Speedo and odometer not working? (SC)

[bobswei]

I've read a few post where the odometer isn't working due to the little plastic gear inside breaking. On this 83', both odometer and speedometer don't work. So, maybe not the gear but wires? I read one post that shows a red/white wire going to terminal A. I have nothing on A. Other wires seem to match up and red/black has good 12 volts. Any ideas?
Thanks for suggestions.
Bob

[fred cook]

Check the sending unit on the transmission. If it is a 915 gearbox, it will be on the driver side near the axle flange. You could have a bad wire there or the sending unit could be missing entirely. It takes a 10mm wrench to install/remove the sender unit.

[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.

[bobswei]

Yea, I found that illustrated photo. Nice job. But I have an update. Swapped a speedo from another SC and new one works fine. So, can a DYI'er repair a speedo on his dinner table?
Thanks for the responses.....

[Dave Colangelo]

Originally Posted by bobswei

Yea, I found that illustrated photo. Nice job. But I have an update. Swapped a speedo from another SC and new one works fine. So, can a DYI'er repair a speedo on his dinner table?
Thanks for the responses.....

Depending on the level of DIY skills you have, yes. I have taken apart the speedo on my 78 a few times to fix the gear issue once and other time to fix the gear motor (it had come lose). I was able to bench test the speedo using my signal generator before reinstalling it which helped me to make sure it was in working order. To do this you will need a signal generator capable of generating a square wave. if memory serves its a 12V peek to peek wave from 0Hz to 300Hz but I could be wrong on those frequency ranges.

The real answer boils down to what is wrong with it. If one of the resistors is toast then it could be a simple and quick fix (if you can see it clearly burnt etc.). There was at least one IC chip if memory serves but I dont know what it did. There are also some capacitors in there. As long as you can test all these things you should be able to fix it on your table. In terms of tools you will only need a small flat head screwdriver to open it up and a soldering iron to remove and reinstall components.

Regards
Dave

[bobswei]

Sounds like I may be able to fix it since I have a screwdriver and soldering gun. I've rebuilt everything on a 911 from engines to paint except for the trans and some electrical. Not good with electrics. So, you can guess, I don't have a signal generator. But I will take it apart today and have a look inside. I've had gauges apart before but only to clean and paint.
Thanks Dave. Great info.....

[porwolf]

Originally Posted by bobswei

Sounds like I may be able to fix it since I have a screwdriver and soldering gun. I've rebuilt everything on a 911 from engines to paint except for the trans and some electrical. Not good with electrics. So, you can guess, I don't have a signal generator. But I will take it apart today and have a look inside. I've had gauges apart before but only to clean and paint.
Thanks Dave. Great info.....

Worse comes to worst you can always send your speedometer to North Hollywood Speedometers. They probably can take it from where you left it, include a new correct worm gear and complete and recalibrate your speedometer. I had mine repaired there a year ago for about $150.-. That included a new 150 MPH/KPH bezel.

[bobswei]

Well, I took it apart and nothing visually wrong. I've used NH Speedometer in the past, on a tach, and turned out very nice and new looking. If I can't find a place close to home, I may do that. Thanks for the responses. Bob

[NYNick]

I recently got the speedo back from North Speedo and hooked it back up…per porwolf's diagram above, and it works…but...only in 1st, 3rd and 5th gear. As soon as I depress the clutch, the speedometer goes to zero. I believe the odometer is working properly.

To be honest, I initially hooked it up per the wire connection of the loaner speedometer my mechanic installed, but that never worked either, at all. So, I've never had working speedometer to compare anything to.

I must assume the Speedometer is good…it was just returned, repaired. Anybody know what's going on? Bad speed sensor perhaps, or connection?

Thanks all.

