Electronic speedometer

Ranchman · 05:09 PM

Novice 911 assembled 76 targa...everything working except electronic speedometer. Closed circuit briefly and speedo deflected; replaced sending unit (ears in) ...still doesn't work. I'm baffled which may not be difficult. Can you give me some direction with next steps please.

Ranchman · 06-26-2004, 05:02 PM

test

Early_S_Man · 06-26-2004, 06:56 PM

Does the speedo have +12 Volt power when the ignition switch is on?

Well, if you pull the two sensor wires from the speedo terminals and hook up a multimeter set to measure low Ohms ... you should see exactly eight switch contact closures and opening when you rotate passenger side rear wheel exactly one revolution. If not ... check the connections in the tunnel beside the shift rod and coupler.

Ranchman · 06-27-2004, 10:19 AM

Warren...thanks so much for your note. The speedo tests out fine, with power when ignition on and deflection when closing the circuit. With the new sender disconnected and an ohm meter attached, there is no reading when rotating the rear right wheel. As I assembled this car from pieces having never seen a Porsche up close, I have no idea what the transmission receptical should look like nor if the sender unit is complete. I inserted the sender into the transmission receptical which is a bare hole capped inside with a female aluminum plug. There are no terminals or connections, so I am assuming the sender is magnetically driven. I inserted the sender with the ears in (resin filled side in). The sender has what looks like female recepticals on the resin side, but again, there is nothing on the transmission to plug into. What am I missing? The problem is either with the sender not being complete or with the transmission missing something, but I have no idea what. Thanks Warren! Rick (Ranchman)

RoninLB · 06-27-2004, 10:49 AM

Hi Rick.
I sent you 2 pages from the P Workshop Manual.. note the "caution" at end of p1. Also on p1 is a reference to the speedo being triggered by the dizzy. Thus the speedo is related to engine speed. The calibration number is on speedo housing. I have no experience with this. sorry

tchavalit · 08-02-2004, 08:44 AM

Speedo NEED HELP !!
Can anyone tell me how electric speedo work ???
from Transmission sensor to speedo.
Did it have something in between, my speedo not working.
Check for OHM from sensor look OK.
Bob
new911junior

IROC · IROC's Garage

My '76 911 has a mechanical speedo. Maybe that's the problem. :>)

Mike

Groesbeck Hurricane · 08-02-2004, 09:00 AM

This only works some of the time, and toooo many people have stated (including all the manuals) that it does not matter, but have you tried swapping the connection at the speed sensor couplers? the couplers are in the tunnel and connect to wires going directly to your speedo. Swap them. It has worked for some of us, me included.

tchavalit · 08-02-2004, 09:28 AM

I was try swap the wire before (2nd unit speedo) still never work.
Bob

Early_S_Man · 10:04 AM

The 'sender' is a simple magnetic reed switch, which is why both wires are white ... it doesn't matter which way they are connected. If the contacts in the tunnel are oxidized ... removal and reinstalling my wipe off the oxidation, but cleaning the electrical contact is what matters, NOT which wire goes to which terminal at the speedometer!!!

The sender is triggered by eight magnets on a carrier wheel bolted to the differential. If you get eight contact closures [measurement should be less than one Ohm] and openings per revolution of the ring gear and rear wheel ... the sender is working properly! It is possible to get the sender in the bore backwards, so, if the contact closures aren't happening, try reversing the position of the sender!

The speedometer isn't connected to or triggered in any way by the points in the distributor, but the electronic circuit driving the speedometer needle is identical in theory of operation to a tachometer triggered by points!