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tachometer wanted...or solution?

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Stoney

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When it rains it pours. I have noticed on my 67 Sprite the last week or so the tach jumping, then stopinging all together dead. With a thump or jiggle of the wires it will sometimes come back to life. I checked the wiring going into it, and it seems solid. This tach uses a male and female wiring feed with another flat style wire clip right together in an almost triangular arangement, along with the traditional gage bulb light. I noticed that the male end of the connection on the tach is crap/flimsy. Before I start breaking it down... does anyone have an extra tach out there this style? or suggestions? /ubbthreads/images/graemlins/england.gif
 
I don't quite follow your wiring descriptions. Start by looking at the gauge face for the letters RVI or RVC. The RVI tachs are earlier and wired in series with power to the ignition coil (typically with a length of white wire looped on the back of the gauge). Some later RVI types have bullet connectors mounted on the back of the gauge, effectively putting the wire loop inside.

RVC gauges are the later, voltage sensing type and are connected with a single wire to the low side of the coil. For both gauge types, there are a number of online pages that discuss troubleshooting and replacing components. You'll find good examples of both gauges on eBay.

In your case it sounds like you've got an intermittent connection possibly from a failed component, failed solder joint, or bad ground. However, even with wire that looks solid, there is the possibility that the end terminals aren't making good contact with the wire and/or the wire could have a break somewhere along its length inside the insulation.

Where in NC are you?
 
Thanks for the reply... I am in the Concord area. The tach is a RVI 1433/01. I'm sure it is a short, but tracking where is the question.

Stoney
 
Mine is an RVI with bullet connectors. I once left the ground wire off the case when reinstalling it and it would only work if I wiggled the case to get it to ground.
 
I'd start troubleshooting based on Trevor's comment and experience. Run an extra ground wire from the case to a good clean point under the dash and see if the tach works better.

I doubt you've got a short. With electronics (even the old stuff) you usually get white smoke with a short circuit. Once a short happens the instrument shouldn't recover by wiggling wires. A cold solder joint or a bad ground connection sounds more likely.
 
Once you figure it out, let me know if you need a good used tach
 
Well gang, good news... a friend pointed me to a person (and specialized mechanic:smile: that had several parts cars, and I traded a later tach that I had and it didn't work with my car for one with the right connectors. So far it works well. Mark another off the list of bugs?
 
My word, what a break.
 
The tach on my '68 MGB needs a slight tap with my finger every time I start it for the first time each day. I thought they were all like that.
 
Yeah Paul, the old tach needed a tap also...but then in time (a week and a half) it went spazo and died.
Next stop: I have to get a deep 6 inch scratch removed next month from the back right quarter... no dent, but my wife has a Saturn "mini-tank" and the two car garage isn't really a two car garage. Needless to say, there are tape lines for guides and a DO NOT CROSS with bright red paint down the center now. I've decided to name the car the "Elizabeth" after the Queen;)
 
I'm new here, but I thought I'd "dust off" this old thread. My Uncle has a '69 MG midget, great car! We upgraded his ignition to Mallory electronic (no points, uses magnetic pick-up coil), Pertronix Hi-voltage coil, and Pertronix Second-strike ignition controller. Set-up works great. One catch: the OEM Tach is not compatible. He has an OEM Ammeter type Tach (connected in series between voltage input and ignition coil). The Pertronix Tach out-put requires a pulse counting Tach (much more accurate, like you'd find on any car now-a-days). Can anyone here recommend a nice aftermarket (that does not require enlarging the OEM hole in the dash), or a MG factory replacement that is Pulse Counting instead of Ammeter design?
 
Wouldn't an RVC type tach from a later midget work?
 
Could you hook one side of the tach he has to the (-) terminal of the coil and the other side to ground?

This is just a wild guess, so all should feel free to shoot it down.
 
Like Trevor said, your first step should be to look for a Smiths RVC type tach from a later car. The RVC tachs are as you describe, voltage pulse sensing. You could also do a swap of the tach internals. I have done this for a couple of people and documented what I did in a PDF. See:
https://home.mindspring.com/~purlawson/files/Smiths%20Tachometer%20Conversion-R1.pdf

If that's too much work and you still want the OE look, see Theo Smit's pages and contact him about converting your tach. (Browse his web pages... they're very interesting).
https://members.shaw.ca/tsmit/tachmod/tachmod0.html

If you want an off the shelf solution and aren't worried about matching the speedometer, consider tachs from VDO and AutoMeter. Measure the opening in the dash and buy something close (but smaller) and expect to make a filler ring to complete the installation.
 
Regarding going between coil (-) and ground, sadly that won't work.

The induction loop on the back of the RVI tachs has nearly zero resistance. Connecting a wire between coil (-) and ground will behave like the points never open. Effectively, the tach circuit would become a permanently closed kill switch.
 
[ QUOTE ]
The induction loop on the back of the RVI tachs has nearly zero resistance. Connecting a wire between coil (-) and ground will behave like the points never open. Effectively, the tach circuit would become a permanently closed kill switch.

[/ QUOTE ]

Oh yeah! Duh!

My brain, she is slow today.
 
Wow, great article. Obviously you have a strong technical/tech-writing background. I'll further discuss the situation with my Uncle, but I think he had in mind replacing all the OEM Smith guages, with an aftermarket sustitute (OEM is very dim, I suppose we could find brighter bulbs if nothing else...). We can enlarge the dash openings to fit standard aftermarket guages easily enough, but I think he would prefer a plug&play set-up if possible. If not, I'll try to talk him into retro-fitting his earlier Ammeter Tach to a later model Smith Pulse-counting Tach. One last question: Are the late model Smith Tachs actually pulse-counters, or do they simply measure average DC voltage? i.e. as the duty cycle increases the Avg. DC voltage goes up. Lastly, if he goes with the OEM late model Smith tach; Specifically which cars qualify as donor vehicles?
 
You might want to paint the insides of all your instrument cans a bright white.
 
I have no idea about how the RVC counters function, but they are clearly marked as RVC on the face. You can pick them up at swap meets for $10, or a little more on ebay.

I cleaned my instrument glasses and painted the inside of the cans, and they were still too dark. There are places selling halogen bulbs and higher wattage incadescent bulbs. However, I went with white LED clusters and prefer the color. And... hopefully I will not need to replace them or worry about the wires melting. Also if you have a dimmer switch on the panel light circuit you may want to bypass it.
 
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