Voltage at Coil Terminals - Help Please

[John P]

Hi,

I've been trying to resolve an ongoing starting problem and have now, I think, eliminated the carb so turned my attention back to the ignition.

Today I checked the voltage across the battery terminals which was 6.1 volts.   Not sure how good this is but the car hasn't been run much for some months due to the starting problems.

If I measure the voltage between the coil SW connection and a know earth I also get 6.1 volts but if I measure the voltage between the SW and CB terminals on the coil I only get 4.8 volts.

I thought the CB terminal was connected to earth when the points were closed so had expected it to be something like the 6.1 volts.   I have crudely tried connecting a wire between earth and the points and also between earth and the distributor body but it's still just 4.8 volts.

Am I misunderstanding things or should I get nearer to the 6.1 volt figure?   I take it that if there's a reduced voltage here it will also be proportionally lower on the HT side.

Any suggestions / advice,

John.

[bob46320]

The coil has a resistance, so if you measure the voltage between the CB and SW contacts, then there will be a drop in voltage. Ohms law ?

[Bob Culver]

it would appear the points need cleaning. On all my old cars I have for decades regularly tidied points with an oilstone.
It is not necessary to elimintae pits. but do not leave oil in.CB to earth should be very low volts.Try just pulling emery paper thru, but avoid leaving grit.

[Chris KC]

John,

First check you have power to the coil by measuring voltage between either CB or SW and earth, should be something in the region of 6V (OK so far).

Then check continuity of the coil by measuring resistance
- between SW and CB should be 0.4 - 2 ohms
- between either contact and the HT terminal should be 6000 - 15000 ohms (mine was 8570 last week).

[John P]

bob46320

I sort of understand what you're saying and if I had disconnected the connection to the distributor before measuring the voltage between the CB coil connection and earth then I could understand it but with the coil still connected I would have thought the CB terminal being directly connected to earth would also be at zero volts so the voltage between SW and CB would be battery voltage.

Bob Culver

This was may first thought but when I tried directly shorting out the points so that their condition didn't affect things there was still no change in the voltage.

Just wondering about the capacitor but I don't really understand how this works.

John.

[Chris KC]

By the way here's a checklist of 40-odd points I generated when I couldn't get my own car started. It's an excel document so heaven knows if I can attach it successfully, if not message me and I'll share it with you directly.

It took me 2 days of testing and replacing stuff to find a very slight air leak at the inlet manifold. New gasket and it started first try on the handle...


  A7 Doesn't Start.xlsx (Size: 18.91 KB / Downloads: 85)

[Bob Culver]

These topics can get protracted. The great virtue of old cars is that can safely and simply arrange a reasonable spark gap for one plug lead, and observe any spark or lack of. Avoid wide gaps which stress system. Possibly relevant,  on at least some distributor types the earthing of points to body is obscure. 
Testing at plug checks rotor etc.
Condensers are a puzzle and normally checked by substituion if feeble or no spark. Any traditional car type or cheap .2 mfd 400 v radio style can be wired externally to just test.

[Mark Dymond]

You dont need much resistance resistance between the cb terminal and earth to see a couple of volts lost between sw and cb. If its not in the points then check the distributor to engine and engine to chassis.

Disconnect the cb wire from the coil and measure resistances rather than voltages, there's less to confuse he issue.

Good luck.

[wild_willy]

John try this:

Check battery voltage. You say 6.1V

Switch on IGN. If battery still reads 6 volt or close to it do the next check. If voltage drops to 4.8 volts then battery should be a suspect.

Now check voltage on coil at SW. Should be around 6 volt still. If substantially lower (like 4.8) then do the next check. 

Connect a wire from CB on coil directly to battery negative. Measure voltage at SW on coil which should read around 6 volts. If voltage is still low then the wiring or Ignition Switch may be faulty. Any high resistance path in wiring connections, fuse holders or ignition switch will cause a voltage drop. In your case this equates to 1.3 volts which is a substantial loss in a 6 volt system.

Best of luck

[AustinWood]

If you have manual advance/retard (which rotates the distributor) the distributor body may not be earthed. I always connect a wire from the distributor body to an earth nearby.
Vauxhall Vivas suffered a similar problem because the distributor was painted causing a bad earth.