I read somewhere that if you have an electronic ignition like petronics[sp?] or Crane or whatever that the loop through the tach might cause a bad tach reading. My tach and speedo seem to be in sync at 22mph per 1000rpm in top gear [BW automatic trans]. But when I'm doing 66mph and 3000rpm, I know I'm NOT going 66, more like 55mpg. How can the tach and speedo be so in sync and yet so wrong? One article suggested that having only one wire, i.e. no loop, through the tach would yield better tach reaadings, but that would mean that my speedo if WAYYYYY off.

Any ideas or experience with this issue out there

Such a phenomenon was mentioned in a very recent issue of Practical Classics. I think the soltion was to connect the tacho to the other side of the coil.

Your speedo is purely mechanical and will not be affected. However, as the C came with 2 different differential ratios, you could have the wrong speedo fitted.

I have an automatic and my differential is a 13/43 3.308:1 and the speedometer is a SN 5226/00 1020TPM. I've read many articles and couldn't determine whether or not I had the correct speedometer or not. Maybe I need to get it rebuilt or at least checked for accuracy.

The MGOC Spares web site lists several speedos. They are all listed by SN instead of Turns Per Mile.
6144/33E,5226/00E,5227/02E,5226/02E,6144/31E,6144/32E,5227/19E ???

I posted a while back a thread from Soyokaze who had found a lot of info on MGC speedometers. According to his data I should have a 1120 TPM speedometer unless I had a 69! I guess I'll just get a Garmin and go with that!

Below is what he posted at:

http://www.mgexperience.net/archive/introduction_mystery_speedometer/486868

1. 68 manual SN 6144/10 1020 or 5226/00 1000 ??
2. 68 overdrive ??
3. 68 auto SN 5227/18 1120

4. 69 manual SN 5227/18 1120 3.7 rear end
5. 69 overdrive SN 5227/10 1120
6. 69 auto SN 5237/00 1020 3.3 rear end

1969, chassis number CD1-4236on and CN1-4266 on should be fitted with a 140 mph SMITH 5227/02 speedometer (MG part no 37H3764). Chassis number up to CD1-4235 or up to CN1-4265 should come with a SMITH 5226/00 speedometer (MG part no BHA4603). References are valid for an "US-config" MGC, i.e. with the US dashboard.

Then there is the MGC Service Parts List AKD 5066, Issue 2. It lists 5 different speedos.
MPH s/n 6144/10 BHA 4598
KPH s/n 6144/11 BHA 4599
KPH s/n 6144/12 BHA 4600 Germany
MPH s/n 5226/00 BHA 4693 USA
MPH s/n 5226/02 BHA 4694 USA for Germany

Lawrence,
I have AKD5066, Issue 6 (i think). Mine lists nine speedos:
MPH s/n 6144/10 BHA4598 (ex USA and Germany) (1)
KPH s/n 6144/11 BHA4599 (ex USA and Germany) (1)
KPH s/n 6144/12 BHA4600 Germany (2)
MPH s/n 6144/31 BHA4864 (OD)
KPH s/n 6144/32 BHA4865 (ex N. America) (OD) (3)
KPH s/n 5227/19 37H4181 (N. America) (OD) (3)
MPH s/n 5226/00 BHA4693 (N America) (4)
MPH s/n 5226/02 BHA4694 (N America for Germany) (???) (4)
MPH s/n 5227/02 37H3764 (N America) (5)

(1) (c)G-CD1-101 to 1493 Canada, 101 to 4235 overdrive, 101 on standard and automatic. (c)G-CN1-101 to 3313 Canada, 101 to 4265 overdrive, 101 on standard and automatic.

(2) (c) G-CD1-101 to 4235 overdrive, 101 on standard and automatic. (c) G-CN1-101 to 4265 overdrive, 101 on standard and automatic.

(3) (c)G-CD1-4236 on, (c)G-CN1-4266 on

(4) (c)G-CD1-101 to 4235 USA, 1494 to 4235 Canada, (c)G-CN1-101 to 4265 USA, 3314 to 4265 Canada

(5) (c)G-CD1-4236 on, (c)G-CN1-4266 on

Bill:

My experience with the tach issues are limited, but usually the problem with the original tach and the electronic ignition systems come from the fact that most electronic systems do not have the solid On/Off firing signal of the points.

And they often use a lower voltage in the systems themselves, so the signal is usually not strong enough for the old tach to "see" the firing signal. Using a single loop on the back of the tach instead of two will do just the opposite of what you want. And most of the MGC tachs have the later internal loop connections so that will not be possible to change. Other methods are needed.

Some systems will offer an added ballast or amplifier to make a stronger signal for the older tach.

Sometimes you need to have the tach rebuilt and adapted to the electronic ignition system you are using.

And I think you are confusing yourself on the speedos. Even though the Parts Manual shows that many speedos they are for too many different applications. For the "Pillow" dash there really are only two listed. One for 1968 and one for 1969. All the others are for KPH or the metal dash.

And there is only one tach listed for the "pillow" dash.

If your speedo is not working correctly (use the GPS to verify that) it is probably in need of service and calibration. And that includes the cable and all the angle drives.

C Ya,
Mark

And attention to the new final drive ratios of the skins of choice on the rear axle. The car was designed for tires probably no longer available on 15 inch wheels also NLA.

---

Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote

The tacho input is a current transformer and the single loop is OK for a points based system that draws well over 1 Amp.
When you go to modern electronic systems or in my case a Capacitive Discharge system the current from the distributor through the tacho pich up is way to low and will affect the tacho accuracy.
With my CD system I had only 240ma going through the tacho pick up so increased the 1 turn to 8 turns of thin teflon coated wire (because I had it and it fitted well) this fixed the Tachometer reading.
I now have an Ignitor Electronic system (made in California) pick up in the distributor and this now feeds my CD system and all works perfectly. I can switch from Ignitor direct to the coil or via the CD system or even put back the dreaded points in an emergency this way.

