Series 5 tacho

[petersimpson]

The tacho on my Alpine does not seem to be well calibrated. At low revs it is very sensitive, i.e. blip the throtel and the tach zones to 2,000+ rpm. But it loeses the sensitivity at high reve, i.e. even flat out the tach does not go above 5,500rpm.

Any ideas?

Thanks

Peter

 

[Jeff Scoville]

More than likely the tach needs going through.
The sensing wire as well as the capacitor inside are more likely failing.

 

[Alpine 1789]

The tachometer can be calibrated pretty easily; there is a potentiometer in it that allows you to calibrate it to be correct at one specific RPM, which should make it more accurate through the range. There used to be some detailed instructions online, but I can’t find them anymore. To the best of my recollection, all you have to do it open it up (give it a good cleaning while it is apart), find the potentiometer, then drill a small hole though the back of the housing opposite the pot. Put the tach back together and re-install it. Hook up a separate dwell/tach so that you have an accurate reading. Then, it’s as simple as running the engine at the speed you want to set it for (I set mine for 3000 RPMs) and using a small screwdriver to adjust the pot until the car tach matches the dwell/tach reading. As I recall, I had to do the adjusting on my back under the dash, so it helps to have a second person to hold the accelerator to the desired speed and read the tachs. You could also temporarily rig a longer harness so that you can hold the tach while you work, but I just asked my wife to help. I put a piece of tape over the hole when I was done to keep dust out.

At least, this is how I remember it working. I am sure someone will correct me if I am wrong.

 

[V_Mad]

Yes, you can drill a hole to access the pot, and calibrate it in situ.

The condition/timing of the points can affect the rev counter. Make sure points are good, and timing/dwell is correct before suspecting the tacho.

Having said that, they do suffer from accuracy problems. I wish mine would read as high as 5,500 rpm!! Seems OK to about 3000 then it goes to sleep!

 

[64beam]

Does anyone know the value of the capacitor in the tachometer that gives the problem?

Thanks, Robin.

 

[Jeff Scoville]

.22uf
Radio Shack p# 272-1070

 

[Andy Kayll]

This Scares Me

Just looked up Tacometer on the board as if I believe what my SV Tacho is saying I have a slip diff somewhere in the drivechain...
At present my gearing is all messed up as the rear wheels are around 6% to small (although 270 wide which is GREAT for grip) but that is how I got the car.
I have no overdrive and (haven't checked this yet but will get my shaft Tacho out in the morning) 5000 Revs seems to give around 75mph most of the time but it can go down to 60mph (seems to be when the lights are on...) The speed is taken from a Road Angel GPS Unit as the Speedo wanders around the dial as it thinks fit (well... +- 10mph).
What scares me, having read previous posts, is how stupidly inaccurate the Tacho seems to be... Given that I am running so close to red line (can anyone give a definitive on valve bounce) and could be 500rpm off disaster I would like to think my Tacho was accurate. Can the Tacho reading vary depending on supply voltage? If so I'm going to fit a modern Silicon regulator (as well as bigger Dia Tyres).
Andy

 

[Wombat]

Andy

I suppose it is possible that the supply voltage to the tacho can affect its reading, but if the voltage is varying by enough to do so than I thinkyou have other problems in the electrical system that you need to track down. Begin by checking the battery voltage with the engine running and the lights off, then on. It should be about 14 volts and should not vary much when the lights are turned on. Also check some of the voltages behind the dash, particularly the supply to the tacho. 40+ years of corrosion can cause lots of problems with electrics.

As for the Tacho itself, it has 40 year old electronics in it, so it is quite probable that the components are drifting or beginning to show signs of failure. Maybe someone out there makes replacement electronics for them ....

An Alpine with stock gearing (3.89:1 diff and 165/78R13 tyres) does about 18 mph per 1000 rpm, so 4000 rpm should give you about 72 mph (or 6% less with the wheels you have.

 

[sunbby]

Just looked up Tacometer on the board as if I believe what my SV Tacho is saying I have a slip diff somewhere in the drivechain...

I have no overdrive and (haven't checked this yet but will get my shaft Tacho out in the morning) 5000 Revs seems to give around 75mph most of the time but it can go down to 60mph (seems to be when the lights are on...) The speed is taken from a Road Angel GPS Unit as the Speedo wanders around the dial as it thinks fit (well... +- 10mph).
Andy

If something is slipping, like the clutch, I would think you could easily hear the difference, and when going on and off the throttle or up a hill you could hear the engine wind up. I've had various vehicles with clutches that went and they sounded like weird CV type transmissions, (think snowmobiles).

My tach behaves as yours, the same road speed gives varying rpm readings, although the differences are not as drastic.

I doubt your car uses 5000 rpm to go 75 mph sometimes and 5000 rpm to go 60 mph sometimes. I would be almost certain your tach is inaccurate. It is probably not supply voltage. There is also a thermistor (resistance changes with temperature) in the circuit to compensate the temperature dependencies of meter movement and transistors. I think the issue with my tach is most likely related to heat combined with circuitry age. You say the reading changes when the lights are on, is that mostly at night when it's cooler out? Have you tried turning the lights on and off during the day? If it really is supply voltage related (and your charging system/battery would have to be marginal) turning the lights on and off should make the tach jump.

 

[V_Mad]

The tach should not vary with supply voltage as it has a built in regulator that allows it to work accurately over the normal battery variation range. But if you are revving your engine close to limits you really need to get your tach overhauled and calibrated, or replaced.

For example, one common fault is that, since the potentiometer allows a very large adjustment range (to cover 4 to 8 cylinders) this contributes to some instability. To improve stability, this needs to be changed to a smaller value in series with a fixed resistor. Another common fault is the 0.25uF capacitor, which can fail and needs replacing.

Dont forget to make sure the proper metal clip (pole piece) is fitted to the pilse transformer at the back of the tach.