Tach wiring problem

I've come across a problem in my MG project that I'm hoping someone can help with. I'm installing an LS1 into the car, along with the engine computer from a 1998 Camaro.

The problem is the tach. Not the speed of the tach, I can deal with that. But I'm just not getting a reading.

Now, it's fairly well known that you need to add a pull-up resistor to LS tach signals to make them 12v pulses instead of 9v. But I'm wondering if my tach is an oddball. It's a 1971 car with what is apparently an "inductive loop" tach based on the information I can find. Here's an awesome snippet from the manual:

Awesome MG manual said: The MGB tachometer is based on the principle of the monostable multivibrator, which has one stable state and one quasistable state. The circuit is normally in the stable state, producing no output. However, when a triggering current pulse from the ignition system is received, the circuit transitions to the quasistable state for a given time before returning again to the stable state. In this way, each ignition pulse produces a clean pulse of fixed duration that is fed to the gauge mechanism. The more such fixed duration pulses the gauge receives per second, the higher it reads. This design requires that the ignition coil current passes through the tachometer on its way to the coil.

I'm going to try to work "monostable multivibrator" and "quasistable state" into conversation tomorrow. Anyhow, the ignition coil current does indeed pass through the tach. Can this be adapted to a more standard tach signal, or do I need a tach from a later MG without this inductive loop design?

The inductive loop type tachs (RVI on the face assuming it's a Smiths) won't work with anything other than a points type ignition as far as I know, unless you want to rebuild the inards. link for conversion

If you want to sticj with a Smiths tach for the looks, get one that is an RVC tach. It should work for what you are doing.

Fixed that link for you.

In reply to peter:

Thanks.

MSD and other have tach converter boxes to keep old style tachs. Not sure if they'd work here but might be worth a phone call.

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I think I'd just go with the later style negative trigger tach. The reason: the inductive loop tachs count the pulses in the wire which supplies power to the ignition coil (that's how the Jensen tach is wired, too). The way I understand it: the points opening and closing cause a much stronger pulse than the electronic ignition systems and the tach circuitry doesn't 'see' all of the weaker pulses. When I went with the Crane ignition in my car, I had to twiddle the calibration potentiometer to get the thing to work reasonably well and over about 6k the tach needle still twitches like crazy. Which is just when you need it most. < The JH community has tried a lot of stuff with varying degrees of success, it keeps coming back to converting the tach to a negative trigger setup.

On the MG, if you get a later ('77-up) tach with the different face marking font you can easily swap the faces to get the numbers to match. I'd like to do that with the J-H tach, but it redlines at 8k rather than 7 and I'm not sure there's enough adjustment in the MGB tach to be able to calibrate it properly.

Perfect. Thanks. That's why I ask these sorts of questions here!

It is indeed an RVI tach. According to my googles, 1972 cars like mine can have either type. I could have spent a lot of time chasing down what I thought was a problem with the LS1 wiring, when it's a more fundamental problem.

The good news is that 1972-1977 (I'll check that end date) tachs are the RVC type, and they're visually identical. There's one on eBay right now for $40. It would drop right in, and the redline markings are actually about right for the LS motor. So I think I see the way forward!

The

Now that I am armed with more knowledge, I have an easier time finding good info.

I found this note on a page about the MG electrical system. Might be worth a try. At worst, it won't work and I am no worse off!

If your inductive tach (64 to 72) doesn't work with your shiny new electronic ignition system try the Crane tip of changing the wire going through the pickup from two passes (one turn) to one pass (half a turn) and recalibrating. You will need to dismantle the tach to get at the pick-up on later versions.

...and the answer is: nope. That didn't work. RVC it is then.

So am I to understand the MGB-GT of doom is now running? Vid or it didn't happen!

Also, late Spitfire tach innards can be used as well.

Well spotted! It is indeed sustaining internal combustion now.

http://www.youtube.com/watch?v=EbhKA9nYUu4

It runs quite happily now that I've fixed a few vacuum leaks. I just finished up plumbing all the brakes - so the only thing between it and a first drive are a bunch of finger-tight bolts, a brake bleed and some surgery on the fenders to clear the tires. It won't be in a position to do much more than drive around the block, but it will be self-propelled!

I've bid on an RVC tach on eBay. We'll see how that works. Come to think of it, I think I have a late Midget tach in the garage, I wonder if the guts would swap?

Here's what they look like inside for those who are curious.

Not sure if it is at all related but when I switched to EDIS in my Fiat The tach stopped working altogether and what I did was crack open the tachometer and trace the input wire. It went straight to a resistor so I jumpered around that and it popped right up. Oddly it is sluggish in the mornings and reads a bit low until the car is warm but other than that it reads exactly what megatune reports.

Turns out my "late Midget" tach isn't late enough - there's an RVI on the face. Thank you Smiths for making it so easy to check!

Update: the new RVC tach arrived. I plugged in it and voila! Gratification. It works without the usual LS messing around with voltages too.

Thanks again for the help on this one.