Rev counter challenge

[Tor]

Hi all,

Trying to replace the tacho on my father's 1973 South African P6B it turned out to have three connectors and my UK-sources replacement had two. The broken one has a male (white-black) and female (white-slate) bullet connector and a female spade (white), the new one misses the female one. Trying to wire up the available connectors I couldn't get the car to fire.



Brought home a dead unit to investigate, and it turns out that the two bullets are for the same wire that goes in one end, runs through a coil-like thing inside a couple of times, then exits the other end.





My question is whether I can make this work: Splice the two wires mentioned onto one bullet connector, or do surgery to fit the missing one and run the wire through that loop thing if there is one, or will either method break the unit or simply not work?

 

[dmcsweeney]

Unfortunately I'm not near my cars or workshop manuals to check but it sounds like your new replacement is from a pre '73 car. On my '72 car the +ve of the coil is fed from the back of the tacho (I think this has only 2 connections), which works the tacho. On my '73 cars the coil negative is connected via the tacho to the points. Again, without seeing my own car or wiring diagrams I would guess that the 2 bullet connectors on the original unit are the coil -ve in and out, and the other a live feed. Look at the loom next to the ignition coil, does the cable from the points connect straight to the coil -ve or into the connect to the loom, and then back out of the loom and onto the coil -ve? If you have the cable going in and out, connecting the points lead directly to the coil -ve should make it run. Someone with a manual to hand may be able to confirm this?
Regards,
Dave

 

[darth sidious]

The tacho on your father's car with the 3 connectors is an RVI tacho, it senses the current pulses in the LT wiring. The 2 terminal tacho is an RVC variant, it senses the voltage pulses/variations on the coil negative terminal to the points wire (i.e. the white/black or white/slate wire that connects to the side of the distributor).

I think 73 cars onwards used the RVC (Voltage sensed) tacho, cars before this used the RVI (Current sensed) tacho.

There's a possible catch with the RVI. There's two methods used on P6's to wire them:-

If I recall correctly, Brian Northampton's 2000TC has a spade coming from the ignition switch for power to the tacho, but the bullet connectors on his tacho are connected in series between the coil - terminal and distributor.

But Fraser P6's 2000TC, only a year younger, has the spade female connector pigtailed with a female bullet connector (and taking a bit of a liberty!) :-



In this case, the tacho is wired between the live supply from the ignition switch and the wire which then goes to the ballast resistor and ultimately to the coil + terminal.

 

[marcus]

Hi Ive just purchased the same tacho as above , can someone please confirm where the wires go to for me please, if you have just wired in yours any advice would be welcomed thanks
i will have to put in new wires have i have nothing to start with so a wire drawing would realy help please thanks
marcus

 

[Tor]

Thanks guys for that. Dave, the points lead (white-black, male bullet at the tacho) goes into the loom and then the female (white-slate) comed back out to the coil -ve. The female connector is the missing one on the new tacho. So, if I understand you correctly, putting the white-black points led to coil -ve would make it run, and the new tacho would work or would I need to use the female bullet (white-slate) lead and connect that too to the coil -ve?

Darth Sidious - according to our guy Alan here, SA cars ceased to have new engineering developments implemented ca. 1973 (due to low sales), which was when the car was built so old/RVI tacho is probably quite right.

I've uploaded a shot of the innards of the two units, in case it's of interest to anyone... No mods can be performed to them by myself, that's for sure... :roll:

Dead, three-pin unit on the left:

 

[dmcsweeney]

Thanks guys for that. Dave, the points lead (white-black, male bullet at the tacho) goes into the loom and then the female (white-slate) comed back out to the coil -ve. The female connector is the missing one on the new tacho. So, if I understand you correctly, putting the white-black points led to coil -ve would make it run, and the new tacho would work or would I need to use the female bullet (white-slate) lead and connect that too to the coil -ve?

Tor, following Darth's comments I'm a little unsure about my own information and would need to refer to my wiring diagrams, which unfortunately I won't have until tomorrow evening. IF I'm correct my method would get the engine running but not sort out your tacho problem. What you would basically be doing is connecting the points cable (just the short section from the distributer) directly to the coil -ve and leaving the other negatives at the coil disconnected. I might be able to give you more accurate info when i get back home.
Regards,
Dave

 

[darth sidious]

I'm not 100% certain either, I'm reasonably sure the later models had the RVC (i.e. sensing via the "voltage-pulses-on-the-coil-negative-to-distributor-wire") tacho, but of course check!

Do bear in mind that P6's don't always follow the quoted wiring diagrams exactly!

 

[Tor]

To recap, I understand that most likely I will end up bypassing the loom by connecting the distributor -ve straight to coil. Then I need a signal from either -ve or +ve coil to tacho. The ignition-switched power supply is fine. Isn't it then a question of whether the RVC tacho (which the new one definitely is) needs a -ve or +ve lead to get a reading?

 

[darth sidious]

To recap, I understand that most likely I will end up bypassing the loom by connecting the distributor -ve straight to coil. Then I need a signal from either -ve or +ve coil to tacho. The ignition-switched power supply is fine. Isn't it then a question of whether the RVC tacho (which the new one definitely is) needs a -ve or +ve lead to get a reading?

I think, but check with the others first, what's needed is a signal from the coil -ve (white/black or white/slate, if I recall). But remember, the 'new' tacho senses voltage, so the signal wire to the tacho is wired in parallel.

 

[Tor]

Result!

Quick recap: Successfully replaced the older RVI tacho with an RVC one. By connecting the points lead straight to coil -ve, together with white/slate that already sits there (it ends up in a female bullet connector at the tacho end), the car starts. The difference is that the male bullet at the tacho end (white/black wire) now is surplus wire.

In our case the surplus wire came in perfectly handy as we fitted a SimonBBC electronic module to a rebuilt distributor at the same time. We extended the surplus lead to connector 19 on the fusebox and used it to feed the red +ve lead to the module (the black -ve connecting to coil -ve like the old points lead did). Car fired up straight away, tacho reads like it should, and now it's off to get the timing set. Yay

 

[darth sidious]

I'm pleased to hear that! Well done, something for us all to memorise/remember if need be!