electric speedo drive testing

[gelmonkey]

Hi All
Bought an electronic Auto Meter speedo last year which self calibrates at the touch of a button and a 2 mile drive.
Problem is all the connections are good, the speedo does what its supposed to do on start up but will not show a reading once calibrated.
I bought a LT77 speedo drive and it will give me the correct pulses to match with the speedo.
The needle jumps up and down on occasion seemingly tyring to show me a speed but is not constant .
Is there a way for someone like me with basic electrics knowledge to test this drive for correct function please.

cheers
P

[Mark]

This might sound like a daft question but have you tried switching the wires around from the speed sensor ?

I remember from fitting Emerald management on a car I had that the crank position sensor had a positive and a negative pole.

Perhaps speed sensors work on the same principle and the polarity has to be correct ?

I'm just guessing though

Mark

[ChrisJC]

I am pretty sure the SD1 sensor has +ve, -ve and signal, i.e. 3 wires. I wouldn't advocate swapping any around unless you're sure.

Have you any data on the gauge?

Chris.

[DaveEFI]

I am pretty sure the SD1 sensor has +ve, -ve and signal, i.e. 3 wires. I wouldn't advocate swapping any around unless you're sure.

Have you any data on the gauge?

Chris.

Only two wires to the one on my SD1 - they can and do fail and I replaced mine recently.

The plug is polarised - but then its the same style as all the rest on the car, so doesn't mean polarity is important. My guess is it isn't - just a simple AC generator with a pulse counting amp in the speedo. Unlike a VR sensor, it doesn't really need to know the rising edge of the pulse. All IMHO.
There's a thread on the SD1 club forum about fixing one running at the minute. By Ramon Alban.

[ramon alban]

I bought a LT77 speedo drive and it will give me the correct pulses to match with the speedo.

Is there a way for someone like me with basic electrics knowledge to test this drive for correct function please.

For information, I recently repaired an intermittent LT77 Speedo Transducer, full details are on a PDF which can be downloaded from here:

http://www.vintagemodelairplane.com/pag ... cer02.html

The article also describes a novel method of testing the unit.

The repaired item looks like this

[gelmonkey]

Hi All
Sorry to bring this one up again but I am having a mare with this problem as it's still not fixed.
I have looked on auto meter's site and found a way to test my sender but still get no reading from it.
From Ramons article I can see that the brown wire is the output to the speedo but does yellow and black wire have to have a 12v feed to it or does that go to earth.
Auto meter say that a 2 wire sender has an output and the other to earth.
Someone help me please as this is driving me nuts.

Thanks in advance
A confused Gelmonkey

[DaveEFI]

On the SD1, the sensor is fed from + 12v. White wire in the loom. Output to speedo is black/red.

[ramon alban]

Like Dave says, this applies to the SD1.

The Yellow/Black wire at the transducer converts to White in the loom and is the 12 volt feed for the electronic circuit from the ignition switch.

The Brown wire on the transducer converts to Black/Red in the loom and feeds the speedo with pulses.

The earth for the electronics is the body of the transducer.

Because you have made no reference to the car or the application, is it possible your engine and gearbox are not actually earthed (yet) so the transducer circuit cannot function?

[gelmonkey]

Hi Guys
The car is a Cavalier Mk1 with 4.3 V8 and LT77.
Engine and box are earthed to very good points on the chassis so dont see any problems there.
The confusing thing is is that just once it worked counting pulses (as shown on the Auto meter website) whilst driving and in calibration mode.
I do not know if the speedo is dodgy or whether it is me being a pleb.
Muscle Manta fitted a very similar electronic speedo to his car and it works fine and is very accurate.
I am going to jury rig the system up tonight on my bench and see if that works using all new wires.
The wires currently fitted are all brand new but there may be a problem that I cant see.
Will keep you updated or be screaming for help again.
Thank you to those who have helped so far.

cheers
Paul

[DaveEFI]

My SD1 wiring diagram shows no ground to the sensor. Nor can I see why it would need one?

[gelmonkey]

Hi Dave
I understand your comments completely.
The Auto Meter wiring diagram however shows that the sender should be earthed.
Bearing in mind that this is a 200 dollar speedo and the fact that I dont want to bugger it up makes me sit on the side of caution and follow their instructions albeit even though it looks wrong!
Also remember that earlier on today I said that the speedo has looked to be calibrating just the once with the transducer earthed and it gets even more complicated and that is why I find myself struggleing with this particular problem becaause nothing seems to makes sense.
I dont normally have this much trouble with basic wiring.
Will let you knowe how I have got on later this evening.
cheers
P

[gelmonkey]

Well still none the wiser.
Have tried with the sender earthed and nothing and the same with a 12v supply to the unit.
Interestingly though when I touched the black and yellow to earth the pulse reader on the speedo jumped up in increments.
Auto meter say that the speedo calibrator reads pulses from the sender which it then converts to a correct speedo reading.
I have seen this thing work on the internet and it looks so simple.
Also because Muscle Manta uses this same type of speedo I thought it would be just simple and straightforward.
How little did I know.
Any suggestions greatfully accepted.

cheers
P

[DaveEFI]

Is there a schematic available for the Auto Meter?

I assume the generator is just an alternator producing a near enough sine wave. Does the speedo give a spec for the input it expects to see?

I'm interested as I'm in the process of building an interface to drive an SD1 speedo on a car with a different gearbox using a sensor on the propshaft. But haven't yet seen the sensor.

[ramon alban]

My SD1 wiring diagram shows no ground to the sensor. Nor can I see why it would need one?

You are right Dave - not shown on the diagram. but I'm lost regarding tyour second observation. Never did transducers as school.

When I was testing my rebuilt transducer off car but connected to the loom I also had a big croc clip from the tranducer body, to chassis earth and just assumed it was needed,

What I did not do was test it with the earth not connected. too late now.

But how does the transducer electronics get its return path from the 12 volt supply?

Is it simply Via the input circuits to the speedo, cruise and or trip computer.

Something wierd happens because cruise still works when the speedo is disconnected - as does the TC.

I have just received from Australia an essay with a circuit, on how these system are perceived to work.

I'll Email you a copy.

[DaveEFI]

I assume it's just a magnet or two whizzing round past a coil or two. This would produce one or more sine waves. Likely more than one, superimposed on one another but phased so the peaks and troughs are equally spaced. In other words a series of pulses. That two wire output could have a common ground or common 12v depending on the speedo electronics (triggers on negative going, rather than positive going pulses).