MGB & GT Forum

The tach on my 67B is off at idle by about 300 rpm, and then 6-800 rpm 3-4K. (confirmed with clip-on tach). Presuming an electronic tach - is there a way to calibrate, at least some, easily (I thought I saw a post about buttons on the back - haven't checked). I haven't checked to see if they put an electronic ignition in the stock distributor (it's definitely not a Pertronix/Mallory) since it runs so dang well - I presume that would require some re-calibration (?).

Edit: Despite the rumors - this is a '67 and on Moss (http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=123897) looks like at least there were mechanical tachs between 62 and 67. I wonder if I actually have a mech tach and need to lube/get a new cable.......

It may be my old ears - but I'd swear there is a rpm-synchronous clicking sound coming from the dash area - which made me think it was a mechanical tach but I haven't dug around much yet. Sound may be coming from the engine/transmission somehow - but it is definitely right in front of me and near the dash/footwell area.



-M



Edited 1 time(s). Last edit at 2015-10-16 10:28 AM by mferris.

Yes, there is an adjustment screw. I can't remember, but it may be inside.

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Cogito ergo sum periculoso

The 3 main bearing cars used a mechanical tachometer via a cable/pinion off the cam shaft. The 64 thru 67 5 main bearing uses a Smiths electronic current sensing tachometer. There is an adjusting potentiometer on the inside circuit board. If adjusting, I would use a car analyzer that can read your RPM to assure correct indication. The removal of the tachometer is not a fun task.

Over the years, the internal electrical tach components will deteriorate to the point that the tach's reading is way off. At this stage of the game, it's best to send it out to be recalibrated with new components. There are several specialists that do this. Nissongers is one of them. RAY

Very common issue with old tachometers.

If your tachometer says Smiths on the face, it is almost certainly electrical. The early mechanical units were labeled Jaeger.

Likely the unit will be an RVI 2401/00B (or similar - the RVI notation indicates it is a current sensing unit). Pull it from the dash by reaching under the dash and unscrewing the thumbscrews that hold the studs on the tachometer's case to the metal bracket. There should be a white wire coming from the ignition circuit in a loop through a clip held to the back of the tachometer (that provides the pulsing ignition current which is sensed by the tachometer).

If you take the innards out, there is a potentiometer (variable resistor) you can adjust. However, you may struggle to achieve calibration by tweaking this alone.

At this age, the values of the resistors and capacitors in the tachometer will have shifted. In particular, there is a thermistor (heat sensitive resistor) which fails with age. This is why your tachometer could be in good calibration when starting cold, but then drifts higher for the same real RPM when the car heats up.

A member on this site called Tom Hayden used to sell a small package of resistors and capacitors along with a circuit diagram to enable you to switch out the tired components for new ones. Here is an excellent article he put together:

http://www.sunbeamalpine.org/downloads/alpine_tach_repair_rev5.pdf

Also, Herb Adler (ozieagle) on this site is a bit of an expert on these things I believe.

Cheers,
Mark

Well - my tach has been off by 200 RPM since it was new - and is still off by that much after 40 years.

At least on the later RVI tachs there is an adjustment potentiometer. What this actually adjusts is the width of the pulse that is generated every time the ignition fires. By tweaking the pot, you should be able to compensate for most age related drift. If things have drifted beyond the ability to adjust for, then it is likely the capacitors that have drifted. Resistors tend to be fairly stable with time (unless they are overloaded - unlikely in the case of the tach).

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Hi Michael,

I suppose having been branded as somewhat of an expert I should chime in.

There are two different RVI tachos, one for positive earth vehicles, which is the one Tom Hayden describes, and a different one for negative earth..

They are both adjustable, with a potentiometer on the circuit board, but the positive earth one can't be accessed without removing the works from the can. With ,ine I drilled a hole in the back of the can, to allow access. Smiths must have woken up to this and the negative earth ones don't need to be disassembled.

I think that your car probably is negative earth, so access is fairly easy.

Also the negative earth tachos only have one electrolytic, that acts as a filter for the triggering pulses. A crook one of the usually shows up as a flickering pointer. The other component that may cause problems is that thermistor, mentioned above. Tom details how to replace.

For you info I have attached an article with the circuit diagram and a photo of the innards.

Herb

Based on Michael's VIN, he has an early '67 model year, which would be positive earth. If it has previously been converted to negative earth, it is likely that the earth and power connection on the positive earth tachometer has been switched and the ignition wire loop reversed (as per the following):

http://mgaguru.com/mgtech/electric/et202a.htm

By the way Michael - beautiful looking car - love the color!

Here's exactly what you need.

http://www.mgexp.com/journal/sweep/7626

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Thanks guys. My car is definitely negative earth (it even has a sticker) and a single 12v battery. But I think the previous owner had a very thorough restoration done and I don't know if it was originally positive earth. See pictures. I'll take the tach out at some point but it is very stable just not accurate enough for my taste.

A 67 tach was originally positive. My 65 positive earth tach reads pretty high, starting at around 200-300 rpm at idle and around 900 rpm or more near the orange area. It drifts a bit with temperature. Likely I'll replace the capacitor, maybe the thermistor and re-calibrate when I switch to negative earth.

My speedo is also wildly optimistic. Off by around 5 mph at 25 to 10 mph at 65. Error is proportionally greater at higher speeds. Reading threads here and other internet sites it seems the speedo is much harder to correct. Renewing the cup bearings or coil spring is beyond my skill set let alone access to parts. BTW it's the correct speedo for a non O/D three synchro. It was frozen when I bought the car. This broke the angle adapter. But, I unfroze the speedo and was able to fix the angle drive.

If you drive by gauges my MGB it is an impressive car, snappy acceleration, high top speed and quick revving. (Ignore the wavering needles). I drive by ear for the most part and just keep up with traffic when possible.

Someday maybe I'll fix it all.

Have fun. Ditto the nice looking car.

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Bob McCoy
1965 Pull Handle

OK Michael,

Your car was positive earth and has been converted. On the tacho it definitely says positive earth.
So Tom Hayden's article is the one you need to work to. While you have the tacho out I would suggest that you black out the word positive, and if possible replace with NEG, using Letraset or similar. This will stop confusion later on.

Herb