XKE S2 Tach Issue

E-Type
1

Hi All,

I am working on rebuilding a friends S2 XKE. I had all of the gauges redone by Nissonger a couple of years ago.

As I now start up the engine, the tach does its job, showing the correct rpm…

But as I adjust the choke as the engine warms up, the tach drops to zero…

Any ideas, comments as to why it does this?

Thanks, Bob

2

Bob,

That is bizarre. Before I even think any more, please let us know what ignition system you are using on this car, and what the Chassis/Car Number is. The reasons for these questions:

  • Later Series 2s were equipped with a ballast resistor in the ignition circuit. This could be relevant.

  • If the original points/condensor ignition system has been replaced with any for of electronic ignition - e.g. Pertronix pickup, 123 distributor, EDIS… it may affect the operation of the Tach.

Before anyone speculates on the source of this behavior we must know the details of the ignition system on the car. One other question, since you say the Tach was “redone” by Nisonger: Do you know what they did? Were they told to refurbish it, modify it for use with Pertronix, or what?

-David

1 Like
3

Hi David,

Car No. P2R13831
Body No. 4R8539
ENGINE No. 7R12251-9
GEAR BOX No. KE13699

The distributer is a Mallory dual point.
It does have a ballast. Maybe the ballast is defective?

Thanks!
Bob

4

Since you tach has been recently rebuilt (almost certainly converted to modern guts) I’d suspect the issue is external. There have been reports over the years of loose or cracked terminal connections resulting in intermittent operation as things heat up. Suggest you check the spade connections, especially at the coil. The wires can fatigue and break where they are crimped to the connector and appear to be sound.

5

Thanks for the information Bob. As I understand it, the Mallory distributor, despite the “magic” inside, it still a conventional points/capacitor type, so that shouldn’t affect the waveform on the Tach line. If the ballast resistor were the issue, I doubt the car would run. Just to clarify, are you saying that the is a definite cause-and-effect relationship between touching and moving the choke knob, and the Tach dropping to zero? If so, and if you pull the choke back out, does the Tach start to work again? The Tach on the Series 2 is electronic, and takes it’s signal from the ignition feed to the distributor. The only thing that I can think of if physical movement of the choke cable has an effect is a bad engine ground. I’ve heard of cases where due to a bad engine ground strap, the ground path has gone through the choke cable. You might also want to trace the path of the signal cable to the Tach in case it passes into the engine bay close to the choke cable. If so, maybe the Tach cable is damaged, and movement causes problems, though I’d expect the engine to stumble then too. Wierd…

6

…strangest thing I ever did hear…

“Adjust the choke” means the rpm’s are dropping? I can’t imagine the tack is not getting enough electricity…

LLoyd

Don’t you just hate those who live in the US for ten years and never bother to learn the language?
I think no one should graduate from grade school without learning either Cherokee, Apache, Ute or Arapaho.

LLoyd July, 2014

7

Hi Guys,

Thanks for the replies. Wire harness is brand new. I actually attached x2 new engine ground cables to avoid any ground issues. When I pull the choke cable back out, the tach still sits at zero.

One add’l note, sometimes the tach works when I put the choke back in, engine warm. It’s the oddest thing!

Thanks, Bob

8

The choke position probably has nothing to do with it - it’s probably the dropping rpm. I’m suspicious of the Mallory dual points. They extend the dwell time. I’d be inclined to disconnect one set of points and see what happens.

9

Bob, does the tach initially stop working at the point the choke warning light goes out, or just rpm change?
Tom

10

I had that thought too, but Bob doesn’t mention that the Tach starts working again when he revs the engine. Bob, can you clarify? We need to determine if the Tach failure is dependent on the choke lever position, or the engine revs.

11

So maybe the thing about the tach and the choke is just that when the revs go up or down, the resultant vibrations simply jerk the sticky tach into operation?? Nothing to do with electronics as such.