Electrical oddity - calling Mr. Kramer!

[MTemp]

Normally I find the electrical side of things reasonably straight-forward and logical, but this one has me confused..... so c'mon John help a man out here!

72E, stock. Recommissioning after full restoration. Battery not connected. Sorting out engine bay electrics.

Coming from connection B (middle) of the cdi box is a short red lead with a male connector, which fits (I believe) to an equally short shrouded female connector coming from the base of the electrical panel near the front 14 pin connector. The wire colour is taped at this connection, but it is open circuit to the red wire on the 3-way fuse holder on the electrical panel, so I think that's right.

Anyway....
Separate the connection:
Across the female to ground - no connection. No surprise there.
Across the male to ground - no connection. Again no surprise I think.

BUT if i go across the two connected together to ground - open circuit! How's that work then?

I have tried various times and it's the same every time. I must be missing something really simple ..... but please embarrass me because I cannot see it.

Mick

[304065]

I dunno who Kramer is but I will give it a try


B lead from CDI = Batterie = +12V.

Looking at L35, the wiring diagram for the "modern" LWB cars. . . we find that B is in continuity with the red wire at the top fuse.

Since you measure open, this means there is an open somewhere. Remove all tape (the factory didn't tape under there) and the break will be revealed.


http://members.rennlist.org/911pcars/SL35-Master.pdf

[MTemp]

Since you measure open, this means there is an open somewhere.

Yes, but the 'interesting' bit is that neither side of the connection measures open separately, only when connected together ....... I cannot see how that can be! If I can run a movie while doing the checks I'll show you this intriguing phenomenon.
There will be an explanation, and I think I need to find it before I put full battery power to the system.

Mick

[MTemp]

Two things:

First I've been advised locally that I've used some imprecise (wrong!) terminology. When I said 'open circuit' above I meant that my multimeter showed a connection - the 'circuit tester' function 'beeped'.

Anyway, problem sort of solved .....

As I said, I was using the 'circuit tester' function on my multimeter last night - the one that 'beeps' when there is a connection.
Anyway I measured the resistance this morning.

Male connector on short red wire from cdi box to earth = 70.8Ω
Female connector to earth = 42.2Ω

Both presumably too high to register as a 'connection' on the buzzer, so I recorded 'no connection' but clearly there is through either the cdi box wiring in one case, or the speed switch etc. on the other

However, providing two resistors in parallel

Req= 1/(1/R1+ 1/R2) = 1/0.0378 = 26.4Ω
Measured = 26.6Ω

Presumably below the 'buzzer threshold'.

Now the only question that remains is are these what they should be .....? No reason to believe not, but if anyone fancies a small test on a cdi/mfi motor......

Mick

[ESully]

You have to realize there are other things connected to the red supply wire feeding the CDI. If you look at a wiring diagram, the feed comes from the line side of fuse 8, which is also jumped to fuse 7 with a strap on the back of the fuse block. There are multiple loads coming off of this other than the CDI, Speed switch, etc. That would put more into the circuit than you seem to be looking at. I can't get the readings you would since mine is a 1973 cis, and I also have a noise suppression capacitor for the CDI, which I don't know if yours has also.
I don't think you should worry at this time.

[MTemp]

Yes I understand that; as I said the electrics are an aspect I normally relish. Any concern I had was born mainly out of confusion that two wires seemingly not connected to ground, could become connected to ground when connected together. After a little thought (should have done this before posting) overnight, and using resistance measurements not just the connectivity meter, it has become clear what was happening. This all started because I touched the speed switch case in error with one lead from the meter when checking my cdi connection to the red wire on fuse 1 on the engine panel fuses, and heard it 'beep'.

A UK colleague has just checked his cdi/mfi car and has the same readings ..... so we can stand down this thread!
Thanks and sorry for wasting 45 viewers time!
Mick