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Jumping Tach

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Here's the problem..
Tach. Smiths # RV1 1433/00 in my 72 MGB with pertronix in 45d4 dizzy.. The tach. works fine until the car warms up.. may 20 min. driving.. It then starts to drift up and down maybe 200 RPM each way, then from time to time it will jump up 1000 of more RPMs then back to drifting... Tried another tach with same results.. Tach is a current reading type attached to the + side of the coil.. Coil is a Lucas sports coil..
Have checked all connections ect.. all good.. Have tried the take a loop out of the wire inside the Tach. Have tried adding a ground to the case of the dizzy.. all without result..
Anyone got a fix for this problem??
 
HI Blake, I am not clear on your statement "Take a loop out of the wire inside the Tach" can you clarify that a bit.Also do you have a Moss catalog on the MG there is a very good description of the tach mods in there.Additionally, as I recall there was a similar problem to yours handled on this forum just recently. It seems that some of these Dizzys are not compatible with the Pertronix unit. You might look in the archives for the discussion.---Keoke
 
hi guys,
I've actually had a somewhat similar problem...

With my tacho, occasionally, the tacho reading wont go past 3500 rpm, i know coz i usually shift around 4000 +.
Do u recon one of the instruments could be a bit sticky?
Any ideas?
 
Here's a reply I recieved from Moma Instruments (Momanm@aol.com)regarding my erratic tach due to Pertronixs
FWIW
Well. Bob, it seems the Petronix people really dont like to deal with English elec tachs. We have talked to them. They used to call it a "tachometer fix" - because of problems, but that goes back quite a ways. We prefer to change out the circuit in the tach for a permanent "FIX" - that runs about $200. Their previous suggestion was to put 2 diodes from RadioShack #276-1144 in series between neg side of coil and tach lead - or call them for their up to date suggestion. I have not heard of anyone having first success, then recent failure as described. We dont like to see people put more $$ into the old circuits of the RVI series tachs - but rather a one time conversion which corrects it forever/regardless of IGN. Margaret
 
Hi,

I'm new here and was searching for threads about Smiths tachometers as I mess with them from time to time. The problem described above isn't limited to Pertronix, you'll probably see it with Crane/Alison (or the Hitachi dizzy conversion) ignition also.

The problem is that the RVI tach is current pulse sensing and it's pretty primitive. You can bandaid it if you want to try... but the results aren't guaranteed and most report failure over time or with changes in temperature. Swaping the guts of the RVI for a later voltage sensing tach will solve the problem.

Most British car owners I know are fairly handy at their own repairs. This is a conversion you can do yourself if you're industrious. I've tried RVI conversions two ways. The first is to fit and calibrate an entire new gauge behind the Smiths face. The other way is to fit a late model tachometer's circuit board to the Smiths movement along with a calibration trim pot. This later conversion worked very well for me using a circuit board removed from a NOS tach I bought as NOS off eBay. The tach was from a late 1980s Pontiac LeMans of all things. I later learned this same tachometer was used in certain Toyotas. It was an easy swap and complete with cosmetically restoring the Smiths case and face took only a day.
 
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