Speedometer reference Sensor replacement!
 

Hi All,
Speedo stopped working recently, did all the tests on the speedo itself, and its working fine on a bench test, checked all the wiring and power etc, all fine, did some further tests on the car and it would seem that its the speed sensor in the gearbox, as I have no resistance reading across the two wires in the rear trans tunnel, which go to the speed sensor on the gearbox, hence I am looking for advice on how to go about changing it.
All helpful input appreciated. :)
Cheers Ant.

If no resistance reading at the sensor, that may be because the axle is in between two of the eight magnets. To rule this out you should maybe try to rotate the wheel while testing the sensor resistance, or continuity since it is only a reed switch that closes 8 times per wheel turn as it "sees" the 8 magnets passing by.
But from what you describe I think the sensor is indeed kaput.

^^^ +1

Need to rotate the wheel one full turn and check that the switch closes the correct # of times. Apparently not unknown for one or more magnets to go adrift from the carrier. Or even for the carrier disk that holds them to break entirely.

But without rotating, could simply be that the switch is just between two magnets. You should get a square wave out of this when the axle is rotating (one side is power, the other is the speedo input signal).

If you still get no continuity while rotating the axle, pull the sensor and put a magnet on it to confirm. Replacing a dead reed switch or bad wiring would be a big win, as opposed to replacing the carrier disk...

Cheers both for your input :) I have to say I was under the impression that the sensor was actually similar to the Crank reference sensors in that; you get a fixed resistance reading across the sensor, obviously from what you have described this is not the case, hence I will do as you suggest and try turning the wheels to a different position, and see if I get anything different as far as measurements etc.
Ant

The "sensor" is a reed switch. It's open (infinite ohms) when there's no magnet under it, and closed (no/low resistance) when there is. One side is power, the other is a square wave fed into the speedo, which counts the pulses.

• Get a helper to turn the axle or wheel. Or roll the car - doesn't need to be in gear.
• Measure continuity.

But if the wheels are off the ground and you have an open diff, make sure to turn the side with the sensor/carrier disk...

Turning the axle and randomly measuring continuity statically is the hard way to do this.

Dynamic test is much better & easier - one full turn and you'll know exactly what's going on.

Rolling diameter of the tire is ~25", so the car doesn't need to move very far. When I was calibrating my Yellow Dog, I checked the sensor for correct operation by sticking a leg out the door and scooting the car backwards Flintstones style.

Cheers Spuggy,
Yes as soon as both you and Wazzz explained that's it actually a reed switch and not a coil type sensor, I understood the kind of measurements I could do to establish if there is on off continuity when the magnets are pulling the switch closed etc, yes I agree its not easy to do these tests on your own, but I have a plan! :)
I will let you know how it goes.
Ant.

Ant, the connector for the sensor is easily accessed under the trap door (4 screws) on the tunnel inside the cabin. You could maybe disconnect it here and connect your ohmmeter on the sensor side, then drive very slowly so you can watch continuity and see if it is breaking and making as the car moves. Even use the continuity buzzer if your DMM has one, so you should hear a series of beeps if sensor is good.

Hi Giles,
That's exactly what I was going to do when I get chance, car is super easy to move now that I have sorted all the brakes! :D
Ant.

Update;
Ok, so checked for continuity on the sensor wires using my multi-meter set on the ohms range, moving the car back and fore I got a very high reading every time the magnets passed the reed switch, however; the reading was in the meg ohm, and I presume should be nearer dead short as the switch closes, so some good news in that the magnets on the wheel seem to be fine, its obviously an issue with the reed switch itself, or the wires going to it, so next step when I get a bit more time, I will be examining the rest of the wiring going to the sensor, and if all found to be intact, then unfortunately it looks like the sensor will need to be removed/exchanged! :(
Ant.

PS, any picks of where to find the sensor on the gearbox would be great!

The speed sensor/sender is located on the transmission case. The one with the bell housing. It's just above the output shaft/axle flange. Same side as the starter hole in the bellhousing, opposite side to the diff cover.

In this thread Speedometer, E Sully posted this picture - look for the yellow arrows:

http://forums.pelicanparts.com/uploa...1728322124.jpg

There's also a thread specifically about removing the sensor remove speedo sensor 3.2 86. However, the pictures in that thread aren't any help providing clues where the sensor is located, because they're too close-up and detailed... :D

Cheers Spuggy,
I wasn't sure which side it was on, so that's exactly the info I needed to make a start! :)
Ant.

