An electrical puzzle?

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Ian Fozzard
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An electrical puzzle?

Post by Ian Fozzard » Sat Mar 28, 2020 11:35 am

I'm tempted to offer a prize to anyone who provides an answer to this puzzle!!

I recently bought a Sealey TL93 timing light which also measures rpm and dwell.

Works fine on my MG Midget but has caused puzzles on my MGBGT. Both original ignition set-up.

The problem: The dwell should be 60 Deg. with a points gap of 15thou. For the midget I get the correct reading 60, but for the B I get 30.

The only difference in set-up is that I connected the light directly to the midget battery (under the bonnet), whereas because the B has batteries at the back under the floor, I connected the light to a connection on the fuse box (with a 13 volt output).

I managed to get the correct reading of 60 for the B by connecting the instrument directly to the midget battery and reading the data from the B's electrics. So it looks like the cause could be some issue with the power supply to the light?

Any explanations anyone?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
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Re: An electrical puzzle?

Post by Peter Cresswell » Sat Mar 28, 2020 4:46 pm

Hi Ian,
I can't remember the year of your MGB and although I don't know the real reason for the low reading when the timing light is connected to fuse box, but might I provide a few ideas.
1. It is interesting that the reading you got is half the expected value, and I wonder if your car has a (ballasted) resistance wire feed to the coil. The coils for the later cars are 6 volt and not 12v but during engine cranking the full 12v is used. So have you picked up a point where the resistance wire is still in the circuit and thus gives a reading of half the expected value. There is a hint in this article (http://www.mgb-stuff.org.uk/ignitiontext.htm#coil click on Ignition coils at the top and you then have to go down the page quite a long way to get to some testing figures for different types of distributor and their dwell angles) that a ballasted coil produces a dwell angle of 27 degrees.
2. Did you measure the voltage at the fuse box, and compared it with the voltage available on the Midget. Might be you have some bad connectors in the feed you chose, or the feed had some other electrical load already.
3. try contacting Sealey and ask them! See https://www.sealey.co.uk/contact-us, then use 'Technical Advice'.
Pete
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Ian Fozzard
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Re: An electrical puzzle?

Post by Ian Fozzard » Sat Mar 28, 2020 5:15 pm

Hi Peter, many thanks for the reply. My BGT is a 1972 car, with everything in the ignition standard as far as I know, although I do have a Pertronix coil fitted (supplied by the Distributor Doctor along with my rebuilt 25D4 distributor).
The voltage at the fuse box is circa 13.2v with the engine running ((third terminal down), and more or less the same as the voltage from the midget battery.
The real puzzle is that I get a "correct" dwell reading from the B by using the midget battery as the power source. Incidentally the dwell measurement on the midget checks out fine at circa 60 Deg. This suggests very strongly (certainty?) that the power source is the responsible factor here?
Sealey did not have anything useful to say by the way!
I will read the reference you have provided, and I will report back if I get anywhere with it.

If you have any further ideas given the information above please let us know!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Ian Fozzard
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Re: An electrical puzzle?

Post by Ian Fozzard » Sat Mar 28, 2020 5:21 pm

P.s. the circa 30/35 degrees I am reading could be one of two things perhaps:
1. Half of the real figure of circa 60 degrees
2. Could be 90 degrees less 60 degrees, i.e. the points open measurement?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
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Re: An electrical puzzle?

Post by Vic Butler » Sat Mar 28, 2020 8:17 pm

It's a pity that the timing light leads won't reach the B's battery.
Have you got a set of jump leads you could use and connect the timing light to them to see what the reading is?
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
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Re: An electrical puzzle?

Post by Ian Fozzard » Sat Mar 28, 2020 8:30 pm

Hi Vic, haven't actually tried that, but I did use my jump leads to connect to the midget battery. I am assuming that connection to the MGBs battery would be just the same as this? I can only think that the power from the fuse box is being modulated in some way and interfering with the digital output of the Sealey timing light? Interestingly that power source from the fuse box gives an "accurate" reading of circa 60 Deg when I connect up one of my old Gunson analogue meters (just a needle on a dial).

Ian F.
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
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Re: An electrical puzzle?

Post by Vic Butler » Sat Mar 28, 2020 10:16 pm

It could be that the meter thinks it's an 8 cylinder engine with the reading of 30 degrees but why I can't explain.
Or, if the dwell angle is 60 degrees then the "non dwell angle" must be 30 degrees (360 - (4 x 60) divided by 4)
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Paul Hollingworth
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Re: An electrical puzzle?

Post by Paul Hollingworth » Sun Mar 29, 2020 3:10 pm

I've always connected my Gunson's autoranger meter positive to the bottom fuse, negative to a headstud on the engine. There's a third wire which goes to the coil on the side that leads to the distributor. Its always worked fine on my 71 MGB. In fact on many cars in the last 25 years. I've just converted to Luminition Magnatronic so no need for it any more.
1971 MGB roadster & 2006 MGTF

Ian Fozzard
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Re: An electrical puzzle?

Post by Ian Fozzard » Sun Mar 29, 2020 5:33 pm

Paul, my old Gunson instruments all work fine on the MGB. They are all analogue instruments, and I think your Autranger is also an analogue reading machine?
My puzzle is with the Sealey TL93 which has a digital readout.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Paul Hollingworth
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Re: An electrical puzzle?

Post by Paul Hollingworth » Mon Mar 30, 2020 11:41 am

Ian- does your timing light have a connection to the low tension side and if so have you got it on the right side of the coil ?
1971 MGB roadster & 2006 MGTF

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