Digital Multimeter Dwell/Revs problem

[stuartgb100]

'71 Sprint, standard ignition system, alternator conversion, negative earth.

Problem: when I connect the meter to the dwell or rpm setting, the display goes haywire and will not give a steady reading. I borrowed another meter and had the same problem.

The car's rev counter does not work....that's to say for a long period it will stay silent, then flicker into life for a short period, but never staying steady. I've checked the wiring (as has an auto-electrician) and we both think there must be a fault within the rev counter.

I'm wondering if the rev counter is somehow interfering with the digital meter? If so, is there a way to bypass the rev counter?

Any thoughts would be much appreciated.

Regards.

[john.p.clegg]

Don't shout at me if i get this wrong,but i'm having a beer at the moment.
There are two types of rev counter,one is in circuit and senses the current passing through the wire to the coil(wire twisted at the rear of the rev counter), the other is just teed on at the coil and senses the on/off volts caused by the points(wire plugged into the rear of the rev counter).
Which type do you have?
When did this start happening?
You say alternator conversion/negative earth?
Do you have the right rev counter?
John(chilling)

[types26/36]

[quote="john.p.clegg"]Don't shout at me if i get this wrong,but i'm having a beer at the moment.quote]

WHY?..... DOES BEER MAKE YOU DEAF?

[stuartgb100]

John,
Have only owned the car for the last 2 months, and inherited this problem from previous owner (his comment: "must be a loose connection").

Cannot contact him at the moment for additional info.

I know he rewired the car way back, and the standard of the rewire IS good (IMHO). He also fitted an alternator, and I inherited old dynamo and control box in a box of 'goodies'. I mentioned this just in case it might have a bearing on the problem.

As to the rev counter:

It's a Smiths, marked 4 cyl, neg earth, and RVI 2419/00 stamped on the face. There are a pair of bullet connector wires in the back (one male, one female) which I assume are the inductive pick-ups. Certainly if unplugged, the engine stops (vbg).

Suspect colour of wires would mean little, since the general colour scheme of the rewire doesn't seem to follow the Workshop wiring layout I've been using.

Does this help?

Thanks and Regards.

[john.p.clegg]

Sorry about this but i've had a few beers now (no it doesn't make me deaf but it does make me get thing rwong) and i'm off to Corfu tomorrow,if your problem isn't solved by then i'll start again
Sorry
John

[types26/36]

It's a Smiths, marked 4 cyl, neg earth, and RVI 2419/00 stamped on the face. There are a pair of bullet connector wires in the back (one male, one female) which I assume are the inductive pick-ups..

My Sprint (72) rev counter is marked exactly the same and from memory also has the bullet connectors, I also did an alternator conversion, the car was originally a negitive earth but I do have Luminition unlike you.
A few years ago the rev counter gave problems something like you are having and a new circuit board was fitted, its fine since then so I would suspect a faulty rev counter.
Why dont you disconnect the coil from the cars electrical system, run one live wire from battery to coil, one wire coil to distributor thereby cutting the rev counter out of the system. Start the car and try your meter
Brian.

[stuartgb100]

Brian,

Thanks for the suggestion, but no luck. Even with the tacho completely disconnected, the readings were still haywire (both dwell and revs).

Is this a general issue (the use of digital meters on older cars) or specific to me, please?

I'll have the tacho refurbished later this year, but in the meantime I'm completely unable to set the dwell, and to check the distributor's advance curve........ bit of a bugger, that.

BTW, I came across an old book of mine a couple of weeks ago, which is dedicated to tuning Lucas ignition systems (probably out of print now).

In the back of this is test data for approx 800 Lucas distributor variants (both vacuum and non-vacuum).

I also have specific test data for the twin cam distributor variants.

Would any of this be of use, or is it already in the public domain?

Regards and thanks.

[mark030358]

Hi there,
Couple of points on digital meters. They have very little in the way of damping. The rpms will not read perfectly steady. Are you reading rpms or frequency, if frequency then divide by 4. Could could have contact bouce, caused by worn points, this will give erronious readings.

hope this helps
Mark

[stuartgb100]

Mark,

Thanks for that.

The readout is rpm direct (ie crankshaft rpm, not distributor).

When I say haywire, I mean a constantly changing display, which can include negative and positive values.

No steady reading at all.

Points are brand new (besides, I'm sampling at tickover, so bounce should not be an issue).

Talking of bounce ....... can you still get those old Cosworth points (rated at 32 oz) which were good for about 8000 revs?

