f3nr15

Hi all;

Started the car up last week after it had been sitting for a few weeks

In doing so, I removed the DME relay and cranked the engine up few times, before replacing it.

As I went, the tach began to malfunction - initially, the needle swung around to 4K rpm as I clicked the ignition to position 2, and then swung back slowly to 0.

It then began swinging erratically as the engine was started.

The car drove fine, and for most of the journey, the tach simply sat at 0 (i.e. unresponsive).

Upon turning the headlights on for the journey home, the tach gained some life again, and started swinging erratically, and all the way around to the 6 o'clock position.

Any thoughts?

Could this have been caused by removal of the DME relay in starting?

Might this represent a more serious charging system issue, or is it likely just a bad tach?

tbennett017

Just the basics from me... but you can gently pull out the tach and check the connections at the back... all tight? All good? I've had erratic readings simply due to loose connectors.

mojorizing

I too pull the DME relay and turn the engine over after the car has been stored.

I agree that you have a connection problem to the tach.

John Boggiano

Bad connection did something similar to me once.

f3nr15

Thanks all - will have a check of the connection tonight and report back.

ras62

It will not be the DME relay but is very likely to be borderline battery voltage. Good luck with getting it sorted.

f3nr15

The plug to the tach was firm - too firm! I could hardly take the damn thing off :-)

I ended up sending the unit out to Hartmut at North Hollywood Speedo, who confirmed that the tach was kaput -

He's advised me to check out the charging system for faults before reinstalling the repaired tach incase faulty charging was to blame for the tach problem in the first place.

Doe anyone have any tips on how to go about checking the charging system?

f3nr15

Okay, so being the impatient bastard that I am, upon receiving the repaired tach from Hartmut I plugged it straight in and fired her up.

Same problem - tach swung wildly, and all the way around to 6 o'clock - so I'm gathering that this is not a tachometer problem.

The car is always connected to a battery tender, so I'm guessing that borderline battery voltage is an unlikely culprit.

I've ordered a new DME relay for the heck of it.

I guess what I'm wondering now is if in fact this is the first sign of alternator regulator failure - I haven't noticed anything else malfunctioning, but the reality is that I don't drive the car frequently enough to know.

If the alternator reg is failing, do I risk damaging the DME by operating the car?

Lorenfb

"Same problem - tach swung wildly, and all the way around to 6 o'clock - so I'm gathering that this is not a tachometer problem."

Great when a repair service (NHS) indicates a problem when there is really no problem, right?

Possibilities:

1. bad engine ground and/or DME/ECM grounds
2. poor connections at the battery
3. electrical noise from a bad alternator
4. electrical noise from the ignition system, spark plug wires, connectors, caps/rotors, bad ignition coils
5. voltage drops in the wiring harness

f3nr15

Loren thank you, that's a great "map" for starting to look at this, really appreciate your input.

I'll let you know how I get on.

f3nr15

Just a follow up to this thread for posterity -

I gave up and dropped the car off to Simon Greenwood at Autowerks.

He pointed out that the strange tacho swinging actually commenced with the first click of the ignition switch, and not just with the engine running, so obviously not then a charging issue.

He also confirmed that the grounds were all in good nick, and that the charging system seems to be fine.

Simon then sent the gauge off to a VDO specialist who indicated that the internal mechanism/processing unit for the gauge was kaput, and not serviceable.

Currently, I've sourced a replacement gauge from eBay (actually from a 993). Still waiting to find out if fitting it solves the problem...