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MG MGB GT V8 Factory Originals Technical - V8 tach conversion revisited

A previous posting in the BBS archives suggested that, for tach conversions using the "Zak" approach of installing a variable 100K ohm pot resistor, a fixed 47K ohm resistor would yield the correct accuracy (without the associated drift).
Has anybody actually confirmed this approach?. Also, does Zak's conversion method apply to 3" dia gauges (up to '76) and also 100mm dia '77 to '80 gauges?

Graham
Graham Creswick

I tried the 100K resistor & it did not work. I couldn't get a reading, no matter how much I turned the pot. Of course, it could be because it was done incorrectly (an electronics shop followed Zak's instructions for me, and it looked right). This was on a 79 B tach. Think I'm going to spring for a V8 tach.

Allan
Allan

I'd recommend just buying Theo's board and putting it in the tach. It's a good upgrade, is rock solid, and will work with any ignition system.

Jim
Jim Blackwood

Sorry, but I'm new here. Who/Where is Theo? I have a '77 tach converted to V8 which works well when it decides it wants to, i.e., intermittantly. It might take better advice from a better signal.

Thx,

Edd Weninger
Edd Weninger

I have a 78B and followed the Zaks conversion instructions with the following modifications.

1- Attached two small wires to the pot so I could get the pot outside of the tachometer case and make adjustment with the tachometer mounted in the MG.
2- With the tach mounted and the electrical connections properly made. I used a hand held tach meter to calibrate the MG tach. Held the engine RPM at a steady 1000 rpm. And adjusted the pot to get the MG tach to read 1000 RPM.

Tach conversion works just fine.

http://www.zaks.com/mgb/

Jim Miller

I have used Zak's method on at least four late "B" tach's and they have all worked fine.
Bernie Posey

Theo Smit is in the Tigers listserv. He and a few other Tiger owners designed a modern drive circuit for the Smith's tach and it is now available for retrofit, or you can send one to him and he'll do the modification. The original board does not have to be physically removed from the tach so it's a pretty simple install and it can run on current trigger or voltage trigger, 4,6 or 8 cylinder. I've got one in my MG and use the Ford EFI/distributorless ignition to run it. It works equally well with MSD systems. You can't really say that for the Smith's inductive system.

If you want I can find his contact info.

Jim
Jim Blackwood

Jim,

Dig it up & ask Theo if he would do an article on it for the Newsletter! I know Dan Masters is ALWAYS interested in material.

On Leon's conversion, I did the fixed resistor version on my '79 MG. It's off a couple of hundred RPM (reads high). Since my resistors have a 10% tolerance I could probably get it closer by measuring a few resistors and using one that has a bit higher or lower (not sure which) resistance.

Has anyone measured the variable resistor after calibration?
Carl

I sent him an email, we'll see what he says.

Jim
Jim Blackwood

I used th ZAK aproach and it worked well with my 302. This year I finally got the $$ and upgrade all gauges to autometer Sport Comps to match my temp and A/F gaues from last year
Larry Embrey

Look at www.engravers.com/tiger/. Its a tiger club they have tech on how to change smith tach for V8 use and modern drive circuits. Steve
Steve Nightingale

I've seen conflicting information on which pins to connect the potentiometer across. One source said to connect across the 4 and 7 pins; another said to connect across the 3 and 8 pins. I'm getting ready to modify a tachometer in my '79 B. Can anyone clarify the correct pins to use?
Rod Holderbaum

I got my engine from a Rover SD1 and also fitted the SD1 tacho in the MGB housing.
This is possible because the pattern of both scales is identical and you can use the original MGB scale. For the speedo, i had to use the SD1 scale and had to cut of a little.
Its a little fittling but works fine

Regards
Peter
Peter van de Velde

Rod use pins 4 and 7 just as instructed on Leon Zak’s page (see above). I calibrated mine by leaving the disassembled tachometer hanging out of the dash with only one terminal soldered, revving the motor to an indicated 4000 and connecting the potentiometer and adjusting it to read 2000. A few tries and I was satisfied it was as accurate as the original tachometer ever was.

