Helpl with my speedometer please.

[smikeb]

First of all I want to say thanks to everyone that has posted informative how-to's on this forum. I bought my 77, 911 about a year ago and have used this site for everything from brake caliper rebuilds to fixing the dome light. I had a long list of items to fix before it could be considered road worthy.

One of the last items is my speedometer does not work. I searched for and read every speedometer thread I could find and learned a lot, but still I can't figure it out so here goes, my first post:

Its a 911 with a 915 transmission, I think. When I bought the car the speedometer was completely dead. So I checked the sender and found that it was installed backwards with the ears in, So I turned it around and tried again. The car is on stands right now, but when I put it in gear and let the wheels spin the speedometer needle just twitches, ie it moves but only about one millimeter. So I disconnected the sender at the real tunnel access port, and tapped the bare gauge wires together. The Needle bounced up to around 30MPH. If I tap them together really fast the needle will get up to around 50MPH. So I figured the gauge was still good.

So next I attached my meter to the sender terminals in continuity mode and found that as I rotate the wheel , I get four beeps per revolution. So at least I know I don't have an LSD. The resistance across the sender is quite low when the reed switch is closed. When I remove the sender and used a magnet to trigger the reed switch I can her it clicking and closing on the meter. I reconnected the sender to the gauge and used the magnet to trigger the sender, and I get the same 1mm twitch from the needle. So then I attached the magnet to my drill chuck to make it spin really fast and held that next to the sender and got the same 1mm twitch.

Thinking it was the sender, I then made a new sender from a reed switch I had lying around. I tried it on the bench with the magnet and in the transmission, but still the needle just twitches, and doesn't go higher than about 2MPH.

So I think I've tested both the gauge and sender independently and they seem to work, but why don't they work together? Any ideas?

Cheers and thanks in advance.
Mike B

[schumicat]

is your speedometer getting +12 volts when ignition on and and is ground wire grounded? I would swap speedometers with someone and see if it works; it takes 5 minutes to swap them just label the wires.

[smikeb]

Yes it gets 12v. Cause it seems to work when I touch the two wires together. Can anybody tell me if that is normal behaviour? ie when I tap the two signal wires together repeatedly the needle bounces up to 30-50 MPH?

[smikeb]

Swapping speedos would be great but I don't know anyone with a Porsche.

[porwolf]

As far as I know your 77 speedometer is not a mechanical one. It responds to contact closures by the sender at the transmission. That micro switch closes 8 times per revolution of the gear behind it. That closure frequency could be simulated by quickly shortening the signal wires:

Bench test electronic speedometer?

If you are somewhat more ambitious you can try his testing method:

VIDEO How to test a Speedometer

[smikeb]

Thanks, I have tested the gauge by tapping the signal connections together. The needle bounces up to about 30MPH. Can anyone tell me if this is normal?

I'll go try using a signal generator and see if it gives the correct reading at 100 pulses per second.

M

[smikeb]

The gauge works perfectly with a signal generator driving it. So now why doesn't the sender work with it. The sender tested fine when I rotated the back wheels manually, it gave 4 pulses per revolution. Any ideas?

[porwolf]

Originally Posted by smikeb

The gauge works perfectly with a signal generator driving it. So now why doesn't the sender work with it. The sender tested fine when I rotated the back wheels manually, it gave 4 pulses per revolution. Any ideas?

Could it be that the sender does not work at higher speeds?

[smikeb]

Getting Very Frustrated Now.

Bought a brand new Porsche sender from PP, and installed it to find I get exactly the same response. When I spin the back wheel by hand the needle just flickers a little bit, but barely moves. Exactly the same as the old sensor.

I have already tested the gauge works with a signal generator, and just by tapping the two wires together.

I read on another post, something about not putting the sender in the hole all the way? Can anybody enlighten me about this idea more?

Thanks
Mike B

[porwolf]

Could your problem still have to do with the wiring between the sender and the speedometer? What about making new, temporary, wire connections between the sender and the speedometer? Just run the wires on the outside of the body through the door opening and test drive the car. See what happens. And just to be sure also run temporary wires between the battery + and - and the speedometer. To be safe add an inline fuse there.

[BoxsterGT]

Possibly an issue with the magnets on the sender plate inside. Disassembly for a visual inspection will tell.

Len

[smikeb]

In response to the last 2 posts. When I tested the gauge with the signal generator, I did it on the car, ie the connection was made at the tunnel behind the seats. So I think that confirms the wires from there to the gauge are ok.

When I test the sender by itself, using a continuity tester, I get 4 pulses per revolution, when both tires are off the ground. So doesn't that mean the magnets are good and working?

What is involved with checking the magnet wheel inside the diff?

Thanks guys. Any further help or ideas welcome.......

Mike B

[ClickClickBoom]

Check the actual hook up to the speedo. Make sure the right wires go to the correct terminals.

[ClickClickBoom]

Check the actual hook up to the speedo. Make sure the right wires go to the correct terminals. If you think about it your troubleshooting is good.
Your speedo works when the wires are tapped together
The sender sends a signal

Now check the signal at the wire end going to the speedo connector.
After that double check all the connections are correct for the spade connections.