Zx Tacho Intermittent

[whodee]

I've searched the site for info on this but found nothing yet. The tacho on my ZX works fine some times and not others. It might be temperature related as during the cold snap it seemed to be working more than not working.

 

I've read elsewhere that a faulty TDC sensor is the most likely problem- that was after I'd taken the tacho out to look at it for any obvious sign of a fault.

 

Can anyone tell me whereabout on the engine I can find the TDC sensor. I've got a Haynes manual which only mentions it in passing- no pics unfortunately. I've searched the web had info on every car except a zx or so it seems.

 

Any help appreciated, ta.

[Shemer]

TDC sensor, or crank sensor, is located on the engine left side, above the flywheel, towards the firewall. I think it's brown colored.

 

But anyway, I don't believe it's this sensor, else you whould have missfire from the engine.

 

I would put my money on a faulty tacho mechanizm. It a known problem in the ZX. I would look for other instrument panel in break yards. Should be cheaper than replacing the mechanizm with new one.

[Guest Colin Hunter]

Is the car petrol or diesel? If it's diesel the tacho works off the alternator. petrol ones are fed from the engine electrics. Can't begin to hazard a guess until you tell us!

[whodee]

Thanks for the quick response Shemer.

 

I'm curious why a faulty sensor would cause a misfire.

 

From what I can see from the wiring diagram it doesn't look like the sensor is wired into the engine management system (such as it is) in any way. Also no mention of it in the fuel system part of the Haynes manual I got.

 

I'll try tracking it down anyway and see if it looks ok. As I've said above I've taken the tacho out and it looks fine, as far as I can tell just by looking at it but something somewhere is faulty.

 

These intermittent problems drive me nuts!

[whodee]

Forgetting the obvious I was.

 

It's a diesel. A 1.9TD Avantage to be exact.

 

Apologies for not mentioning it up front and thanks for being interested enough to ask Colin.

[Shemer]

You won't be able to see if the tacho mechanizm is faulty, since the problem usualy lays in the electronic board which is part of the tacho, not in the needle, motor or what so ever.

[whodee]

Something else I should have mentioned upfront is the age of the car, it's a '94 reg one with just over 150k on the clock.

 

The behaviour of the tacho is intermittent as I said but it will operate for a few minutes at a time. Having taken the tacho out to have a look at it and not found anything obvious I'm inclined to remove the sensor and see if there's anything wrong with that first. Which is why I was asking for its location. I haven't completely ruled out the instrument or circuitry yet.

 

I was interested to know why you thought a faulty sensor would cause misfiring tho'. From the wiring diagram in the Haynes manual I've got I can't see anything connecting the sensor to the fuel injection system.

[Shemer]

my bad, not missfire, but stall. no crank turning signs, no ignition timing point to ECU.

 

http://en.wikipedia.org/wiki/Crank_sensor

[Guest Colin Hunter]

Seeing it's a diesel the tacho will be driven from the Alternator. There may be a dodgy connection at the alternator so check there first. Have a look in your manual. I'm sure the diagram will point you in the right direction. I don't have a Diesel ZX and that diagram won't be in the petrol manual which I do have.

 

The 1.9 TD ZX has neither ECU nor crank sensor. The fuel timing is controlled by the mechanical fuel pump and the amount of fuel injected is controlled "solely" by your right foot! That's it! no fancy electronics on this one lads! Just good old fashioned mechanics!

 

Cheers! Hope this helps :)

[Shemer]

You're right, once I saw it's Diesel, I wanted to write that the whole mambo-jambo doesn't exist in there.

 

Anyway - I still check the tacho mechanizm. many of the ZXs I've met had the same problem. you'll need a specialist for that, nothing you can do at home unless you're familiar with this.

[whodee]

Job done.

 

I tracked down the sensor eventually, with a little help.

It's located on the back of the bell housing flange. Not easy to see unless you can get under the car easily, the pit in my garage comes into its own here!

 

The sensor looked a little cruddy but not enough I thought to prevent it working. However, I cleaned it up and the connector on top of the engine and hey presto! A working tacho.

 

How long it continues to work is another thing but it certainly points the finger at the sensor I think. And it's a lot easier to get to than the tacho itself- I don't fancy taking that out again.

 

Thanks for your contributions guys I appreciate it.

[Guest Colin Hunter]

Well. Wrong again! I was under the impression that the tacho was driven off the alternator. Most diesels of that era were. I just assumed the ZX would be too! Sorry! :D