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MG MGA - 'B tach problem - developments

A few weeks back, I started a post describing a problem with my new 'A with a 'B engine (1GB) and tach (RV12401/00B) that was reading approximately twice what my ear says it is. At 60mph the tach is reading around 6000rpm. I guess I'm the only 'A that'll cruise all day at 7000rpm in top gear!

Here's what I found when I pulled the tach out of the dash. It look like a DPO did some rewiring when putting the 'B tach in. the wire going to abnd forming the loop on the back of the tach is a relatively heavy pauge wire. Could the gauge of the wire be affecting the magnetic field generated?

Also, since the direction of the loop has to be reversed to convert such a tach from +ve to -ve earth operation, is it possible that the DPO has the loop running the "wrong" way? The car is still +ve earth and the tach is marked "4 cylinder Positive Earth".

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

Refering to the L/H drawing (before cutting) on the above page, should the taped wire go to the key or the coil? It must make a difference or we wouldnt be cutting and reversing the connections!

I do have another, identical RV12401/00B in stock, off the shelf (isn't it wonderful having so much "stuff"!) But before I install it to see if it works differently, I wanted to get opinions on the current setup wrt wiring.

Thanks in advance.

TTFN and cruising all day at 6000rpm!

Derek Nicholson

If the tach works at all it is wired correctly. You may need to remove the case for access to the inside at back. There should be an adjuster screw there where you can re-calibrate the output indication.
Barney Gaylord

Thanks for the answer. I've taken apart my spare tach and am looking at a white plastic wheel with what looks like a slot designed for a screwdriver blade. A slight variation of the tach that appears at:-

http://chicagolandmgclub.com/driveline/0505/mgb_tach.html

in the following picture:-

http://chicagolandmgclub.com/driveline/0505/pcb_hole.jpg

my white plastic wheel is located where the black "thing' is to the left of the brown resistor. Another difference I notice is in the colours of the band on the resistor. The above picture seems to have brown-black-brown-yellow. Hard to tell. But mine seems to be white-brown-red-brown. There's also a green wire already in mine that they say should be added. I'm starting to wonder if the tach he's working on is an RV12401/00B!

I know they can be adjusted, but a 2:1 ratio just seems a bit more than an adjustment.

Also, as I said initially, I'm concerned that the wire is, perhaps, going into the loop from the wrong side. It must make a difference, otherwise why cut, swap and reconnect.

Damn, I hate electronics. Maybe I should just put in a mechanical tach.

Derek Nicholson

The picture you refer to is the "other" odball tach which is more rare, presumably a later issue which had a short production run before the switch to negative earth and alternators.

That adjustable potentiometer has quite a wide adjustment range. Give it a tweak and see if it goes far enough to give the proper engine speed reading. Also send me a picture if you can. I can never find a picture of the inside of this more comon tach.
Barney Gaylord

Derek, -- forget the pictures. I have a RV12401/00B tach (not working) which I can use for the pictures.

The tach conversion article in the CMGC newsletter is the "other" type of pos earth tach. I don't know if the model number is same or different. I will check with the owner to verify the model number of that tach.
Barney Gaylord

Okay, I checked, and here's the skinny. Derek has a RV12401/00B with the green wire. I have a RV12401/00B with the green wire. Our club tech coordinator has two RV12401/00A also with the green wire, and I don't know what the difference is between A and B.

The tach with internal direct ground and no green wire also has model number RV12401/00B as shown here: http://chicagolandmgclub.com/driveline/0505/mgb_tach.html
This one also has different resistance value for the main power resistor, along with other internal differences. But it still has the same model number as the ones with the green wire.

The logical conclusion (I hope valid and logical) is that the tachs with and without the green wire have the same model number because they are functionally identical and interchangeable. The difference between the A and B models will be something else which would affect interchangeability (perhaps a small difference in appearance of the face plate). Bottom line then is that you have to look inside the instrument case to know if you have the one with or without the green wire.

I have been taking close up pictures of the model with the green wire, which I will be posting on my web site very soon along with current instruction page for positive to negative ground conversion.
Barney Gaylord

Well, ain't this sweet! I finally swapped me spare RV12301/00B for the misbehaving one in the car.

Guess what? It's even worse now. At a very slow idle, I'm showing just under 3000rpm. I haven't even tried driving it on the roadbecause, assuming the screwup is linear, I'll be pegging the tach at around 45mph in 4th gear.

And the plot thickens. I've just opened up the one that was originally in the car and it appears to be a third variant with the RV12401/00B on the face.

The pot in the picture:-

http://chicagolandmgclub.com/driveline/0505/mgb_tach.html

is all black. The tach I had apart first had a white pot.
The one I just took out has a white and black pot. I guess the good(?) news is that it does have a green wire running the same as my first tach.

So, I wonder how many different versions there are with the same part number?

Maybe I'll just put in an aftermarket gauge.

Barney, I'll send you 2 or 3 photos.

TTFN
Derek Nicholson

For what it's worth, the RV12401/00B in my posession has the pot with black over white knob, along with the green wire. I was just doing the minor surgery to reverse polarity to day, and I should have the pictures on my web site shortly.
Barney Gaylord

Hi Barney. I've a couple of follow-up questions:-

1. Any suggestion as to which way to turn the potentiometer? CW or CCW?

2. There is what appears to be a ground wire installed from one of the screws that attaches the works in the tach housing. Is this standard?

I guess I'll be able to hook up the "guts" of the tach out of the "can" while I make an attempt to adjust it. I'll try the tach that was originally installed as it's "only' reading twice what it should!

TTFN

Derek Nicholson

Well, it seems to have worked. I hooked up the tach out of its can, complete with a ground wire. Turning the potentiometer CCW, I was able to get it to read 800rpm at idle. Just a guesstimate, as I don't have another tach to check it against. But, "close enough for government work".

TTFN

Derek Nicholson

Typical Government work. Over budget!

Took the car for a little spin (to pick up the mail). Seems my ears need recalibrating. At 2000rpm, the speedo was indicating 44mph.

You don't suppose I've found a way to cruise at lower revs without the expense of a 5-speed transmission conversion?

What's a 1600 with 165x15 tires and 3.9 diff supposed to be turning/1000rpm?

TTFN

Derek Nicholson

After a few days work I have the electric tach installation information on line here: http://mgaguru.com/mgtech/electric/et202c.htm

I had a chat with John Twist, and I still can't say which way the induction loop has to go, and not sure it would be the same for all original positive earth installations. I would guess going from switch to coil try clockwise loop for positive earth and anti-clockwise loop for negative earth. If not, then reverse it. Just be sure it works before final installation of the induction wire.
Barney Gaylord

What are the symptoms if the loop is reversed? Does the tach needle just sit there and not move?

Derek Nicholson

If the loop is backward it just doesn't work. If the battery polarity is switched it also doesn't work. In either case no harm done, and when you get the connections right it should work again.
Barney Gaylord

This thread was discussed between 22/09/2006 and 26/09/2006

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