Any experience of using a dwell meter?

[1969Mk3Spitfire]

I have recently bought a Draper DMM which includes dwell and tacho for 4,6 and 8 cylinder engines.  I've just replaced the points and wanted to check dwell (compared to the time-honoured method of simply relying upon the feeler gauge).  I hasten to add that I have managed without such a device for my entire motoring career.

Connected the com (black) terminal to earth and the dwell (red) terminal to coil/contact breaker. The DMM will not hold a steady reading.  When first connected, it displays around 40 deg (my car spec is 40-42 deg) but very quickly counts down to zero, over say, 10 to 20 seconds.  When I change to setting to 4 cyl tacho, it reads approx 2 x rpm but reduces to about 1.2x actual rpm.  

All very strange. Not sure if this is operator incompetence, faulty meter or an indication of a potential issue with the ignition system.  The cars runs well.

How does this compare to your experience?

 

 

[NonMember]

I've not used a DMM like that but my timing light has dwell and tacho features. I don't see that strangeness - both readings appear to be stable and reasonable.

Your DMM's readings are obviously bonkers and your connection method seems reasonable, so it may be a faulty meter. It's unlikely to be an ignition system fault, as zero dwell would definitely stall the engine!

[johny]

Exactly how I use my old Gunson analogue tester but yours probably has a battery in it so if low could that cause this problem? Time to ask Draper methinks....

[Pete Lewis]

after 50 years without one    and now you have one and its worry time      

if the gaps right the dwell is as designed    whats to worry    ...........a must have  you must not have      put it in the bin    sleep easy 

Pete

[dougbgt6]

Pete,

Ditto.

Doug

[1969Mk3Spitfire]

I’m generally quite happy to not know engine oil pressure or dynamo charge rate etc but I have a bit of an obsession with DMMs. Any excuse and I’ll buy one……..I need therapy.

I’ve already contacted Draper. Their interweb Help desk was very quick to respond but couldn’t help so passed my query to tech support.

This is the response:- “you can’t use them functions on a negative earth in the same way as you usually would”

I’ve very politely and courteously asked the Help desk to refer my query back to tech support as I didn’t understand the comment.

The points have been set with “a fag packet” so I’m sure that dwell angle is fine………..sadly, I’d like to measure it and not impressed that my new meter doesn’t work. 

[chrishawley]

I assume you have the Draper DMM300. Just so happens I was using mine today to confirm the manual adjustment of the points.

Instruction manual is not at all comprehensive. So: Black lead to COM, red lead to V/Ω. Turn on meter and check 'hold' and 'max' functions have not been turned on. With meter set to '4-cyl' but not connected to anything the meter should read 90 degrees dwell (i.e. 100% dwell time for 360/4). Start engine. Black lead down to a firm earth e.g battery negative terminal and red lead to coil negative (i.e. points side of coil). Should now give a stable reading of dwell  which fluctuates by only half a degree or so.

Of course the meter is supposed to work on negative earth systems!!!!

Apart from a duff meter the only other things I can remotely think of which might mess the dwell measurement are a failing condensor or wrong coil (e.g. 6V 'ballasted' coil on a non ballast system) but since the car is running fine that wouldn't seem likely.

 

[1969Mk3Spitfire]

Thanks, Chris, yes, it is the DMM300.

I don’t think it’s the condenser. As I wrote above, I have a bit of an obsession with meters so I have an LRC DMM, too. I’ve measured the condenser at 0.21 micro Farads which is pretty much bang on spec 😀

[Pete Lewis]

be suspicious of any condenser with an orange wire   theres little inside 

Pete