Help from electronics geniuses

[tuttle]

I'm at my wits end trying to find a fault with my tacho.
Symptom: At startup the tacho appears to read tick-over normally,occasionaly fluttering very slightly.As gentle accelerator is applied needle will drop,however,when out & about driving normally the rev range up to 2-2.5k is fairly consistant.Above this needle drops again.
Other stuff:Car is a Japanese import('95 R33skyline gtst)The dash gauge is the speedo & tacho single unit type working on a single pcb.
Voltage etc readings from the dash end & including the necessary kph-mph converter & de-limiter appear to be withing manufacturers' limits.I'm 99% sure the sensor is working ok as I can still get a good real-time(digital) reading from one of the ecu screens.
Question:Is it possible to test the unit out of the car & how?
Any info/suggestions gratefully appreciated.
Thanks in advance.
Matt.


>> Edited by tuttle on Thursday 9th June 21:49

[dilbert]

I'm no genius, but I'd say that it sounds like a dodgey connection somewhere.

I'm guessing that the tacho is a calibrated ammeter (analogue dial), and it is sent either a variable voltage or a variable mark/space pulse train, in proportion with the engine speed.

If the ECU knows accurately what the engine speed is, you need to find out is the tacho gets it's information from the sensor direct, or indirectly from the ECU. If it's indirect, then the problem must be on the guage side of the ECU.

You can probably test the tacho guage it's self using a potentiometer and a car battery. If it's the wiring, then it's down to pins and a continuity tester.

[steve_D]

Dashboards suffer with dry solder joints. I fixed a Renault 25 dash buy soldering more than 30 broken joints on the back of the dash.

To run the unit on the bench will require holding the sensor next to something steel (bent 6" nail) running in an electric drill.

Steve

[tuttle]

Thanks guys,looks like a weekend of tracing along all the wires/connections.
SteveD The 6"nail in a drill.Could you explain that proccess a bit more please.
Thanks again.
Matt

[steve_D]

tuttle said:

Thanks guys,looks like a weekend of tracing along all the wires/connections. SteveD The 6"nail in a drill.Could you explain that proccess a bit more please. Thanks again. Matt

Some sensors are reading a magnet set into the flywheel. Others, which I am assuming you have, are looking for change in general magnetic fields. This could therefore read a hole in the flywheel as a trigger or it could read the heads of bolts on a drive shaft (speed sensing or ABS).
If this is what you have then bend a couple of inches of a nail over at right angles, place it in the drill chuck so the nail head is pointing at 90 degrees to the drill spindle. Secure to your bench/workmate. When the drill is running place the sensor close to the nail head that is flying around. The whole thing needs to be pretty secure as the sensor may need to be as close as 1-2mm. There is plenty of potential for flying sensors, ripped off fingers and the like so be very careful.
A variable speed drill will mimic various engine speeds.

If you have the magnet type then you could try gluing a magnet to the nail head. Not tried that one but it should work. Same warning as before but now read magnets ricocheting of the garage walls.

Steve

[tuttle]

SteveD,That makes sense-tho a little precarious for the fingers! I'll try that out & post up any 'interesting' developments.Many thanks again chap.


>> Edited by tuttle on Saturday 11th June 17:17