tach-less

[Sopwith_Camel]

ok i have just tryed wireing up the tach. which has not been working for years.
i have a 72 engine a but a 67 dash so larger tach.
ive tryed it now with the igniion fed in to the tach with a male bullet connecter then used a female out. to the Pos+ side of the coil.
ok first off i didnt smoke the condenser in the destributer so that was good.
but the tach did nothing.
and when i turned off the key the car kept running. and when i turned off the battery kill switch the car kept running.


help!?
steve

 

[dklawson]

I assume that the '67 would have had a Smiths RVI tach. If so, it is wired in series with power going to the coil (2 white wires connected to a loop on the back of the tach). Later RVC type tachs use a single sense wire connection to coil (-).

Which letters do you find on the front of your tach (RVI or RVC)?

 

[jlaird]

Now what I wonder is has it run out of fuel yet?

 

[Sopwith_Camel]

I turned down the idle till it stalled. i should have just blocked the tail pipe.
no its not the one with the loop a years ago last time i worked this project.
this one is has the internal loop so a male and female bullet conntecion,
ok looking at the moss box that the tacho came in RVI2430/01
hmm i bought it two years ago... yes i have fixed everything else on the car. and have broken a bunch on the jag.

steve

 

[dklawson]

tach. which has not been working for years.
fed in to the tach with a male bullet connector then used a female out. to the Pos+ side of the coil.
but the tach did nothing.

and when i turned off the key the car kept running. and when i turned off the battery kill switch the car kept running.

Sorry, I missed those wiring details when I read your first post. The connections alone confirm this is an RVI tach. You have wired it correctly for an OEM installation. I suspect your continuing running issue and dead tach issues are not related.

The RVI tachs are inductively coupled to the ignition coil wiring so a failure of its electronics should neither provide power to keep the engine running, nor interrupt power to the ignition system. The quick test is for you to disconnect those two white wires on the back of the tach and plug them into each other. With the white wires connected the tach will not work but the ignition system will. If with the white wires connected the engine continues to run with the key "off", that says that either the ignition switch is faulty OR (I assume you have an alternator on the later engine) that the diode pack in the alternator may have failed. Test number 2 if the engine keeps running is to stop the engine (by disconnecting the coil (+) wire) and unplug the alternator. Do not unplug the alternator with the engine running. Start the engine again and turn the ignition switch off. If the engine keeps running, you have an ignition switch problem. If the engine stops normally, that says you have an alternator problem. The fact that you can turn the battery kill switch off and the engine keeps running suggests the alternator is at fault (See footnote below). Most parts stores will bench test alternators for free.

The tach itself may not be working for a couple of reasons. First, it is old. Second, RVI tachs generally do not work with electronic ignitions. If you have an electronic ignition (not mentioned yet) that could be why the tach has never worked.

Footnote on the battery kill switch. If this is a simple two-pole switch (battery cable in, battery cable out) and you have an alternator... do NOT use it to try and stop the engine except in an emergency. There are FIA certified kill switches with multiple contacts to disconnect the battery, disable the ignition system, and dump alternator output through a dropping resistor. Those can be used to safely separate the battery from the car's wiring when an alternator is used. The problem with a simple 2-pole switch is that whenever you unplug/disconnect an alternator from the car's electrical circuit you remove the load that the alternator is providing current to. This can create a voltage spike that damages the electronics inside the alternator. The FIA switch handles this quite differently. In short, don't disconnect the alternator from a car's wiring using a simple 2-pole switch or by unplugging it with the engine running. Only disconnect the alternator with the engine off and/or when using an FIA certified battery switch.

 

[Bayless]

Thanks Doug. Some good information there. Some stuff I didn't know or at least didn't understand.

 

[Sopwith_Camel]

wow that is an essay, thanks
ok sun is comming out an i will start with the white to white
answering a few questions
no the tach is new (been on the box on my shelf for 2 years)
yes it has an altinator.
no it dose not have elctornic ignigiton
yes its just a two pole battery kill switch, unforturalty on the neg side.

 

[Sopwith_Camel]

ok the white to white with out the tach in the loop no change from yesterday.
maybe i wireed in white lead from the iggnigiton to the wrong place? there are a lot of white wires comming together under the dash,
ok back to "normal" white ignition wire to coil Pos, car starts and shuts off no problem, and white/black striped wire from tach to Pos of coil.

gas leak in the B, so thats next on the list

 

[dklawson]

white to white without the tach in the loop no change from yesterday.

back to "normal" white ignition wire to coil Pos, car starts and shuts off no problem,

and white/black striped wire from tach to Pos of coil.

There are some issues here. Let's address them in reverse order.

There MAY be a white/black wire that goes from coil (-) to the distributor. Though a white/black wire is used with the later RVC tachs... there should NOT be a white/black wire going to an RVI tach. If you have a white/black wire going to your RVI tach... remove it and tape it off. More info on RVI tach wiring below.

I don't know how many white wires there will be on your particular installation. In Lucas wiring codes, white is "switched" and "unfused". All original/factory white wires should be "hot" with the key in the run position.

You listed some of the tach wiring in your first post. Based on your statement in the post above about a white/black wire on the tach I want to clarify the RVI tach wiring. Forgive me if this is what you already have but I want to make sure we are talking about the same connections.
For an RVI tach there should be...
First white wire from the ignition switch to a bullet connector on the back of the tach.
Second white wire from the remaining tach bullet connector to coil (+).
Green wire (switched and fused) to the spade lug on the back of the tachometer. This will be the spade lug next to the bullet connectors. It is on an insulator plate and should not be confused with the ground connection spade lug sometimes spotwelded to the gauge case.
Black ground/earth wire connected between the tachometer case and the car's chassis ground.
(Some gauges use a spade lug spot welded to the gauge case for ground. Others use a ring terminal placed over a gauge mounting stud for the ground connection. Regardless, make sure the tach has a ground).
White/black wire from coil (-) to the points connection on the distributor. (NOT part of the RVI tach circuit.)

 

[Bayless]

You said "a lot of white wires coming together under the dash." That doesn't sound right. There should be very few, maybe as few as 2 or 3, all coming from the ignition switch. One goes to the coil (or tach) and the other to the fuse box and, depending on the year, maybe one to the ignition warning light. If you really have several, I wonder if the DPO has buggered the wiring some. If so, we are shooting in the dark here.

 

[Sopwith_Camel]

yep
thats what i had when the car was running and not stoping. and the tach not working.
i did try the first white wire in to the tangle of white wires hanging down under the dash with no luck.
i wish i had stumped up for a new wiring harness when i build the car ten years ago.
steve

 

[Sopwith_Camel]

not as much as my jag the cloth wire coverings have faded and the plasic underneath is all black.
the first time i wired it up I put it together with the white coming off the igition switch. and put them back into the jumble of whites that had a brown leading in/out.

I was suppose to put a new wireing harness in the B this summer but with work starting tomarrow and the jag engine in bits it looks like next summer for that.
steve