I got my speedometer fixed and it’s dead

[Planter91C2]

Sent it out for repair, installed today and speedometer and odometer are dead. Still have ABS warning. Guy said he fixed the speedo odo. Any suggestions?

[brianlay]

Might find more expertise for your car on the 964/93 forum
https://forums.pelicanparts.com/porsche-964-993-technical-forum/

[silverlock]

Who’d you send it to?

[Planter91C2]

Nichol’s said it was working when he sent it back to me

How can I check for power?

[Bill Douglas]

Are you sure it's wired up correctly?

Correct connectors to the right terminals on the back of the speedo.

[spuggy]

Originally Posted by Planter91C2

How can I check for power?

With a voltmeter?

A 911 speedo has a terminal marked '+' (red wire, switched 12V), another marked '-'. (blue wire, ground - or negative, if you like) - these are the 2 power connections.

The terminal marked '31b' (green wire) is used for the sensor input, sometimes referred to as VSS - for Vehicle Speed Sensor.

There are 8 magnets in the carrier ring on the transmission, so 8 pulses from the reed switch per complete revolution of the axle/wheel. This is most probably a ~12V sort-of-square wave (although I could have sworn it was 5V when I stuck a DMM on mine many years ago).

The 'A' terminal, if you have one, is an output pulse used to drive various things depending on the model/year; OXS counter, possibly some cruise interaction, some cars apparently have a tie-in to various fan control doo-dads. Mine isn't currently connected to anything and I never put a scope on it, so can't really say...

So, 8 pulses from the sensor for every complete revolution of the axle/wheel. However, even for a specific tire size & profile, diameter can (and very likely will) vary according to brand/model...

My particular rear tire happens to have a nominal diameter of exactly 25", giving a circumference of 25*3.141 == 78.52".

There are 5280*12 == 63360 inches per mile, so 63360/72.52 ~= 806 complete revolutions of the wheel per mile.

Oddly, Tire Rack quotes a completely different # of revolutions/mile for my 255/40/17 Falkens, but I have math on my side (so long as the 25" diameter is accurate). If you don't trust the specs, you can certainly use a chalk mark and roll the car to check the loaded circumference. Given that each complete revolution is 8 pulses, 806*8 == 6448 pulses/mile.

So pick an arbitrary distance - say 50 miles. 6448 * 50 == 322400 pulses; this would add 50 miles to the odometer reading - and if the speedo saw that many pulses in an hour, it should read 50 MPH.

Or thereabouts. Don't forget the legally-mandated optimism (In certain markets, it was legal to read up to 10% high - but it could not read lower than the true speed. At all. Ever).

Also don't forget that the factory speedometers were calibrated for the exact wheel size and tire profile/brand fitted as original equipment (hence the different part #'s for the original speedometers). So some variance is almost inevitable, unless you're running the original wheels and 40 year old tires...

(806*8*50)/(60^2) == 89.55 pulses/sec - or Hertz. So set the function generator to a 12V square wave and 90Hz, and connect it to the VSS input - speedo should read 50 MPH. Ish. In that ball-park, anyway.

You can adjust any inaccuracy introduced by aftermarket wheel sizes/different profile or brand/model of tire by using a speedo re-calibrator. I like the Yellow Dog, personally; my speedo reads 1-2% off "real" speeds in the range I care about.

[Planter91C2]

I just started the car and turned on the headlights, all gauges lit up including speedometer. I assume power is not the problem?

This is the connection :

[john walker's workshop]

One of the terminals in the plug should light up a test light, with the key on.

[spuggy]

Originally Posted by Planter91C2

I just started the car and turned on the headlights, all gauges lit up including speedometer. I assume power is not the problem?

The 964 dash lighting circuit is almost certainly entirely separate from switched power to the speedometer. About the only thing that demonstrates is that you probably have ground (if the lighting circuit uses the grounded instrument can).

The lighting circuit working to illuminate the instrument proves nothing about switched 12V power feed to the speedo itself - what the instrument needs to work & register speed. The other thing it needs is the VSS pulse. If either are not present, neither the speedo nor the odo will work.

Do what JW says to verify 12V supply, leaving only VSS to be checked.

Or remove the speedo and bench-test it, and then work backwards to the car wiring if it checks out.

[Planter91C2]

Is there a connection between the Speedo and odo not working and my ABS warning light and beeper?

[pmax]

Originally Posted by Planter91C2

Nichol’s said it was working when he sent it back to me

How can I check for power?

Who's Nichol ?

[spuggy]

Originally Posted by Planter91C2

Is there a connection between the Speedo and odo not working and my ABS warning light and beeper?

No idea, it's a 964. Maybe you have a separate ABS fault - and VSS to the instrument is routed through the ABS, so not working?

Seems unlikely to be caused by the speedo itself - the photo shows a keyed plug w/ retaining clips, and 4 terminals.

As illumination isn't provided by bulbs in separate holders, we can SWAG the instrument needs:

• ground
• switched 12V (instrument)
• switched 12V (illumination)
• VSS

Doesn't look like anything magic there to me.

[Planter91C2]

Originally Posted by pmax

Who's Nichol ?

Sorry, nichols speedometer in NC

[Planter91C2]

I think the bulbs are in each gauge

[jbell959]

Here are the connections if you're looking at the back of the speedo



The gauge lights working just tell you that you have a good ground to the gauge. Next I would check to make sure you have 12V on the designated pin when the key is on. If that pin has 12V then your problem is either that you are not getting the speedo signal or the gauge is no good.

Not sure where you are located, but I'd be happy to test if for you if you run out of ideas.

[Tyson Schmidt]

The later 964 used the ABS sensor to trigger the speedo instead of a sensor on the ring gear.

So if you have an ABS code, you might have a dead ABS speed sensor. If I remember correctly, it uses the left front sensor for the speedo.

[Planter91C2]

Originally Posted by Tyson Schmidt

The later 964 used the ABS sensor to trigger the speedo instead of a sensor on the ring gear.

So if you have an ABS code, you might have a dead ABS speed sensor. If I remember correctly, it uses the left front sensor for the speedo.

Thank you, I removed and cleaned all abs speed sensors. The front right was filthy. Cleaned and brushed to look like new. They were magnetic and held the metal dust.

Put everything back together and same thing, abs warning beep and no speedo or odo

I unplugged the battery and will let it set overnight, any other way to reset the abs?

I would think cleaning 30 years of grime would help ?

[Planter91C2]

Hello everyone,

Finally got this fixed with a new abs relay. I assume the fry in the speedometer may have burned out the relay. I cut the old relay apart and there was no burned or obvious issues.

FYI the abs relay does rattle when shaken, I’ve asked this for years and no one helped, now we know.

Thank you all for all your help, it is great to finally have everything working