Graham that system only works on the later ’77 through ’80 tachometers that have the chip.
George Champion

Rod,

Definitely pins 7 and 4. Just done it on my '78 BGT and it worked well.

Be brave; good luck.

Peter
P L Hills

George,
So this tach conversion will not work on pre-'77 tachometers, or did I misunderstand your last statement?
Jake

Jake you did not misunderstand. The pre ’77 tachometers used a different system. I don’t recall seeing a chip in the early ones. Still, there may be a place where a potentiometer could be used, but it would be different. I wonder how the stock MGBGT V8 tachometers differed from the standard tachometers.
George Champion

George and PL, Thanks for the advice. George-I especially like your suggestion for calibration. Sounds like a simple and accurate method.
Rod Holderbaum

The reason I ask is because the tach shown on Zak's page looks like the '68-'76 style. Has anyone tried this conversion on the mid style tachometers?
Jake

Jake I’m glad you kept asking about this. I assumed that the big change in tachometer design in ’77 brought on the solid state circuit with the chip, but another look at Zak’s site shows he has a ’75. Some times it’s good to be wrong.

I just took apart an old ’72 tachometer and found no chip as I found in my ’77 or in the pictures on Leon Zak’s site. However I did find a yellow potentiometer that most surely would adjust the tachometer output. I have no way of testing this to see if it’s true and if it adjusts enough to reduce the reading to the V8 level. If it can’t then perhaps adding a second pot in line or a resistor there would do the trick.
George Champion

Guys, while we're talking tachs: just a reminder if you install an MSD box in the car -- maybe someone will find this in the archive and save a headache. My tach (I found an OE factory V8 tach, but it's exactly the same principle as the early-70s CB units) was taking its pulse readings off the negative side of the coil, just as you would with a 4-banger ... it was never re-wired when the MSD unit went in. The tach worked fine but it seemed to read very high -- checked it against, e.g., a timing light with an inductive tach -- and it ranged from 15% to 60% high depending on who knows what. So I mailed it to Nisonger and said, "calibrate". They said it was spot-on. I stuck it back in. Same thing. Repeat. Then they wisely asked whether I still had it wired to the neg side of the coil. Well, sure, I said ... but that's exactly what not to do. The MSD really does send out multiple sparks and if the tach reads the resultant "sputter", it will sense way to many sparks. All you have to do -- duhhh, this is so obvious -- is wire the tach up to the tab on the side of the MSD box, which cleans up the signal to one pulse per rpm. Works perfectly now. You guys probably would not be so dense as me but just in case.
David

George,
Thanks for checking on that. The tach I was planning on using is a '73. I'll crack it open and see what I can find. I guess I could rig up a custom mini harness to read two tach off my '72 BGT and then adjust the one to be half of the other. I'll post the results on my webpage.
Thanks again
Jake

Is there any reason that the potentiometer couldn't be put in line between the coil and the tach rather than between the appropriate pins on the tach as Zak has nicely shown?
Rick

The reason is that the potentiometer is adjusting the pulse duration that the meter (tach) reads. Puting the pot in the coil line won't effect the pulse.
Regards
Tony
Tony Bates

I found on a 73 tach there was enough adjustment and then some to adjust for a 6 cyl. You may be able to replace the pot with one of a higher value and get enough adjustment.
John

No news from Theo, but Tom Hall now has boards in stock: Tom Hall [modtiger@attbi.com]

Jim
Jim Blackwood

How about using a fixed resistor instead of a pot in the tach itself? If you knew how much resistance was needed, would this work? From Zak's site, it appears that the resistance needs to be twice as much as a 4 cyl would require, cutting the reading in half. If someone who knows more about this could reply, I would appreciate it. Thanks.
Jake

Jake - I'm not sure exactly what is underneath pins 4&7 in the existing chip, there could be a couple things going on, but I wouldn't bet on the fact that the resistance needs to be twice as much as a 4cyl because we don't know just what is happening.