Nick

[porwolf]

Nick, from what you describe I suspect a wiring problem from your speedometer to the sending unit at the transmission. Since the sensor wires go very close to the shifter linkage in the tunnel the wiring problem may lie there. Lose connections, wire connections touch the shift linkage? Maybe you can get some help from these threads:

1980SC Speedometer Problem

Speedometer Problems

[NYNick]

38 minutes after I post a question, porwolf answers with a possible solution.
20 minutes after that, I'm test driving the 'repair', to see if it fixed the problem.
BINGO! Works perfectly now!

The insulation on both spade clips was gone on one side of each, so I taped them up. I then spotted a very small frayed spot on one of the wires, and taped over that as well.

Speedometer now works like a dream. Thanks porwolf! I think the frayed wire was rubbing on top of the housing. You're the best!

Nick

[The cap'n]

Originally Posted by bobswei

Well, I took it apart and nothing visually wrong. I've used NH Speedometer in the past, on a tach, and turned out very nice and new looking. If I can't find a place close to home, I may do that. Thanks for the responses. Bob

Palo Alto Speedometer is the go-to place in Nor Cal. Both are as close as your post office or the big brown truck.

The Cap'n

[NYNick]

OK I'm back.
Speedo worked fine until I dropped the car off for a radio install, oil change, minor stuff. Now, no speedo. Check the wiring behind the unit against the diagram above…perfect. Checked the wiring in the transmission tunnel/linkage….looks fine. WTH?
I admit, I did pry it out and turn it a quarter turn, but that's it. What could be wrong now? It's a newly rebuilt speedo?
Thanks in advance.
Nick

[The cap'n]

If the radio guys had it out and installed the wires wrong, even for a second, it could be toasted. Check the brown wire for continuity to ground, and the (+) wire for 12v.

The Cap'n

[redranger]

Pull the speedo out and check the wiring, something might have come loose, or the ground is bad.

There are a bunch of ground wires behind the ashtray, I have 3, check those, possible radio guys pulled it off accidentally. Can't remember if that's where the speedo ground is, though

[NYNick]

Wiring is correct on the speedo. Doubtful they pulled it to do the radio. I'm suspecting the ground as well….but unsure how to check.

[redranger]

You need to check for continuity between the ground wire on the back of the speedo and where the speedo grounds to the chasis. I think, but again am not sure, that it is one of the ground connections behind the ashtray.

[NYNick]

Update;
None of the above. It was the speedometer pickup on the transmission. OE Dated 1977. We changed it out and we're good to go!

Thanks guys.

Nick

[A_Cunningham]

Hi.
I am trying to sort this 1978 sc and speedometer not working. The speedometer was rebuilt at Palo Alto. I have a schematic for the 78 from Pelican which shows a brown/red from the sensor going to 31 b. I checked continuity to the sensor connections. Ok on the one to neg. and to the brown red going to 31b--put that on there. Some wiring had been burnt and there were two wired together and hot and I put them on + There was an extra brown/black and I put that on what was left--A. The speedometer did not work. WTH?
I see now that the brown black may have something to do with the deleted cruise control ala "speed switch" on the schematic?
On the working 80 I see a white red wire to 31b and the brown red going to A!
I see here in the photo that there is no wire on A in 79. So the cruise control on the 78 was discontinued in 79?
Anyway, I put good working speedo out of 1980 side by side and measured the continuity between +12 v and A and 31b and - and they were different. Like a 5.00 ohms between the working 80 + and A and nothing on same two on the rebuilt one. I'm thinking that by putting the brown/black from that I found in there fried the rebuilt. Damn.
How does one make sure the sensor on the trans is working correctly?
I am an old guy of 76, but just rebuilt my third 911 engine and trans...but this is above me--as well as how these forums work. I hope this gets to someone with answers. Thanks for any help. ALC

[cadman930]

I am having the same issues, looking into stripping the speedo and doing the cap and resistor replace.

Checked the sensor by setting an ohm meter on the two sensor wires (disconnected from the back of the speedo) then rolled the car backwards a little ways and saw the meter go from 1.2 Ohms to open circuit indicating that the sensor was spotting the magnets move across its front then away again.

Hope this helps.