911 Speedo transplant for 914 (long), calibrate for your own tire size... Options

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There have been a number of threads here and elsewhere ranting about 914 speedo error, cable issues, tranny pick-off issues, etc, etc. I had my own speedo crisis recently, and I decided to bail on the whole ancient mechanical kluge. Thought I’d describe what I did in case any of you guys are in the same boat, and are looking for an alternative.

My speedo cable snapped a couple of weeks ago after I rotated the gauge in its mount for better view through a Momo steering wheel. I had an electronic 911 speedo (mine was from an ’84 Carrera) and found that it fits in the 914 gauge cluster perfectly. An electronic speedo is essentially just an electronic tach with a different dial face and a sender that transmits wheel (rather than crank shaft) revs.

What stands out compared to the 914 mechanical speedo is the 911’s electronic speedo has an adjustment pot inside it that allows (relatively) easy recalibration if you’ve changed your tire size. There are threads on the Pelican 911 BBS that cover calibration, so I won’t cover those details here.

The primary challenge was to cook up an appropriate “sender”, something I unwittingly had already done for a previous track video data project.

(IMG:http://www.914world.com/bbs2/uploads/post-2-1065087044.jpg)


Just like the mechanical speedo, the stock “sender” for the e-speedo is built into the tranny. In the 915 at least, there are 8 little magnets embedded at equal intervals in the ring gear. A reed switch on the outside of the tranny case is “tweaked” every time one of the magnets pass by, generating a stream of “switch closure” pulses which the electronic speedo converts to a speed readout.

Home Made Wheel Speed Sensor

My approach to fabricate a sender was to use a commercial proximity sensor to “watch” 8 metal “flags” clamped to an inboard CV joint (the flags are the yellow things in the pix). The flags are just ½” cutoffs of Home Depot ¾” galvanized angle, clamped to the CV with a large hose clamp. The flags rotate past the proximity sensor, causing the sensor to generate the pulse stream for the speedo. You’ll have to fab a bracket, and mount it to a convenient stud on the tranny case. I located mine as high up as I could to minimize road crud exposure. The whole assembly is shown below. I used 1” strap iron for the bracket shown, the sensor requires a ½” hole.

(IMG:http://www.914world.com/bbs2/uploads/post-2-1065087133.jpg)

I used a Cherry Corp gear tooth sensor, model number GS100701 which I got from DigiKey (www.digikey.com) for ~ $30 (the Porsche sensor is over twice the cost, naturally). (Digikey’s part number for this item is CH398-ND). This is a common sensor and is available from multiple distributors.

It’s important to get the flags spaced evenly around the circumference of the CV joint, and to “fly” close to the sensor tip. You can make a flag spacing template by wrapping a skinny strip of paper around the CV body, marking where it overlaps, then folding the strip in half (with the mark lined up with the edge), then folding it in half twice more. Mark the creases, re-wrap on the CV, and transfer the marks.

Clamp the flags loosely on the CV with the pipe clamp, line them up on the spacing marks, then slowly rotate the CV and make sure each flag “flies” past the sensor closely but without whacking the sensor. The surface of the flag should be parallel to the surface of the sensor tip. You can “calibrate” the distance by sandwiching a dime in between the sensor and the flag(s), then removing the dime without disturbing the flag. Tighten the hose clamp when everything is lined up.

While the addition of the e-speedo is recent, this sensor rig has been on my car and trouble free for over a year now, with multiple DE’s and AX’s under its belt, not to mention at least one ice/salt/sand/ storm.

Electrical Hookup

You will have to splice enough extension wire onto the three 36” leads coming out of the sensor to get from the tranny all the way up to the speedo. Make sure to route the wiring away from the exhaust system and away from moving parts, and make sure the splices to extend the sensor leads are waterproof. I’d suggest using SAE-J1128 spec wire (try your local FLAPS that sells to the trade, or http://www.kayjayco.com/catP125CWire.htm, among other sources), 18 - 20 gauge is fine. The PVC cr@p that your low-end FLAPS sells won’t tolerate the oil and heat (or salt, or ice) very well.

The sensor needs 12v and ground, and provides the direct output needed for the speedo. I terminated the leads with ¼” quick disconnects (Radio Shack), and used spade lug “doublers” (Radio Shack) to tap the switched 12v off the tach, and the ground wire that came up to the original speedo. You’ll also need to make a short jumper to bring 12v over from the back of your tach.

The connections are as follows:
- transfer the indicator lamp assemblies directly from the old speedo to the new speedo;
- add a lug doubler between the tach’s 12v lug and its existing harness wire;
- add a lug doubler to the speedo’s 12v lug;
- the JUMPER wire you made goes from the 12v on the tach to the 12v on the speedo;
- the BROWN wire from the sensor goes to the +12v lug doubler on the speedo;
- add a lug doubler to the ground lug on the e-speedo;
- the BROWN wire (from the orig. speedo) goes to the ground lug (-) on the e-speedo;
- the BLACK wire from the sensor connects to the lug marked “31b” on the e-speedo;
- the BLUE wire from the sensor goes to the (ground) lug doubler on the e-speedo;
- the “A” lug on the electronic speedo is not used.

(IMG:http://www.914world.com/bbs2/uploads/post-2-1065087070.jpg)

My “new” speedo reads the same as the old one, ie its about 5mph high at 80 mph (on 205/55 - 16 tires). My sensor rig drives another (more accurate) system in addition to the e-speedo, so I’ve not bothered to fool around with recalibration.

I believe (based on Pelican’s catalog) the electronic speedo’s from the 1976 - 1989 model years (911) should be usable, though speedo’s from a turbo might have a different calibration setting. The one I used came from a 1984 Carrera. Look for a speedo with individual lugs (as opposed to a single connector, which is probably from a C2); one of those lugs needs to be labeled “31b” (the DIN standard designation for a periodic-switch-closure-to-ground type of signal).