If the resistance is in series with the output then increasing the resistance could adjust the tach, but my guess is that the added resistance is in parallel with another resistor thereby decreasing the ultimate resistance. I would bet on this and that it is adjusting a pulse circuit as Tony stated.

Either way, you definitely could use a fixed resistor, but I don't what the value would be. Maybe if someone is working on one they could measure it and let us know what the value of the pot is adjusted to.

The other circuit I put up on my site
http://www.zaks.com/mgb/leonsmgb/this_is_the_circuit_i_use_on_my_.htm took the 8cyl pulses (4 per rev) and reduced them by half (2 per rev). That circuit will work with almost all tachs and retains the original accuracy (or inaccuracy). It worked without problems, I only switched because after I heard about the resistor idea I couldn't help but try it.

Leon....
Leon Zak

Radio shack sells a counter IC chip, LM555 or something like that. It makes a good tach drive circuit, much better than the old circuit and not much to it.

Jim
Jim Blackwood

Leon,

Great to see you here. Thanks for the tach conversion help. Sure would like to read about your Holley TBI in the next V8 newsletter. Hint, hint. :)

As for which pins to solder the resister to on the chip, I guess we are using the same ones since it works. The actual pin or leg numbers on the IC chip are 3 & 8. Just go by Leon's pictures and you will get it right.

BTW, I used a 47K ohm fixed resister. My tach reads about 300 RPM high at 3000 RPM. Some day I will fine tune it. I think there is enough variation in the resistors due to the 5-10% tolerance to dial it in closer.
Carl

So the other day I put the pot across the chip in my tach and tested it today with disappointing results. The problems that think may have caused the failure are 1. I tested the tach in a '72 and pulled the pulse input from the neg side of the coil. The tach in this car wires in differently, maybe that affected it. 2. The pot is bad. It is a 100K 1 turn pot. 3. The tach is bad. When I turn on the car, ignition in the on position, the tach jumps to 1000 and will not move even when the engine is revved. Any thoughts? I'm planning on testing it on a '79 tonite if I get a chance, so that should take care of #1.
Jake

Jake,
I am also experiencing the same phenomenon of the tach indicating 1000rpm without movement (this on a '76 tach that I converted a couple of weeks ago). The tach was working fine on 4 cyl pulses before the conversion and the pot was also pre checked for varying resistance per Leon's instructions. I wonder whether it's now a defective 100Kohm resistor or whether too much heat was applied during the soldering operation and effectively screwed up another component.
In the meantime, I'm operating with a spare 4 cyl tach until things get sorted.
Graham Creswick

Thought I add my recent tach recalibration success on my 1977 B. Based on Zak's instructions and other input from the BB I went with a transistor. I got a 47K ohm 1% tolerance transistor ($0.09). The electronics shop measured the resistance at 47.5K ohm. I soldered it between pin 4 and 7 as instructed. Zak's pictures were helpful. They showed that he bent some of the components in the board to gain access to the chip. (I was somewhat concerned about ruining someting). Test results were performed using a digital SnapOn(?)digital timing tach device (a friend has it). At 1000rpm digital readout was 1050, at 2000rpm digital readout was 2050, at 3000 digital readout was 3000 and at 3500 the digital readout was 3500. So I'm happy with the outcome.
Rick

One more time.

It's pins 3 & 8.

Pin 1 is marked on the IC chip. It's the top pin on the left side. The rest of the pins are in ascending order counter-clockwise. Leon's pictures do a great job showing the correct pins to use.
Carl

Sorry about indicating the wrong pin numbers.
Rick

I pulled the first potentiometer out of the tach, and tonite im planning on putting in a fixed 39K and a 20K multi turn pot in series. I have access to a scope, so tonite were going to try to get it to work. Ill post results if they are favorable.
Jake

This thread was discussed between 15/04/2003 and 20/05/2003

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