• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Tach Electrical Connections

Share this page

Ohiobugeye

Ohiobugeye

Senior Member
Country flag
Offline
Another question. I have a Smiths RV 2401 008 positive gr. tach that I am converting to negative ground and using in my Bugeye. I have looked at the various wiring diagrams but I can't seem to find where the electrical wires go. I'm not using a standard wiring harness for the tach so I have no real idea where the wires go to or come from to install the unit.I have attached a photo of the connections on the tach and labeled them "A" and "B". Looking at a Midget mk 2 diagram it appears that "A" has a white wire that is looped and goes to the ignition switch and also the SW connection on the coil. The other connection "B" is the one I have no idea where it goes. Has any one installed this tach and can help me out? P.S. I think this tach was installed in Sprites, Midgets and B's from 1965 to 1968.
 

Attachments

  • 17292.jpg
    17292.jpg
    27.6 KB · Views: 628
One minor correction, the back of the tach shown in your picture is an "RVI" type, not "RV".

Succinctly, the RVI tachs are current pulse sensing and are wired "in series" with power passing through the ignition coil. Let me suggest you start by looking at this link from Speedy Cables:
https://www.speedycables.com/page34aaaa.html

I cannot tell from your picture so please elaborate with text. The spade lug you have labeled "B", is it on a small insulator plate so it does NOT touch the gauge case? (It should be insulated but I can't tell from the picture). If so, the spade lug gets a switched, 12V supply from the fuse box. The CASE of the gauge serves as an earth connection. The induction loop is made of white wire and can be connected as you describe... sort of.

On cars where the RVI tach was installed at the factory, a white wire leaves the ignition switch and is looped on the back of the gauge. The other end of that wire goes off to the "HIGH" side of the coil's low tension connections. (I do not like using terminal numbers/letters). This second end of the white wire is power "in" to the coil. The other coil terminal will have the standard white/black wire going off to the distributor.

On cars where no such wiring was already in the harness, there is a second wiring option available. You can run BOTH white wires through the firewall to the coil and distributor. You remove the white/black wire between the coil and dizzy and you replace it with the white wire going to the tach.

Regardless of which wiring method you choose, if the tach reads "funny" start your troubleshooting by reversing the two white wire connections so the current flows the other direction. If that doesn't work, sometimes making a second loop on the back of the tach helps. Remember, these gauges are old and the transistors inside are "tired". There are also articles on the web that tell you modern replacements for the transistors and the Mylar capacitors if you decide further internal repairs are required.

Lastly, if you plan on fitting an electronic ignition or have already done so, do not invest time fitting this tachometer. RVI tachs do not generally work with electronic ignitions though there are a few exceptions.

Keep us up to date on your progress.
 
Thanks for a very informative reply. Yes the tach is a RVI not an RV, I read the I as a slash mark, on close examination it is indeed a RVI. You answered my question fully, also I have converted the ignition to an electronic system.....guess this tach won't work after all. Do you know of a tach that will work ( and fit the dash cut out ) on my system? 1959 bugeye (4" gauge), Negative ground, 1275 engine, electronic ignition.I know the original tach is an option but an expensive one by the time I buy the correct generator, reduction gear, tach and cable I'm looking at well over $500.00 for just one gauge, ouch. Thanks Again, Don
 
Check Summit,E-Gauges online. VDO makes one. Was here on the forum earlier this year. E-gauges had the best price.
 
Ohiobugeye said:
I have converted the ignition to an electronic system.....guess this tach won't work after all. Do you know of a tach that will work
Click to expand...

Why won't this tach work? Just run whatever wire feeds coil through it first, right?
 
Unfortunately, the RVI tachs expect the duration of the current pulse they detect to be a certain length. Electronic ignitions handle dwell differently than points ignitions. More often than not, RVI tachs wired in series with an electronic ignition do not read correctly. You can always try it... there is nothing that will be damaged.

If the tach doesn't work, look for a later RVC type tach that uses a single wire connection to the "low" (points) side of the coil. You won't likely find one the right size, however, you can often transplant the RVC movement behind the face of the RVI tach to achieve what you want.

Let us know if the RVI tach works for you and what you decide to do.
 
I'm currently in the same boat. Am trying to get the rvi tach to work with my new ele distributor. The tach can be modded to work by professionals, but some people say it can be made to work, by altering the loop of wire on the back of the tach, either add loops, or remove loops to single pass.
 
