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MG MGB Technical - tach testing?

I have a remote tach/dwell meter that I sometimes use when working inside the engine bay. The connection of such meters is rather simple. Just wondering how one could (should) make a similar connection(s) to test function on OEM (Smith) tachs, if removed from the dash?
One other question - the tach out of a '79 B has 3 connectors on the back. One is an obvious ground (flat male spade, riveted to case). The other 2 are separate, but close together - insullated from the metal case. One of these is a spade connector and the other is bullet type connector. What feeds these 2 connectors? I can't tell from the wiring diagram.
Steve Buchina

One is for the light?
Dan Robinson

Steve. I think you will find that one is for the power and the other for the connection to the distributor to sense the frequency. I think the bullet is for the dist.

To run the tacho out of the dash would simply require an earth, power and the input from the dist.

Tony
Tony Oliver

Depends on the tach. RVI (on the face-plate) tachs used up to and including 1972 are current triggered. On these the coil wire runs through a transformer-type pickup. On early versions the pickup was external and a continuous loop of white wire from the main harness is routed through it. On later versions there are male and female bullets on the back of the tach and corresponding bullets on two wires coming out of the main harness.

From 1973 on RVC voltage-triggered tachs were used which have a single bullet that a white/black from the coil -ve connects to.

All the variations have a standard-sized spade for the 12v supply (another white wire on early cars, green after that) and a smaller spade for a black ground connection. These last two are to power the tach electronics.

For the later RVI and the RVC tachs if you want to test them using the distributor i.e. the engine then simply connect up the 3 or 4 wires, there shouldn't be any way of getting the wires mixed up.

For the earlier RVI tachs with the external pickup as well as the possibility of a 'foreign' tach having been converted or not, your car converted or not, you also have to get the loop of white routed through the pickup in the correct direction to drive the tach. getting that the wrong way round won't cause any damage, but getting the 12v and ground supply the wrong way round might.

To test a dashboard tach on the bench you need a way of generating pulses as well as a 12v supply. The 12v supply for the tach electronics needs to be 'smoothed' i.e. from a battery, connected to the two spades *with the correct polarity*. 64-67 tachs used +ve ground, 68 and later -ve ground. However many 64-67 tachs will have been converted to -ve ground, so you will need to use the same ground polarity as the car they came from.

The trigger pulses can be derived from many conventional battery chargers. If you have an RVI tach you need to have a circuit from the charger +ve, through the tach pickup, through a suitable load such as a 3 ohms resistor or coil, back to the charger -ve. Higher value resistances may not result in enough current through the pickup to trigger the tach electronics.

For an RVC tach you need to have a load connected directly to the charger terminals, and as well as that the +ve charger connection to the bullet on the back of the tach, and the -ve charger connection to the earth or ground spade on the back of the tach. You should be able to use a higher resistance load such as a bulb for these.

It then depends on whether your charger uses a full-wave rectifier or a half-wave. The full-wave will deliver 100 pulses per second which equates to 3000rpm, and the half-wave 50 pulses per second which equates to 1500 rpm, and in theory this is what you should see on the tach.

Paul Hunt

Thanks Guys -
On a rainy afternoon and being somewhat bored, I put together a few odds and ends and came up with something to bench test my distributors. It seems to work, at least I can check the dwell, plug wires or a loose plug. Just trying to figure how I can use this set-up to check tach function and perhaps distributor advance.

I'll take some pictures tomorrow and post in the evening. With your help, would like to define some protocols for wiring the tachs (early and later models) to this appratus - don't want to fry the internals.

Steve Buchina

Did much the same thing myself with a variable speed drill and calibrated disk with pointer. Initially created it for a distributor spring project, but it was very useful for setting dwell when I changed points - much easier than on-car for various reasons. I say *was* as I am reluctant to use it any more, having destroyed two digital multi-meters that were each only a year or two old. I don't know whether the differences in wire lengths and impedances were causing HT to get into the meters or what, but I'm reluctant to use my third multi-meter the same way and risk blowing that up as well. For the same reasons I wouldn't use a dashboard tach either - unless I landed on a free one.

Tachs should be connected as per my previous post, the only tricky area is working out whether an unknown quantity originally positive ground tach has been converted to negative or not. The only way may be opening it up and seeing just how the circuit board is connected to the spade and case. If the car it came out of is known to you, and the tach was working before you took it out, then it should just be a matter of determining what polarity the car is.
Paul Hunt

I took the pictures, but can't get them to upload. I get to the upload window, browse to the picture file and click the submit button - then it just sits there doing nothing. Perhaps tomorrow.
Steve Buchina

You have to click Submit on the Browse window, then Close on that window, then the picture should appear with your message and you click Submit again. However I've just tried it and a couple of times nothing happened after the first Submit, just an hourglass if I hovered the cursor over certain parts of the Browse window. Other times it was OK.

How big is the picture? There may be a maximum size, try scaling it down to 800 pixels in the largest dimension.
Paul Hunt

Another try.


Steve Buchina

Lets try another.


