The thought of counting torx screw heads, on a Lobro joint as a way of measuring car speed and distance did not exactly grab my attention, as a quality engineering solution. It obviously works, and is SVA compliant and there are hundreds of Westfields driving around quite happily with no apparent problems. However, it is in the dirty part of the car, and subject to a harsh environment, as it collects road dirt. Not much better is the solution, adopted by some Westfield builders of mounting a slotted disc between the prop shaft and the diff. Then manufacturing, and mounting an angled bracket for the sensor, making sure you get the gap between the disc and the sensor right, so that you can accurately count the pulses. Then there is the problem of calibration. Most accurate should be to utilise the gearbox speedo drive.  We could use a mechanical speedo, but we did not like that idea either. How can we drive an electronic speedo from our gearbox?

We pondered the over the question. The MT75 gearbox supposedly comes with an output suitable for either a mechanical or electronic speedometer. Ours was definitely mechanical. We wanted to use this output but with an electronic speedometer. Andrew had acquired a catalogue from "Merlin Motorsports". "Merlin" advertised, VDO, Stewart Warner and Racetech instruments. More importantly, they advertised a transducer to convert mechanical speedo drive from the gearbox to electronic pulses suitable for Stewart Warner Instruments. Stewart Warner's site on the Internet could not enlighten us further. We began to think about using Stewart Warner instruments with this speedo converter/transducer. We noted that the Merlin advertisement specifically mentioned that VDO instruments were SVA compliant. We liked the Racetech instruments for style but they were mostly capillary or mechanical operated

On a visit to Racetech about Steering Wheels and SVA, compliance (more of this later) Andrew discovered that someone had just returned their Racetech instruments having failed SVA. The reason for the failure remains unknown. To be sure about SVA compliance, it looked like we had to use VDO instruments. On our way back from a trip to Westfield, having purchased the instruments, we called in at Merlin Motorsports at Castle Comb. "Does this converter/ transducer work with VDO instruments?" we asked, "Definitely, in fact it is the easiest solution. Just punch in a number and it is calibrated" was the reply. The Hunter transducer type 65H comes with a short length of speedo cable (you need to specify Ford or Vauxhall gearbox etc.).

Back at the garage, we set about fitting the speedo transducer. The short length of speedo drive cable from the gearbox to the transducer fitted ok but was a tight fit in the tunnel. In addition, where do we mount the transducer on the end for security? There is plenty of signal cable on the transducer, Andrew thought about running the cable down the tunnel and pick up the Westfield transducer leads. Not a good idea since the output signal would travel twice the length of the car. We decided that when we are ready, we would connect it directly to the back of the speedo

Back to the mechanical end of the transducer. We were unhappy about the tight bend in the speedo cable immediately after exiting the gearbox. Westfield recommend the use of a angle adapter (for mechanical speedo cables) Discussing this adapter got Andrew thinking about whether we could mount the transducer directly on the gear box. There looked to be enough room

The transducer has a short length of speedo drive cable on one side and an adapter on the other. The adapter is a tube 32 mm. long, with 2 plastic nuts either end. Down the centre of the tube is a square drive similar to the gearbox speedo connection. The guy at Merlin said the transducer is made for Taxi's, to tap in to the speedo cable and take off an electronic output for their meter. It looks like, if it could be adapted it would provide a direct connection to the gearbox. The problem is the MT75 gearbox to speedo cable connection is a trumpet shaped flange with a square drive in the middle. It is held in place by a circlip. We had an old piece of outer speedo cable, which had been used to blank off the connection to the gearbox. We decided to experiment. The taxi adapter was surplus to requirements if we could combine this with the trumpet end of the speedo cable we would be in business.

We had some 10-mm. aluminium tube, which looked as if it would go inside the taxi adapter and the "trumpet". The trumpet end was cut just outboard of the crimp holding it to the cable outer. It was then shortened to a point where the narrow end was a force fit over the 10-mm. tube. The "taxi" adapter was cut in half. The 10-mm. tube was too large to go in to the "taxi" adapter. The 10-mm. tube was longer than we required so we turned an electric drill in to a makeshift lathe. With the opposite end of tube from the trumpet in to the drill, chuck. We carefully turned down the tube to size, using a flat file, to a force fit the inside of the taxi tube. The tube was cut to insert a good depth in to the trumpet and the "taxi" tube. We placed one of the plastic unions over the taxi tube, so that it would connect to the transducer. We placed the 10-mm tube and trumpet on the opposite end and pushed to fit. A good deal of force was needed to get them to join. We pulled the tube together using a 10-mm bolt through the centre of the assembly and holding the head of the bolt in a vice then tightened the nut. Until we had achieved an overall length of 32 mm, leaving the 10-mm tube 3 to 4 mm recessed from the flanges at each end. It is unlikely to come apart once fitted but would be subject to some vibration. As insurance, we sealed the various joints between the tube and the trumpet with super glue

Our first attempt at fitting the modified adapter to the transducer showed that we had too much of the 10 mm tube inside the taxi adapter and this was preventing us getting a good seat when the two were joined together. We set about making another one with a slightly shorter insert and this worked fine.

The transducer is a "Hall effect" sensor. The question was how many pulses per revolution would it produce? We conducted an experiment. Connecting the Transducer to 12v and ground and an oscilloscope on to the signal lead. Turning the drive by hand, we counted the level changes for one revolution, and came up with and answer of 4. Next, we connected the drive to an electric drill and spun it up. Measuring the time for four pulses with the oscilloscope, we calculated 5 pulses per revolution. However, the drill was supposed to run at 900 r.p.m. However, there is no guarantee on its accuracy. We settled for our first observation as being accurate.

Now we had to do the calculation for the number of pulses per mile to produce the number for calibrating the VDO speedo. Westfield and others have documented how to calibrate the speedo Usually it involves moving the car a fixed distance and counting the pulses or turns on the speedo drive and then calculating a number to be programmed in to the speedo. Our way should be easier and possibly more accurate.

To calculate the final number you need the following

1.The number of pulses per revolution of the transducer

2.The gearbox speedo drive ratio

3.The diff gear ratio

4.The number of revolutions per mile of the tyres.

Item 1, we had worked out as described above (4 pulses per revolution)

Item 2, we had specifically asked for whilst the gearbox was being refurbished (22/7) MT 75 gearboxes can be fitted with a variety of drive ratios. If you do not know this, you are stuck, but the Westfield construction manual (p108) does have a procedure for calculating the drive ratio without removing the gearbox.

Item 3, is usually stamped on a tag attached to the diff. (ours was a limited slip diff. ex Sierra 4x4 (3.62)

Item 4, Revolutions per mile of the 6 most common tyre sizes (see table above, in our case 205/50 x 15 = 898)

Now put these in to the formula

In our case to calculate the number: 898 x 3.62 x 7 x4 = 4137 pulses per mile.
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Only time will tell, when we reach the instrumentation stage, if we have got it correct.