S W speedo prob

[Daveyboy]

I have a S W winged speedo and a 700r4 trans elec pic up and speedo is elec.

Here is the prob.

If i start the car the speedo will go from 20 to 140 and all around and click off the miles.
I have put a 1k restistor in on the ground and then to the frame to ground (from the yellow wire at the trans pick up to the frame)
What is going on?
And how do i fix it?

[Fat Cat]

I have a S W winged speedo and a 700r4 trans elec pic up and speedo is elec.

Here is the prob.

If i start the car the speedo will go from 20 to 140 and all around and click off the miles.
I have put a 1k restistor in on the ground and then to the frame to ground (from the yellow wire at the trans pick up to the frame)
What is going on?
And how do i fix it?

Are the wires from the speedo to the pickup twisted into a twisted pair. If not they can introduce attenuation into the circuit and give you abnormal readings. Becuase the pickup is producing AC current it needs to be secured against wire crosstalk.

[enjenjo]

I would call SW to see what to do. I think the GM sender outputs 4000 ppm, and you need 1000ppm on a SW speedo. You may have to install a pulse buffer.

[Daveyboy]

I did call them and they told me to put the 1k restior in.

The speedo runs and runs with eng running and NOT moving
I will call them again and see

[enjenjo]

If the speedo it showing  a reading with the engine running, but the car not moving, you are reading the tach signal. My guess you have the tach wire, and the speedo wire in the same  bundle, and are inducing a signal from one to the other. I had this problem on a car I built last winter. seperating the wires cured it.

[Daveyboy]

It does not go up and down with eng speed just goes all over from 20 to 160

[enjenjo]

It does not go up and down with eng speed just goes all over from 20 to 160

Typically it will start at about 20 mph, slowly go up, then back down, and then up again. It is computer controlled, and is trying to resolve the tach signal as a speedo input. It is trying to match the meter reading to the signal it is receiving, unsuccessfully.

So it is a spurious signal. Since the car isn't moving, it can't be the speedo signal, since there is not one being generated. The other choices we have are tach, most likely since you have a tach too, or alternator static from voltage variations.

If the speedo sensor wire, and the tach signal wire run parallel for any distance, more than a few inches, it will induce a signal into the speedo sensor wire.

To check this, disconnect the speedo sensor wire at both ends. replace it with a temporary wire that is not in the harness, and see if it still malfunctions.

In any case, it is not likely that the speedo is defective, they just don't fail this way.

[Daveyboy]

I will try that on monday night   Thanks

[Dave]

the other option is go to radio shack and get some of the little thingys that knock out spurious emissions.(rf chokes /Ferrite beads) hang a couple on the wires and see if it stopps the rf that your getting in the speedo signal line.. also did you ground the speedo cable at both ends? Im assuning it is already set at the speedo transducer end but if you grounded the end at the gauge then remove that ground as it will help induce rf into the speedo unit in the dash.. (GROUND AS IN THE SHIELD OF THE 3 WIRE CABLE)
Dave  

[Daveyboy]

ok i ran a NEW wire from the purple wire at the trans sender to the speedo and i have the same prob.
Now what?

[enjenjo]

What happens when you disconnect the purple wire from the sender? What is the yellow wire connected to?

[Daveyboy]

yellow is now on the frame
And when the purple wire was off it did the same thing

[enjenjo]

I would  check that the speedo is connected correctly then. Is it properly grounded?

Electronic speedometers work by reading a sensor input, and matching the needle position to preset parameters. So when the power is turned on, the power pulses, and the needle will jump, then return to zero. If it does that it is working. If the needle starts moving when the engine is started, but the car does not move, it's reading a spurious signal from somewhere else. most common is tach feed, or the alternator. Since we have ruled them out, I would start checking grounds. if the tach, and the speedo are grounded to the same ground, if it's not a good ground, it may feed through the speedo. On a glass body, you may have to run a seperate ground from the sender to the gauge to exclude any other signals.

[enjenjo]

You ever get this figured out?

[Daveyboy]

Well this is where i am at as of now.  
We put a 10k resistor in on the feed line from the trans and that fixed the bouncing when not moving. But the speedo is so far off i cant calibrate it. They say my trans is putting out 200k pulse per mile and the speedo can only handle 16k  
So i had to buy a box from Dakota digital to split the pulses down.  
The car is at the upostrey shop so i cant try it now
sorry for the bad spelling but i cant think its 5 am and i am going to bed now