Speedo Transducer fitted to the MT75 Gearbox

Towards the end of the engine testing we decided to test the speedo transducer.  We had made the adapter some time ago but had not fitted it to the gearbox. Fitting it was a bit fiddly, under the car, but Andrew being slimmer than me, managed to locate the adapter and secure it with the circlip. We routed the cable up beside the quick shift mounting securing it with cable ties and in to the car via the main wiring loom rubber grommet.

To test the transducer installation, I connected the oscilloscope up to the transducer output. We ran the engine and selected 2nd gear.  The wheels turned (The car is on axle stands). We could display a square wave output from the gearbox speedo drive / transducer that varies frequency with engine speed, so far so good. . Andrew temporarily mounted the speedo on our makeshift (cardboard) dashboard.  We connected up the transducer and switched on. Using the procedure in the Westfield build manual we set the pulse calibration to 4140 (we calculated 4137, but you can not adjust the final digit.) an error of less than 0.1% . Assuming the Westfield data of 898 revs per mile for our size tyres is correct. Time to check it out. We started the engine, a flicker on the speedo but no speed indication. We checked all the connections still nothing.  We then set about questioning the design and the function of the transducer and trying different connections on the back of the speedo all to no avail.  I was certain that the basic design should work.  Our speedo was obtained from Westfield, as part of the instrumentation kit.  All of the other instruments came in VDO display boxes with instructions.  The Speedo came in a plain white box with no instructions. The Westfield manual has no information on the Speedo wiring other than the basic wiring diagram, denoting plugs and wiring colours.  (There is a diagram of the back of the tacho).  The information in the boxes of the other instruments is quite detailed this is what we need. We needed to contact Westfield or VDO, but it was too late. 

Westfield Factory Configuration

I called Westfield early next day about the speedo problem.  I got the predictable reaction from their instrument guru (a Mr. H?).  "You can not do it. The sensor must be of the right type voltage etc." "We use an Earth feed and an earth return", he said - sounds odd, I had measured 1K ohms across the VDO / Westfield transducer yesterday, which suggests a voltage is involved somewhere.  "If you do it our way there should be not problem". At this point I probably upset him by saying that counting nuts on a Lobro Joint was not a good engineering solution in my opinion.  VDO's telephone number in the Build Manual is out of date. "Mr. H" didn't have a new one and couldn't transfer me to Chris Masters. I called Chris direct.  " The new number for VDO is 0121 3261234 or 0121 3261156" was his immediate reply.

I contacted VDO this time a prompt answer rather than the eternal ring of the old number. I was soon talking to "Rick" and told him the problem. He asked for details about the instrument (luckily I had it with me, serial numbers, type numbers and physical appearance and dimensions, presumably to correctly identify the instrument.) There was the expected proviso that the only information they could give was for use with a VDO transducer. This we knew and understood, but we required the technical info on how to connect the speedo and any alternative configurations that were possible- one of which might be what we are looking for. "I'll get back to you in an hour" said Rick.

Our Configuration

True to his word we spoke again in a hour.  Rick said there are two ways to connect the speedo one for a Hall effect transducer and the other for an inductive transducer. He would fax me the information.  The information arrived in about 10 minutes.  Now which configuration does Westfield use?  The Inductive, - the transducer is connected between pins 7 and 8 on the back of the speedo. We have a Hall effect transducer. The installation looked promising. The only combination we had not tried, yesterday, was the signal wire connected to pin 8. Because in Westfield terminology this is the "feed" to the transducer, in other words the input. We had measured approx 2 volts on that pin.  Well according to VDO this is where the signal wire should be connected.  (See diagram right) We would soon find out if it works  

We Connected the Speedo, according to the diagram provided by VDO. Starting up the engine and put it in gear. The Speedo reads. Checked the Speedo reading against the chart and indications are positive, well within the expected range and tolerance. 

Whilst traveling the web Andrew came across a useful programme for Gearing / Speed calculations http://hometown.aol.com/dvandrews  . If you dial in the numbers for an MT75 and a diff ratio of 3.62 (Which is what we have) you get the following answers:

Yep! it's theoretical but it gave us a guide when checking speedo transducer, under no load conditions, enough to say we are close.  In the real world, "Honestly Officer I was not on the limit in 5th gear - 161 MPH."  Pheee- ew!!.  0-60 could be quite interesting too.