Steve Buchina

Prettier than mine :o)


Paul Hunt

Paul,
Kind of neat that we came up with the same basic design. Mine works fairly well for testing distributor function and settings (e.g., dwell), but it has some problems. The pulley drive system doesn't give me a stable timing mark (note black timing disk at bottom of drive shaft - belt slippage I'm guessing) and having only 3 speeds, presents some limitations on what I can do with it. I have a small variable speed motor hidden away somewhere that I want to try. Thing came from a lab auction and is fitted with a rheostat and rpm meter.

Noted your timing/advance wheel - how is it hooked up? What are you using for your 12V power source? As to measuring voltages, I've been using an old analog (?) meter that I also pick up at an auction. Think it came out of physics lab and probably dates from the 60's.
Steve Buchina

Steve - mine isn't very stable either. I originally put it together to do some centrifugal spring analysis, but it isn't really accurate enough to get meaningful measuremens between different springs so I abandoned it. If only I could get my hands on a Sun distributor machine ...

The timing wheel is sitting on top of an old Black & Decker right-angle drive I have had for years, with an additional 'straight through' output shaft to drive the distributor. I did it that way because the right-angle drive also gives a 2:1 reduction, however I turned it round to give a 1:2 uplift to drive the timing wheel, which replicates the real-world relationship between distributor and crank pulley. Quite neat, I thought at the time, however some of my instability is probably due to backlash in the gears, so maybe not so neat after-all. I was able to get over some of that by putting a resistance brake on the timing wheel in the shape of a Mk1 digit!

Power source comes from an old 12v battery with a charger connected. Basically the charger supplies the current and the old battery smooths out the pulses from the charger. I've got a couple of old analogue meters, both dating back to the 60s as well. One I bought myself in my teens, the other (a genuine AVO) came from my employer when I started work in telecommunications. They are both fine (they are on their 3rd and 2nd set of leads respectively, the rubber insulation on the others having rotted over time), I was using the digital instruments for their tach and dwell functions rather than voltage.
Paul Hunt

Paul,
Yes - a Sun machine would be nice. Last one I saw was up at University Motors. John Twist pointed it out, but didn't demonstrate. What all does it do - exactly? Course, trying to make one from scratch can be an experience to enjoyed. Somewhat like a puzzle - I think. Keeps the mind active.

Guessed you might have a right-angle drive hidden behind your wheel. Didn't know that B/D made one with a through shaft, though. I have an old one myself, but it just goes at a 90.

Did a little searching last night and today, but still haven't found that variable speed motor I mentioned previously. Did find a bunch of other stuff that I forgot I had, though. Think I need to construct a Lost-Found-Forgot cabinet/room to put such stuff in. One of the items found today was an oscillating/shaker platform (lab auction). It has a small, but strong drive motor fitted with a rheostated speed control and rpm indicator. Appears to run from 0 to about 3,500 rpm. Think all I would have to do is modify the drive head on the motor shaft. It's now fitted with a disk shaped head with an offset drive peg (i.e., causes platform oscillation). Shaving the peg off and slotting the disk should give me a good distributor drive.

As a power source, I've been using a very old battery charger. Think it dates from the late 50's or early 60's - belonged to my dad. The other day, however, I heard some electrical popping coming from inside the case - so it's now retired. I'm now using a Radio Shack 12V power supply - seems to work OK.

Back to tachs -
I've got a bunch of tachs that I need to test. Most are the older, positive ground variety (3-synchro cars). A few are from the negative ground B's. Knowing nothing about electronics, except what a resistor does, I haven't a clue on what one shouldn't connect. If I take some pictures of the internals, can you offer some advice on how to test - safely? THANKS
Steve Buchina

Ah, B&D didn't make one with a through shaft, I modified mine :o)

Basically the Sun machine spins the distributor at speeds controlled by the operator, has a strobe positioned above the degree wheel at the base of the distributor, and has other meters displaying voltage, speed and dwell.

Happy to help with tach testing, you might want to take this offline, email address should be visible with my posts.
Paul Hunt

Paul,Steve; my advice would be to stop by any car dealer in your town that has been around forever, somewhere tucked away in the backroom, often in the parts room, you will find a distributor machine covered with dust and in the way. Make an offer, thats how I got mine. Ric
RIC LLOYD

Paul,
I'm not seeing an email address with your posts.

Ric,
Good suggestion - I'll poke around.
Steve Buchina

Steve - you're right, I was going by the personal info displayed when *creating* a post. It always used to be, together with my web site address, but someone keeps mucking about for some reason and I occasionally have to go back in and recreate them from scratch.

Any road up as they say round these parts, it's http://www.mgb-stuff.org.uk/
Paul Hunt

Ah, email and web address back again.
Paul Hunt

Thanks Paul -
It will be a week or so before I can get back to you - going out of town to visit family.
Steve Buchina

Just noticed I gave the web address instead of the email address! Must be lack of beer, spotted it now I've just got back from the pub.
Paul Hunt

This thread was discussed between 13/11/2008 and 21/11/2008

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