Speedo died last night....

[gjmascoli]

Started up the car last night and while still idling the speedo needle blipped and hung up at some random speed. As I started moving, the speedo was worthless, needle mostly lying limp at 0 mph, but random blips and getting hung up at a random speed.

Odometer dead, trip Odometer dead.

Background:
- Sent the speedo to NHS this past winter to get it calibrated because it was reading higher by 5-7 mph
- Speedo has worked fine for the past month, about 300-500 miles driven

I do have electrical gremlins:
- electrical noise in the radio/speakers when I brake
- right front blinker illuminates when I brake
- occasionally the headlights illuminate even when the switch is in the off position
- dome lights sometimes go on when I open door, sometimes not

Does someone have a simple instruction on how to clean all the ground points? Where are all the ground points?
Could this be related to a hodge-podge of bulks from over the years?

Ideas?

[fred cook]

There is a little white gear inside that drives the odometer and the trip odometer. If it breaks, the odos quit working and a piece of the gear can jam or partially jam the speedo. Since you just had NH work on it, give them a call and see if their work is still under warranty. Since you are a recent customer, they may fix it and only charge for the part(s). Good luck.

[gjmascoli]

Thanks Fred, I do think that is an option to go back to NHS but the symptoms seem to be electrical noise.

On my drive home tonight the needle bounces around a lot then would hang at some random speed.

I'll go back to NHS but first I want to try to trouble shoot the potential gound issue, but I'm hoping for some pointers, I'm not much of an electronics guy.

[lpsalsaman]

You can check the connections behind the speedo itself, then move to the tunnel where your shift linkage is and check the wiring in there. In the tunnel is where the speed impulse sensor connection and the speedometer connection meet. If not the speed impulse sensor unit in the transmission itself could be bad. Finally have NHS recheck it again...Good luck!

[Drisump]

The "hanging up" bit is the strangest behaviour. The signal comes from the speed sensor and is translated into movement on the gauge. The odo is powered by a motor that has nylon gears that are a source of trouble in our cars. Like was said earlier, if both the speedo and odo are non responsive, either the wiring, connections or the sensor may be at fault. That said, my guess would be a bad connection to the gauge since it has only been 500 miles since re installation. Cheers

[gjmascoli]

When I get a chance I'm going remove the leads and to hook a voltmeter up to the leads and see what noise I am getting.

Also forgot to note that I replaces the brake light switch last year and I started getting more electrical noise through the radio when I step on the brake pedal. So problem may be there.

[dap930]

While checking the wire connections, jack up the rear of the car and use a VOM to check that there are 8 pulses per wheel revolution from the reed switch mounted on the transmission. You can check this at the reed switch connector in the tunnel. There are 8 magnets mounted on a disk inside the transmission differential that actuate the reed switch when magnetic flux is present. If one or more of the magnets dislodges, it will cause erratic speedometer readings. I hope your problem turns out to be other issues.

[gjmascoli]

Thanks. Painting my house this weekend so not making any progress on the speedo, but keep the ideas coming!

[gjmascoli]

So I got out there today and tried to do some trouble shooting. I disconnected the various leads one-by-one, place a Voltage meter between the lead and the spade connector on the Speedo and took Voltage measurements as I drove down the road. Here's what I got:

Connector that has a Single Red Wire with Black Stripe: 13V

Connector that has a "double stack" that connects to the speedo spade and includes the Black Wire with Brown stripe, as well as a Brown with Red Stripe: 12V

Connector that has a "triple stack" that connects to the speedo spade and includes the Brown Wires (ground?) and Brown with Red: 0.17V, basically 0. Perhaps this means there is something broken inside the Speedo and I have an open circuit?

So......am I getting the correct signal to the Speedo and my Speedo died for some reason? Time to send back to NHS, or am I getting a bad signal to the Speedo?

Sorry, I can't upload an image as it wants the URL of the image (I'm not savvy enough to know how to move a picture to a URL....is that like FaceBook or something?)

[dap930]

You can not accurately check the speedometer operation with a voltmeter when the car is going down the road. You would need an o-scope. This is because the reed switch at the transmission grounds a 12 vdc signal to the speedometer. The speedometer is looking for 12vdc pulses. A voltmeter would average these readings out, an o-scope would show the actual pulses being generated. Inside the speedometer is circuitry with a pull-up resistor. When the reed switch is open, the speedometer sees 12vdc across the pull-up resistor. When a magnet inside the transmission passes by the reed switch, the switch "makes" and grounds the 12vdc signal across the pull-up resistor, so the speedometer sees 0vdc. In this manner the 8 magnets in the transmission provide 8 pulses to the speedometer per wheel revolution. The speedometer converts these pulses into a voltage to drive the meter movement.

The red/blk wire provides 12vdc to power the speedometer. The Brown/red wires go to the reed switch. One of these has 12vdc on it, the other goes to ground (provided by the brown wire). So, to check to make sure the reed switch is working properly, connect the voltmeter (set to read ohms) across the brown/red wires. Jack up the back of the car, and slowly rotate a wheel to see if 8 pulses show on the meter per wheel revolution. This test is easier done at the reed switch connection the in the tunnel where the shift coupler is. Just disconnect the wires from the connector and meter the wires going to the transmission. If this test passes, then the electronics inside the speedometer a likely the issue, and you may have to send the speedometer for refurbishment.