The single pass loop was standard. The double loop is used more often when the transistors are not working well anymore and the additional loop of wire can create a stronger magnetic pulse to trigger the transistors. You can certainly try any of these methods without risk so by all means give it a go! There is also some information on a Sunbeam Tiger web page (somewhere... I didn't save the link) that discusses putting an additional capacitor inside the case to improve the performance when electronic ignitions are used. I've discussed this with Theo Smit in Canada. He tried the additional capacitor without much luck and has subsequently developed his own circuit board which will allow you to power the RVI movement as either voltage or current sensing.

Visit Theo's web site. If you do the work yourself, his upgrade can cost about the same as a new VDO tach.
https://members.shaw.ca/tsmit/tachmod/tachmod0.html
 
Ouch that's an expensive way of doing it.
I'm currently looking/bidding on fleebay for an RVC in UK (hands off it's mine, I saw it 1st).
I've done an electronic ignition conversion on my BE & the current RVI tacho is bouncing all over the place. I just missed out on one for ÂŁ33.50
Look for midgets & MGB's around 1972 or later.
Adrian
 
What kind of ignition are you guys using? If I finally get around to installing my Crane XR700, can I expect my tach to go weird? It's already bouncy.
 
Jvandyke, to the best of my knowledge, RVI tachs just don't like the different dwell produced by electronic ignitions regardless of make/brand. It certainly won't hurt to try your RVI with the Crane ignition. However, if it's already bouncy it probably won't get any better.

Hairyone, I guess the expense is all relative. Theo's board isn't priced badly if your goal is to keep an original looking dashboard. For comparison, look at the price of quality tachometers from racing supply houses (don't compare the price of those tachs or Theo's conversion to the price of the bling tachs from China). I hope you win your auction. I've picked up a few nasty looking tachs off eBay. Fortunately they usually clean up rather easily and most bidders are put off by the outward appearance.
 
In the Crane instructions, it shows to REMOVE a loop to get better accuracy. I've done that plus I drill a hole in the back of the tach to access the potentiometer to tweak the adjustment.... ..

That said, I got my present tach off ebay and it is heat sensitive. It will read a different rpm in the afternoon than it did in the morning, just from ambient temperature difference. .. I should get it refreshed, but it's close enough.
 
Bill, I suspect you have a bad capacitor. Mine was doing the same before i turned it around for the neg earth. It was the Hunts A301A MP. Made one up out of a old trashed pc monitor.
 
I'm gonna send mine off to Nisonger Instruments to get it modded. Its 275.00 plus shipping to get it done, and there is a 4 week turnaround time, or for an additional 40 bucks I believe, they will do it after hours and it will ship in 5 business days, check out their website, they specialize in Smiths gauges,
 
Update on my tach, I found several web postings regarding the compatibility problems involving electronic ignition and the Smiths RVI tach's. I found Doug Lawsons excellent article regarding Smiths and Jaeger Tachs on the web but it is also posted here on the forum. I truly feel we have the best knowledge in the entire world helping and aiding us right here!!!!!!!!!
I have decided to try my hand at putting the inter workings of a 3 1/2" RVC tach into the casing of my 4" RVI tach. I broke my RVI unit down and looking at Doug's article I think it can be done. We'll see.
Doug, forum user name DKLAWSON has written several informative postings regarding this post, Again Thanks Doug and everyone else who has answered this post.
In answer to Ivandyke's question regarding what type of ignition we are using, mine is a negative ground Pertonix Ignitor system.
 
I didn't realize there were issues with tachs and electric ignitions, good to know. I still have to fab up a shutter for my Crane but it's on the list, probably worth the effort.
 
eeep.

jvandyke, I'm far from a purist (every car in my sigfile except the Benz has DCOE Webers mounted) but you just deep sixed that poxy DGV thingie and retro-fitted "proper" SU's to it.
What's so ~wrong~ with points and condensers? :jester:
 
Thank you for the kind words. I'm glad the PDF document helped but I understand that what I documented isn't for everyone.

The post above by Chad points out that instruments and instrument repair are not inexpensive. The $275 restoration from Nisonger may appear expensive to some but there is a lot of work involved and I'm sure places like Nisonger earn every penny. Remember that your time is also money. Theo Smit's solution falls somewhere in the middle of the possible expense range and will work well for many people who like DIY work.

Bill L., I agree with Lee that you may want to replace the Mylar/polyester capacitors (and perhaps the transistors) inside your gauge if you're seeing a lot of change with temperature. Thanks also for the information regarding Crane ignitions. I'm curious though... doesn't removing "one loop" change the system into a simple wire passing in close proximity to the gauge case? On late model RVI tachs the induction loops are inside the case and all you have are two bullet connectors on the back for connecting the white wires. I'm not sure how you would easily change the number of loops on a late model RVI.

Let us know what your final fix is and how it works out.
 
Back
Top