortunately there was no damage but it was quite a shock and I was cursing myself for not reading the manual first! By trying to cut corners and not remove the rear exhaust section, I had created another issue. I now had to find a way of pushing the rear mount back into position in order to remove the last two bolts. However the exhaust was in the way and was now supporting one side of the rear mount that had been pushed down by the spring.

By chance I found a thin block of metal which fitted in the narrow gap between the silencers, enabling the bracket to be jacked back into position. Phew, but it was a nervous time removing the last two bolts. The angle drive was then unscrewed with mole grips and removed, while levering the gearbox over a few millimetres with a short length of 2″x4″.

Supporting the gearbox while
trying to jack the gearbox mount
to compress the support spring
The driven shaft from the gearbox had
been pulled out of the faulty angle drive
Note: round shaft, squared at one end

On initial inspection the angle drive did appear to be broken. The square shaft that engages with the driven gear in the gearbox was detached from the angle drive. Although this could have been caused during removal. This shaft looks a though it is made from a very tightly coiled wire which gives it some flexibility and is squared off at the output end. The internal end is left rounded and is simply held in place by an interference fit and it was this that had failed.

SNG Barratt didn’t have any in stock and wouldn’t for several weeks. So I called Speedograph Richfield to see if they had any available. They didn’t but during this conversation I found out that not all angle drives are equal. The correct one for E-Types has a ratio of 1:1.27. Other makes and some other Jaguar models had a 1:1 ratio. Externally they are identical so I needed to make sure I sourced the correct ratio.

Searches on angle drives in the E-Type Forum confirmed the variations. Hmmm …. could it be the angle drive causing the low speedo readings rather than my speedo needing to be re-calibrated? The test reading of 56mph multiplied by a ratio of 1.27 gives 71mph. Too close to the speed indicated in the lead car to be a coincidence.

Sure enough, the ratio of the old drive was found to be 1:1. A rolled up Post-It had been inserted into the old angle drive to check the rotation of the output drive for one revolution of the input shaft. The same test was repeated on the new angle drive to confirm a correct 1:1.27 ratio before it was installed on the car. The replacement angle drive had a shorter drive shaft so it should be easier to fit as it won’t require the same amount of clearance.

Checking the ratio of
old angle drive
Exercise repeated for new
drive before fitting
Newer drives have shorter shaft
– much easier to fit!

I still hadn’t got to the bottom of why the old angle drive had failed. Did it just fail or was it caused by other components? The cable was removed from the car and all appeared to be in order. There were no kinks and it was operating reasonably smoothly by hand.

It was a good opportunity to clean and re-lubricate the cable and the inside of the sheath. I wiped the cable with some silicone lubricant as some advised that grease can cause binding problems further down the line. Grease or oil will also have a tendency to migrate up the spinning cable which acts as an Archimedes screw, potentially causing damage to the speedo itself.

The cable was reattached to the speedo to check it could still rotate freely and not bind. It couldn’t be turned at all! The speedo’s input drive had seized. This explains why the weakest link in the chain had failed – the interference fit of the round shaft into the angle drive.

The conclusion is the seizure in the speedo would have stopped the cable from rotating. This in turn would have stopped the output of the angle drive from rotating. With the angle drive locked and the input shaft still being driven from the gearbox, the round end of the input shaft would have failed rather than the square end.

With the cause identified, the new angle drive and cable could be fitted back on the car. The exhaust was removed first so the gearbox mounting bracket could be easily jacked into position. It was also a good opportunity to finally fix my wonky tail pipes.

Something I should had done at
the outset – remove the exhaust!
Access to the gearbox mounting
bracket is much easier
Jacking the mounting bracket
into position to fit the bolts

All the instruments and gauges had been professionally restored many years ago and safely put in storage until needed. So I was rather disappointed the speedo had failed after only 500 miles. It will now have to been sent off for repair.

The input drive into the speedo performs two tasks; i) driving the needle and ii) driving the odometer/trip distances. At the internal end of the input shaft is a worm drive and an input disc containing permanent magnets.

The worm drive simply rotates another gear wheel which then drives gears for both the odometer and trip distance mechanisms.

In close proximity to the input disc is a similar sprung disc containing magnets to which the speedo needle is attached. As the input disc starts to turn, its magnets attract those on the needle disc causing it to turn. The faster the input disc rotates, the greater the torque on the needle disc due to the magnetic attraction.

Brass worm gear turns nylon gear
that drives odometer/trip distance
Input disc rotates within the
needle disc

A hairspring on the needle disc counters this rotational force, stopping the needle disc from free wheeling. The amount the hairspring coils is proportional to the rotational force and therefore road speed.

Speedograph Richfield can either calibrate the speedo back to the factory settings or calibrate it to your specific vehicle. I’m fairly sure the new angle drive will resolve the fact that it was reading 20% too low and so the factory settings would be fine. However I’ve also provided the measurements for it to be calibrated to the car, so they can determine if there is a discrepancy between the two.

To do this, they need to know the type of tyres in order to calculate the rolling circumference and the number of revolutions of the speedo cable for 6 revolutions of the driven wheel. The tyres were checked to ensure they were the correct pressures and a chalk line put on the rear tyre to aid measuring exactly revolutions.

To help count the revolutions of the speedo cable, I cobbled together a pointer made from matchsticks which slid onto the square end of the cable. All very high tech! The car was pushed forward with one person counting the wheel revolutions and the other the cable pointer. The average of three measurements was 8 full turns and 290 degrees (+/- 5 degrees) for six revolutions of the rear tyre.

Mark to accurately a full revolution My matchstick speedo cable pointer!

It is now with Speedograph Richfield who have indicated it should be returned within the week. A pretty good turn around. So fingers crossed this will be the end of my speeding troubles.