An electrical puzzle?

Technical MGB discussion
Ian Fozzard
Posts: 370
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: An electrical puzzle?

Post by Ian Fozzard » Mon Mar 30, 2020 1:07 pm

Hi Paul, yes the TL93 will only provide a reading when connected to the correct terminal on the coil - connected to the other one there is no dwell reading at all.
As I've said earlier the problem seems to lie with the power source. When connected directly to the midget battery it gives the "correct" reading of circa 60 degrees on the MGB. It also works fine on the midget as well, where of course it is always powered from a direct connection to the midget battery.
I have read through the reference that Peter supplied earlier (thanks Peter) and am a little wiser, but nothing has leapt out and provided a solution so far.
At least I can get the light to work on both MGs now, as long as I remember to use the correct power source, I may well leave it at that instead of tying my brain in knots!!

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

ChargedAutoGT
Posts: 29
Joined: Wed Nov 21, 2018 9:37 pm
Forename: Graham
Surname: Moore

Re: An electrical puzzle?

Post by ChargedAutoGT » Thu Apr 09, 2020 11:28 pm

ian

How did you connect the unit to the fusebox? a standard supplied croc clip?

I suspect yes & the issue may be the quality of the connection you have to your power source. Ever tried to jump a car battery and it won't - then you waggle the clips and it works? When you put your mater on the fuse you have a good connection but even then you sometimes have to do a bit of scraping with the probes.

Why not put a spade with a trailing wire onto the fusebox (assuming you have not already) then connect the gun to that. I have a feeling all will then work.

If the gun is not getting the voltage it needs, all bets are off and readings are meaningnless. of course you may be lucky and get some 'accurate ones' but if you don't know when you can rely on the numbers then they are not much good.

Last year I was testing a Gunson Gas tester. All was fine until i got to RPM measuring. It seemed to be about 1.5 times what i should be getting. I was using an external battery. When I connected to car battery all worked. It was comparing voltage readings to what it was getting. So the two were out of phase. Now this is of course different to your issue but a similar headscratching moment until the 'light came on'.

Do please report back....

Ian Fozzard
Posts: 370
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: An electrical puzzle?

Post by Ian Fozzard » Fri Apr 10, 2020 11:09 am

Hi Graham,
I think I have come to the end of the road with this one, not managed to identify the actual cause, but found a perfectly good work around.
The problem is solely with the power supply and the dwell reading when using that power supply (from the fuse block). I do use a crocodile clip but that works well for all readings except dwell, and is providing just over 13 volts with the engine running. I can only assume that there is some other electrical variable at work which interferes with the dwell measurement on the digital readout of the Sealey light. My old analogue dwell meters when set up in exactly the same way give a correct reading (correct inasmuch as the dwell is 60 Deg. with points gap set accurately at 15 thou at the cam peak).
If I use a direct supply from a 12 volt battery (in this case my midget battery) I get perfectly good, believable, readings from my BGT and from my midget, so I am happy to leave it at that point. Would be very good to understand what is going on with the fuse block power source, but beyond my attention span I'm afraid!!

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

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