I thought that the gearbox had two sets of speedo gears 67 to 69 and 69 to end as this is what the BLMC Service Manual shows.
This is Bull S**t. There was only ever one set of gears in the gearbox for the speedo.
I discovered this only last year when I tried to get the 69 and on ratio (as in the Workshop Manual) only to find that all that the factory did was to change the speedo itself to one with a different TPM (turns per mile)ratio.

Hello,

Slightly off topic, I still retain the original ignition system on my 68 MGC. After rebuilding my engine, the tachometer now reads higher revs than actual, by a wide margin, like almost twice. I have confirmed with multimeter. Any thoughts as to why and how to fix?

Regards,

Dean

My car's tach went to Nisonger Instruments in 2006 for a much needed rebuild. Nisonger called and suggested an electric tach conversion to match the tach with the Petronix electronic ignition that was to go into the cars original, and Jeff Schlemerized, dissy. The tach is now rock solid and dead on when tested against a remote tach. The conversion is a bit pricy but worth the money in my opinion. The tach operates like a tach on a modern car. Worth a call to see what they can do for you. Nisonger repaired the C's speedometer too and now the rpm and mph match the book figures.

Chuck


http://www.nisonger.com/electric-tachometer-conversions.htm

"Nisonger converts Smiths RV Series Electric Tachometers for use with virtually any Electronic or Points Ignition Systems.

In recent years, an ever increasing number of British car owners have been updating their cars with electronic ignitions. Unfortunately, the Smiths & British Jaeger RV Series "Loop Tachs" used in many of these vehicles are not truly compatible with any electronic ignition systems. To resolve this problem, Nisonger has developed new circuitry that eliminates the Loop Assembly and allows these Tachometers to be used with virtually any electronic or points ignition systems without changing the outward appearance of the original equipment Tachometer. As is the case with all of Nisonger's rebuilds, the converted Tachometer is guaranteed for 1 year."

I totally agree with the above inputs about converting current loop tachos to voltage pulse input tachos as then they will work correctly with electronic ignition systems.

No car should be running the old Kettering points and capacitor system today with all the excellent systems from Petronix and Crane. Aside from being maintenance free they provide constant accurate timing through the rev range as there are NO points bounce problems at high revs and much better spark delivery at all revs.

I was amazed at the coil input voltage peaks at the coil + that my CD system produced. I bought home (on loan) from HP a 75 mhz CRO and on the positive coil terminal I saw +600 and -200 volt peaks. No wonder CD systems will run with fouled plugs. There is one drawback with CD ignitions and that is that there is a symptom of lean mixture miss if the car runs lean. The spark duration is too short to fire a lean mixture. I like the "Ignitor" from Petronix as the driver for my CD system, fitted when the LUCAS solid state Ignition amplifier and it's hall effect pickup failed.
If my CD system fails (I built it in 1970) I will go for the really good Crane Ignition system.

Hi,

Before playing with the loops inside the tacho, or converting it to modern electronics (at a cost...), I would try adjusting it with its potentiometer which you can adjust with a small screwdriver - there might be enough "play" to compensate for the %50.
You access it from the back of the tachometer, it's under a small rubber plug.
Worked for me both on the B and the C.

Regards,

Koby

Agree with most of the above comments. I am still old school and like my points, dizzys done by Jeff S. There is some adjustment as Koby pointed out. Nisonger does nice work and I think there are a couple more shops that can do the job. I just got back a set of MGA gauges from Nisonger and they are a work of art. I sent them less than a stellar set so they do nice work but not cheap.

Ron

Morris at West Valley in CA is also a very competent US re-builder of the UK car gauges.
West Valley Auto Electronics (was WV Instrument Specialists)
19314 Vanowen St.
Reseda, CA 91335
818-758-9500
http://www.westvalleyautoelectronics.com
Nothing against Bob at Nissonger, but I have always found him to be more expensive and turned all my rebuilding over to Morris. Mrs Lucas at Moma in New Mexico, completes the short list of time honored and capable re-builders.
MO-MA
1321 2nd St NW
Albuquerque, NM 87102
(505) 766-6661

I know nothing about these folks:
North Hollywood Speedometer & Clock Co.
6111 Lankershim Blvd.
North Hollywood, CA 91606
Phone 818-761-5136
Fax 818-761-4857
http://www.nhspeedometer.com/

---

Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote

Just tried the http://www.westvalleyautoelectronics.com link and it in not valid. Called the phone number and talked to Morris. Correct link is www.westvalleyinstruments.com. Also, the cost is MUCH less than the previously referenced source for these conversions. Morris says his conversion will operate on any type of ignition, both positive and negative ground. Worth following up if you are looking at doing this. Just goes to show that Robert knows his stuff.

---

Larry
San Diego area, CA

---

Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote

Robert
I will give them a try on my next speedo/tach needing work.
Thanks for the info.

Ron

Does anyone have a table of tach indicated rpm vs road speed understanding that the numbers in the table will depend on whether the gear box and differential are standard or not and that wheel size and air pressure will cause some differences?

MPH X1000 rpm 1968
Std 24
Od 27

MPH X1000 rpm1969
Std 22
Od 24

Automatics should be the same as late std cars (22mph).

From data 3 in shop manual

Ron



Edited 1 time(s). Last edit at 05/05/2013 10:32AM by ron neal.

Thanks Ron. I didn't think to look there. I have it. However, my only has the 1968 data. I need to wait until very late when I have the road to myself to take a run partially because my speedometer also does not read steady. Just another repair to get to!
Brian