Rev counter behaving badly

[dlgis]

Hi All, my rev counter is jumping around from 5-1500 rpm, and doesn’t go higher than that. It was working fine but 2 things have been done since it was. The car has been completely rewired, and I cleaned the face and glass of the rev counter and fitted a new shiny bezel. I’ve tried fitting a good known working rev generator, and that doesn’t make any difference - still jumps around between 5-1500 rpm.
Can i safely test whether the rev counter is working by connecting the 2 wires to the generator straight to a battery? I seem to remember reading that this would result in a constant rpm reading?

Thanks

[mgcjag]

Have a read here.... viewtopic.php?f=2&t=16474&p=135321&hili ... st#p135321

[Jack the lad]

DO NOT connect the rev counter to a battery, you will destroy it. If you have an electronics engineer local to you, they will have a signal generator which they can safely connect to the rev. counter and they can vary its output to test the full range of your rev. counter. A common multi-meter, (Ebay a good one for £10) will enable you to test the generator output, but having changed it for a known good one, seems to be OK.

[Heuer]

If it is an S1 then there is a generator on the rear right hand cam cover. It works by generating a voltage of 1V AC per 100 RPM. They are prone to failure as the magnets get weak. To test remove the generator (three Allen screws - no need to remove the cam cover) and attach the spindle to an electric drill with a known (or stated ) maximum speed. Connect an AC voltmeter to the contacts. Holding the generator spin up the drill to maximum speed and note the voltage. If your drill is rated at 3,000 rpm you should see 30v on the meter. Any lower and you need a replacement.

[abowie]

It works by generating a voltage of 1V AC per 100 RPM.

You can also test the gauge by connecting it to an AC power supply.

Home alarm systems use 15V AC plug packs. One of these should produce 1500RPM.

Speedograph Richfields can convert your tacho to a more modern circuit board. This can be run by simply connecting the tacho to the negative terminal on your coil.

Note that the gauge has both electrical and mechanical components; it my be as simple as dirt in the mechanism.

[dlgis]

Thanks all I’ll follow up on the suggestions and report back

[dlgis]

I’ve tried running the generator with a 1600rpm drill. That didn’t give a consistent rpm figure on the gauge, the readout just hovered around 500-1000 revs unless I blipped the drill when it would momentarily go up to around 2500rpm. Also tried to take a pic of the generator dog drive - does it look ok?

I’m leaning towards there being something wrong with gauge

[Heuer]

The generator works using magnets to produce a AC voltage pulse. The magnets lose their strength over time which means the generator does not provide sufficient pulses. Before messing with the tachometer, and repairs can be expensive, invest in a new generator (£186) which should look like this:



Your photo seems to suggest something is broken.

[mgcjag]

I have read somewhere..JagLovers.. I think about repair to that drive dog....probably worth a search...Steve

[chrisfell]

I found two things wrong with mine. One was the generator the other was the needle pivot.

The generator.

The original generator didn't produce a high enough voltage. At first I thought it was because the coil had moved relative to the driven shaft. It had moved, but this wasn't the cause of the poor voltage. I became used to the fact that revs never really matched where the gearing and engine speed suggested it should be. Then one day the rev counter suddenly stopped. One of the wires connecting the coil to the terminal block had come away. There was no way my fingers of butter were ever going to reunite the wire with the coil, so I bought a replacement. This did produce the right voltage.

The needle pivot.

I'd noticed early on in my ownership of my E that the revs counter was sluggish, slow to react to changes in revs. A drop of watch makers oil delivered by a sewing needle to the pivot end did the trick. (3 in 1 Oil is a perfectly good alternative).

Several tens of thousands of miles later, all is good. The rev counter reading is where it should be.

[Tommd]

It works by generating a voltage of 1V AC per 100 RPM.

You can also test the gauge by connecting it to an AC power supply.

Home alarm systems use 15V AC plug packs. One of these should produce 1500RPM.

As I read this, yes the tach generator produces 1 VAC per 100 rpms.
And yes, one can connect the gauge to a 15VAC transformer to test the gauge.
However, since the transformer is to "simulate" the 15VAC from the tach generator and for the tach generator to be producing 15VAC, that would happen when the camshaft is turning at 1500 rpms. If the cam shaft is turning at 1500 rpms, then the crankshaft would be turning at 3000 rpms.
Therefore, with the 15 VAC input to the tach gauge, the reading on the tach gauge should be 3000 rpms, not 1500 rpms.
Tom

[Jeremy]

Speedograph Richfields can convert your tacho to a more modern circuit board. This can be run by simply connecting the tacho to the negative terminal on your coil.

That's what I did. Original tacho wasn't working and when I sent it to Speedograph they told me they no longer repair the original circuitry. Generator is now purely decorative. It's nice having at least one gauge in the car that's both rock steady and accurate.

[dlgis]

How much (roughly) did Speedograph charge for that ?

[tim wood]

Consider these;

https://spiyda.com/tachometer-electroni ... dules.html

I fitted one, fairly straightforward and good support

Tim

[dlgis]

Thanks Tim, interesting! Is this conversion suitable for a 3.8 with 123 ignition?

I’m starting to wonder whether my problem might relate to having 123 ignition and the car having been rewired since the rev counter was last working properly. i.e. could there have been an electronic component fitted by a previous owner who fitted the 123 that made the rev counter work properly with the 123 and that got binned when the old loom was ripped out

[tim wood]

Hi Darryl,
I’ve no experience of fitting this with a 123 distributor. I think that Spiyda has previously had concerns with connectivity with 123.

I think that it would be a good idea to speak with Chris @ Spiyda. I found him extremely helpful.

Tim

[dlgis]

[Jeremy]

How much (roughly) did Speedograph charge for that ?

£35 plus vat for complete overhaul including repainting needle and bezel and replating the case. That was back in 2017.

[PierreW]

.
"Therefore, with the 15 VAC input to the tach gauge, the reading on the tach gauge should be 3000 rpms, not 1500 rpms.
Tom"

I confirm these values as I tested the tach gauge with variable DC voltage inputs: here the results, comparative between Etype and MK2

[christopher storey]

I hope you meant variable AC inputs!