Hello all. since this is an appropriate threat I thought I would chime in with my experience and being stumped. I have a 79 SC with a recently rebuilt transmission. Note Speedo was working well before removal and rebuild. Post rebuild Speedo stopped working. Ordered a new one and replaced. which did not resolve the issue. I went through the process listed in this thread with multimeter set to automatic Ohms. tried rotating the wheel with meter connected just to sensor and got 0. ran magnet past sensor with it out of the case and still nothing. tried it with the old sensor (although I think wires are frayed) and 0. Speedo connections are good and was never touched. Could the new sensor be bad as well? any input would be helpful. best

when I say "a new one" meant Speedometer Sensor

If the speedo sensor stopped working at the time of rebuild, one possibility would be an issue with the magnet carrier disc. If it were installed on the differential under the shim ring, rather than over the shim ring, the distance between the disc and speedo sensor could be a problem.

Assuming the meter is connected and set to measure resistance correctly, then ... either both your sensors are bad (not impossible, but does seem unlikely) or the magnet you tested with isn't strong enough to close the reed switch.

I'd make sure the magnet is powerful enough to operate the sensor (eg not a fridge magnet).

Maybe you have a magnetic tool dish, tool shelf or power strip? Or an inspection light with a magnetic base? These tend to have quite powerful magnets... Or a rare earth super magnet, rated to hold 400 lbs for magnetic "fishing", is about $10 on Flea Bay.

If a strong magnet won't close the sensor, the sensor doesn't work.

Thanks for the replies all. In searching further I realized that I installed the sensor backwards. Initially I put it in with the tabs facing in toward the transmission when on fact they should be facing outward. Will get car back on the lift this week and flip it. I’ll report back with the results.

Hi All,
Update; the new sensor arrived this week, first checked it with a magnet to make sure it was working properly before fitting it, all good, next I set about removing the old one, that however took longer than originally intended, due to it being completely stuck, it had obviously been in there a long time, and the UK weather being what it is tends to corrode much of what's underneath over time, anyway, I was hoping to get the old one out in one piece to examine it, however; after much lubrication, and plenty of prying it eventually came out unfortunately in bits [see attached picks] anyway, after cleaning out the recess in the gearbox housing, the new sensor was gently tapped into place, next I threaded the new wiring into the rear tunnel compartment, now for the test, placing my multi-meter set on ohms on the two new connectors from the sensor, I managed to move the car back and fore a small distance each way, which was enough to move the magnets in the gearbox across the sensor and make the reed switch inside open and close as expected, so even though I haven't driven the car yet to be absolutely certain everything is fine, I am quietly confident! :)
Ant.
http://forums.pelicanparts.com/uploa...1747240699.jpg

http://forums.pelicanparts.com/uploa...1747240730.jpg

http://forums.pelicanparts.com/uploa...1747240747.jpg

http://forums.pelicanparts.com/uploa...1747240762.jpg

PS, The oil on the side of the gearbox near the drive shaft flange is penetrating oil and not a gearbox oil leak! :D

:):):)

Went out for a drive today, speedo all good now. :)
Ant.

Time for a beer. :)

Indeed! http://forums.pelicanparts.com/support/smileys/wat6.gif

Thanks again to both you and Spuggy for your help on this one :)
Ant.

Heh. I was going to say "it's 5:00 PM somewhere" - but given the time zone you're both in, it's even appropriate/socially acceptable :D

:cool: you got it working Ant!

:) :cool:

Huh, never thought about that. Many reed switches aren't sensitive as to orientation.

But if one side/terminal were fixed in the package, then it could matter a lot - any magnet action would be moving the mobile side further away from the fixed terminal if it was inverted...

Fingers crossed it's that easy!

Installed new sensor correctly and no-go. Cannot seem to get any ohm readings while either out and running a magnet or installed and wheels turning. I used a metal pickup wand magnet and it does not register. Going to with ordering another sensor with the assumption it is bad (it’s a Uro) and on from. There

You have to swipe the magnet close to the rear of the sensor while watching your ohm meter response, its a bit fiddly to get the right orientation of the magnet, but if the sensor is ok, you will get the switch to make or break with a good strong magnet.
Ant.

UPDATE. Bought a second sensor. Tested on bench with positive results. Installed and it works!!! Bad new sensor. Thank you all for the feedback and the help. Side note helps to have a lift as the swap becomes a 10 minute job !!!

Wow, that seems unlucky... Glad you got it sorted!

:)
Good to hear you got it sorted!
Ant.