MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Dwell Meter

Anyone have experience of using a dwell meter with Positive Earth?

I've tried 2 digital types and neither work for dwell or tacho. They're the 2 lead type where one goes to the coil/dizzy connection and the other to chassis.

Changing polarity gives no joy so I wonder if they only work with negative earth? Both had manuals and neither mention Pos Earth.

The old analogue gunson's Sparktune II is meant to be OK with Pos Earth, anyone advise if it's worth tracking one down or other models?

Thanks, Rich.
Rich

If I'm not mistaken a dwell meter is just an analog volt meter with a different scale. It's connected between the distributer side of the coil and ground with it's (+) lead to the coil and the (-) to ground. Every time the points close voltage is established and the analog meter tries to display it. The points open before the meter has time to display the total voltage, then tries to drop to zero. The points close again repeating the cycle. The meter stabilizes in one spot depending on RPM's. Hooking an analog meter up backwards seems like it would give you the necessary polarity to make the needle deflect in the direction you wanted it to.
tom

Rich,

I have a 25 year old analog elCheapo special that works fine on positive ground--as a tach. I connect black wire to soliniod, red wire to ground. Very handy to have in the engine compartment rather than having to look at the facia for info. Has a low rpm setting that allows me to set rpms within 15 to 20 which of course helps with setting the timing

I've never used it as a dwell as I don't know how to hook it up. Was looking for the archived thread on this subject--discussed around Jan of this year, but could't find it. Black wire to the coil, eh? Which side, cb?

Paul
Paul Hanley

Just tried my new/old dwell meter. It works! Connected black to cb and red to earth. Unfortunatly, it read 70. Book says 60 +/- 3. Does that mean my point gap is too big or small? Thought I was dead on @ .015.
Paul Hanley

Paul; Too small.

Clifton
Clifton Gordon

I think it depends on the instrument you are using. Some digital instruments show 90 degrees when connected the normal way round for negative earth to a positive earth car, which would mean the points are closed all the time, which they obviously aren't if the engine is running. Reversing the connections, if there are only two wires, *should* work. If you have an instrument with two wires which connect to the 12v supply and a 3rd to the coil you may have to reverse the power supply wires, then it may indicate the complement of the actual dwell i.e. 90 minus 60 e.g. 30 when correct. This may also happen with two-wire instruments, so try connecting it between the coil -ve or CB and 12v (i.e. a brown or white at the fusebox) instead of an earth. The 'sense' wire always goes to the CB connection, as that is the one that should be switching between 0v (ground) when the points are closed and 12v when they are open.
Paul Hunt

Paul Hanley (getting too many Pauls on here). I would suspect that your gap is too small rather than the meter reading the compliment of the dwell angle as Paul Hunt suggests. A 10 degree discrepancy between dwell meter reading and point gap set with a feeler gauge is not uncommon, nor is it a large discrepancy. If you were reading 20 degrees or 40 degrees, I would agree that your meter was probably reading backward. I would also thing that hooking your meter from CB connection and the 12 volt line, would read the complement of the angle. I have a 3 wire dwell/tachometer like Paul Hunt talks about and it does, in fact read the complement on our TD which is positive ground. Cheers - Dave
David DuBois

Thanks,

I agree with everyone's thoughts but neither digitals work at all whichever way I connect them, they both throw out jumble figures for a few seconds then go blank.

They're both 2 wire type. I can only think they're taking both power and sensing from the same connections, need to be powered up the correct polarity, and this is then is wrong for their sensing.

Problem is I don't have a neg earth points type car to test them on so hoped someone might have been here before. No matter, I'll get an analogue as Paul Hanleys works and take the digital to my next car rally to test on someone elses.

Paul, dwell/gap contradiction can happen, often with pattern points. Perhaps open the points 2 or 3 thou and see if it comes down nearer 60. If not try a new set of Lucas.

Dwell sets coil charging time but also the point at which the heel contacts the cam, and hence the opening speeed. If dwell's too great the points will open slowly, as the heel will be nearer the cam peak rather than on the ramp. I don't know at what point it would become a problem.

Rich
Rich

Regapped to .017 and dwell reading was 62. Leaving well enough alone as she's running as well or better than ever.

When I said I had an analog meter, I meant a dial face with a swinging needle--like a audio meter. It has 2 wires plus the spark wire pick-up.

Rich, I don't know what it would cost to post this gizmo to you round trip--likely more than the instrument's worth--but you're welcome to contact me via email to investigate.

Well, off for a 40 mile run to enjoy the 75 degree weather and to get the engine hot enough for a valve adjustment. Cheers

Paul
Paul Hanley

Paul

I'm pleased your adjustment came right. Thanks for the kind offer of the meter but as you say probably not worth the postage. I gather the old Sparktune will do pos earth and they're going through eBay all the time so I'll get one. Lucky devil, temp climbed into the 60s here today but it was good to see the sun :-)

Rich
Rich

Paul - If you can say for sure that the gap is 0.015 or 0.017 with the distributor you are a hell of a lot better than I am. That is why I always use the gap as a rough setting and then refine it with the dwell meter. Dwell readings are just an electronic measurement of the amount of engine rotation that the points are closed, which is an indirect measurement of the gap when the points heel is on the peak of the distributor cam. Essentially, gaping the points and setting the dwell accomplishes exactly the same thing. Cheers - Dave
David DuBois

Tricky to check the gap on the car but easy off. Also best to check on all 4 lobes as a dwell meter will give an average and won't show problems like an off-true spindle. Expect a thou or 2.

If you've never done it, hook the timing light to all the cylinders in turn (make a new mark for 2 and 3)and, if needed, average the timing as long as it doesn't pink. Sometimes throws up something important - for example, pinking might only be happening on 1 cylinder and it may show why - but usually just bear any variation in mind and err on the safe side.

Rich
Rich

I agree that the dwell will average out the settings for all four lobes but .002" variation !! I had a car years ago with a Delco dist which read .010" and on the opposite lobe .020" The dwell meter or a new dist was the way to go.

I have an old Gunsons analogue meter which is known to be accurate and can be used to set the dwell with the distributor turning. Remove the cap and rotor connect the meter and remove the spark plugs. Turn the motor on the starter and adjust the points in the slot. Fully tighten the screw and recheck if you wish with the engine idling.
Iain MacKintosh

Hi Iain

The 1 or 2 thou is what I got checking a good diz just installed.

The point of checking on the 4 lobes for gap is that a dwell meter would not give a clue to even a 10 thou discrepancy such as yours yet the cylinder timings would have been different.

Rich
Rich

Agree entirely Rich, if there is timing scatter then the whole thing is pointless (excuse the pun). Timing will be averaged out and performance lost.
Iain MacKintosh

Rich. You have an excellent point. I have measured several points cams and the best ones will show about .001" of variance between the lowest and highest lobes. "Bad" ones will show about .005" difference. One points cam and distributor shaft combination showed .045" between the lowest lobe and the highest (bent dizzy shaft was part of the problem here). Needless to say, the run out of the points cam and distributor shaft can affect timing on all cylinders making "spot on" for number one and something else for the other three. At least once every few years, measuring the points gap on each lobe is a great idea. Les
Les Bengtson

This thread was discussed between 09/04/2005 and 11/04/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.