Regards.

[types26/36]

is this a general issue (the use of digital meters on older cars) or specific to me, please?................................................................

Well I have a digital test meter and it works just fine, it registers the rpm in variations of 10 rpm. It also shows dwell but in % but I dont know how relevant that is with electronic ign. It used to show dwell when I had a points system.


completely unable to set the dwell, and to check the distributor's advance curve........ bit of a bugger, that..................................................

Revert to the time tried method of setting the point gap


BTW, I came across an old book of mine a couple of weeks ago, which is dedicated to tuning Lucas ignition systems (probably out of print now).
In the back of this is test data for approx 800 Lucas distributor variants (both vacuum and non-vacuum).
I also have specific test data for the twin cam distributor variants.
Would any of this be of use, or is it already in the public domain?.........................................................................

Well the manuals give some info although somewhat confusing, some people on this list have tried verious settings and advance curves with modified engines and could reccomend settings but I for one would like to see your info, if you can publish on here that would be great.
Brian

[stuartgb100]

Brian,

As I'm sure you know, reverting to the old method of setting points gap is really only even marginal when the distributor is new, let alone 30 odd years old .... but it will have to do for now!

Here's a sample of the Lucas ignition data which is specific to the Lotus Twin Cam engine, as opposed to the Elan family:

Quoting direct:

"No fewer than 5 different distributors were used on the twin-cam engine, all with varying degrees of advance. Since they all appeared to function on the same basic engine, how could they work correctly with as much as 12 degrees difference in advance between them? Unfortunately, once again, the various workshop manuals are misleading, and the earlier manuals are wrong.

Before unravelling the complexities, and explaining why different distributors were used, one particular aspect must be cleared up, namely that Lucas distributors 23D4 and 25D4 were current at the same time. The 25D4 had a vacuum retard mechanism, while the 23D4 did not. The early 25D4 (as stated in the workshop manuals) is not the same as the late emission/European 25D4. Distributors used by Lotus on the twin-cam engine are shown in the table below."

23D4 ref 40953 introduced 18/Sept '63 Ignition advance (Distributor degrees) 11-13 degrees. Fitted to Standard Weber, Elan, Lotus Cortina. All domestic (UK) Strombergs on Elan, +2, Europa.

23D4 ref 41189A introduced 6/March '67 Ignition advance as before 6-8 degrees. Fitted to all S/E engines on Elan, +2, Europa, L/Cortina S/E, Escort twin-cam, all domestic Big Valve engines.

23D4 ref 40930 introduced early '62-63 (ceased early '64) 12-13 degrees
1498cc T/cam, early 1558cc T/cam on Elan, L/Cortina.

25D4 ref 41125A introduced 3/Jan '68. 6-8 degrees. Federal exhaust emission Strombergs only -- all types, Big Valve Dellorto, domestic and european,ECE 15 regulations.

25D4 ref 41225 (vacuum retard removed). 6-8 degrees. European, late-model domestic Big Valve engines, pre ECE 15 regulations.

The current Lucas model is the 43D4, which is a direct replacement for the 41189 and 41225 only.

The static ignition-timing settings listed in the different manuals vary, but the true static settings are as follows:

1498cc/early 1558cc Weber (B type cam) dist ref 40930 7 degrees BTDC

All Weber standard (B type cam) 40953 12 degrees

All Stromberg domestic (UK) C/D type cam 40953 9 degrees

All S/E Weber (C type cam) 41189A 10 degrees

All Big Valve Weber/Dellorto domestic 41189A 12 degrees
(with a D type cam)

All Stromberg Federal emission 41225A 5 degrees
(with a C/D/E type cam)

All European specification Dellorto 41225A 5 degrees
to ECE 15 regulations
(with a C/D type cam)

All Big Valve Dellorto, domestic and 41225 10 degrees
European, pre ECE 15 regulations


Now to detail: the advance curves for all types. Please note that in the manuals, Lotus got confused between crankshaft rpm and distributor rpm. Distributor rpm is half crankshaft rpm; in some cases, static settings have been added, in others they have not! the following tables are the final settings, ie. static settings have been added, so maximum advance is exactly as it says..........."

to be continued ASAP.........

At this rate there'll be another 4 parts or so, but only if it's of interest.

Regards.

[marcfuller]

Very interested! Thanks! Looking forward to the next chapters.

[types26/36]

Carry on! great stuff
Brian