SC Speedometer

patkeefe · 05-22-2021, 07:39 AM

Speedometer isn't working. I tested the Hall sender wit a magnet and ohmmeter; that works OK. My Bentley wiring diagram for instruments shows one side of the Hall going to 31b, and the other side going to - (I assume ground). I have +12V and I do not have cruise control. Picture, from left to right is -, 31b, A and +12V. the blank between A and +12V is empty, labeled B (probably some leftover Mercedes or Volvo junk from 1978 at the VDO factory).

How are the wires supposed to connect? I think I have a wire grounded in the tunnel.
I'm trying to put the car back together and get all the little things working (that I didn't care about when it was a track car).
TIA
Pat

island911 · 05-22-2021, 07:48 AM

If it's not working, check the sending unit connector.

This is w/ cruz ctrl (81 SC)

island911 · 08:12 AM

Also, that is a 1978 speedo.^ (150mph - the US 1981 had an 85mph as OE)

IIRC the sending unit is not a Hall circuit but rather a simple contact switch that gives a ground pulse with every magnetic pass. Anyway, checking continuity, back to the sender is a wise thing to do. I had the sending unit wire break, due to rubbing/vibrating against a sharp edge.

patkeefe · 05-22-2021, 08:18 AM

Yes Dr Island, it is a simple switch. Regardless, it works. I’m checking wiring now.

jbell959 · 05-22-2021, 08:19 AM

Your wiring doesn't look right. That solid red wire looks like it was added at some point and one idea of your sender is not connected. Try removing the red/black wire and moving the single brown/red wire to that position. Then connect the other brown/red along with the ground wire to the ground connector on the speedo. If that doesn't work, check to make sure you have 12v on that solid red wire. The red/black wire may be your 12v wire and the solid red was added for something else. I'd need to dog out the schematic to be sure. But the 2 brown/red wires are coming from the sender and one needs to be connected to ground and the other to the signal input on the speedo where you have the red/black connected.

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island911 · 05-22-2021, 08:23 AM

Nicely annotated pictures here: https://forums.pelicanparts.com/porsche-911-technical-forum/813009-speedo-odometer-not-working-sc.html
with what looks more like your wiring.

Quote:

Originally Posted by porwolf

Here are the wire connections to my '79SC speedometer. Hope it helps. Also check with the wiring diagram and see whether the +12V power to both your instruments comes from the same source somewhere. The problem could be there.

jbell959 · 05-22-2021, 08:43 AM

That's a good picture. Follow that and remove that solid red wire and hopefully that does it. Tape up the red wire if you leave it unconnected so it doesn't short against something

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patkeefe · 05-22-2021, 08:49 AM

I have the car in 3rd gear. I’ve spun the engine over, and have my ohmmeter cinnected directly to the speedo sensor output. The circuit is not being broken like it was when I rested the sensor with a magnet. Am I missing something here?

island911 · 05-22-2021, 08:56 AM

Assume the wire is broken unless you know that it is not.

3rd gear? The switch should give 8 clicks per wheel revolution, so you shouldn't need to spin it all that much.

patkeefe · 05-22-2021, 08:59 AM

Thanks. I spun it in first gear also. No made circuits. I’m trying to remember if the speedo ever worked since I had the transmission rebuilt

island911 · 05-22-2021, 09:00 AM

IIRC I tested my speedo by splaying out the end of a multi strand wire and fanning that across the contact, to spoof the speedo pulses. -this should make the speedo come up a bit.

I made a jumper wire for that.

island911 · 05-22-2021, 09:03 AM

30hz gives 15mph

patkeefe · 05-22-2021, 09:37 AM

Jbell-I added the red wire, as I needed a 12V source. That is for sure a 12V DC signal going to the speedo.

I am still bothered by the fact that I am getting nothing from the speedo sender with wheel revolutions. I will check it again with a different ohmmeter after lunch.It used to work OK, until I had the engine and trans out.Never touched the speedometer gauge or wiring. Maybe it went bad sitting in the garage for a couple of years...

island911 · 05-22-2021, 10:36 AM

The sending wire often wears thru the insulation and/or breaks. (typically not the most controlled of wire mountings)
I would check the signal lead at the speedo. If it does not go to ground with the wheel turning, then the wire is broken. If the signal lead at the speedo remains grounded (wheel turning) then the insulation is broken thru and is grounding.

oh, and simply give a visual check back by the tranny.

patkeefe · 05-22-2021, 02:57 PM

Well, the speedometer annoyed me to no end. And, I can't find the problem. Tomorrow, I am going to mount a magnet in my battery drill, somehow, and hold it near the sensor until I get the speedometer to function. Similar to island911's suggestion above. Then, I''ll put the sensor back in the trans and see if it works.

My question of the moment is, what makes the sender do the off-on sequence? Are there magnets mounted on the ring gear or something? This seems to be some well disguised fact, for which I can find no information.

jbell959 · 05-22-2021, 03:13 PM

Quote:

Originally Posted by patkeefe

My question of the moment is, what makes the sender do the off-on sequence? Are there magnets mounted on the ring gear or something? This seems to be some well disguised fact, for which I can find no information.

Yes, there is a disc inside with 8 magnets. There is a picture in this thread

https://r.tapatalk.com/shareLink/topic?url=https%3A%2F%2Fforums%2Epelicanparts%2Eco m%2Fshowthread%2Ephp%3Ft%3D395994&share_tid=395994&share_fid=844&share_type=t&link_source=app

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patkeefe · 05-22-2021, 03:53 PM

jbell959, you are a saint! I have no idea how you found this, but thanks!

patkeefe · 05-22-2021, 05:57 PM

Is it conceivable that someone forgot to put the magnet ring in? Say, when the trans was getting rebuilt and having an LSD installed?

patkeefe · 05-23-2021, 09:54 AM

Here is the test bit I made with a plastic container lid and a magnet. Mounted it on an arbor, and spun it near the speedo sensor to verify it works. Repeated this:
a-with only the sensor leads connected to the bell function on my Fluke multimeter
b-with it connected as it was (that was grounded, so rewiring was definitely in order)
c-jumpered from sensor to gauge (easier to switch the wires around) with the Fluke at the jumpers at the speedometer.

I got it working, after moving some wires around. Final configuration is as per the photo.
Started the car, on jack stands, in gear, no MPH. But after a few seconds, I started to get what appeared to be valid MPH. So, the sensor might be bad, due to the intermittent nature of the speedometer reading and some quirky "misfires) on the bell when testing with the drill. Sensor is mounted with the "castle" tabs facing out, and the relatively flat side in. I give up for today. "It was running when I left", which I learned from the service guys at my old company.

jbell959 · 05-23-2021, 09:59 AM

I don't think that red/black should be on the ground connection. I think red/black should have 12v. I would check that. Do your gauge lights work? Unless somehow that red/black is ground, I don't see a ground to the speedo case? There should be a sold Brown on that last position where the red/black is.

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