ballast resistor

I noticed my 1960 Mercedes getting harder to start on cold mornings. Had gradually deteriorated over my 7000 mile trip.

Ignition system uses external ballast resistor: Beru 1.8 ohm. Not switched, so always in the loop. It's in line between ignition wire and coil.

I measured battery voltage with engine off. 13.5 volts.
I measured ignition voltage from switch to ballast resistor. 13.5v
I measured voltage from resistor to coil. 5 v.

Seems like quite a drop.

I bypassed the resistor. Engine fired, started, and ran perfectly.

Then I decided to check the resistor itself. Resistor is marked 1.8 ohm, which meets spec for this car.

But using my volt/ohm meter, resistor shows 5 ohms.

Can a resistor "mutate"?

Thanks.
Tom

I've never seen one do that. Generally my experience has been that they either work fully or don't function at all. They tend to have a wrapped ceramic piece between the two terminals to provide resistance and drop the voltage and if the ceramic fractures, it quits period.

Thanks Mike. Here's the resistor:


Note it's marked 1.8. I just tested it again (removed from car) and it still shows 5 ohms.

So the engine was trying to fire on 5 volts. When I removed the resistor from the system, engine fired immediately. But with no ballast, points are going to fry eventually.

Tom

Sounds like you might want to throw one at it if it's cheap enough Tom.
Are they expensive?

Don - not all that expensive. But the plot thickens!

Engine starts and runs perfectly after I removed the resistor. But I wondered if the coil itself has a problem.

It's a Bosch "red" coil. Spec is primary resistance 1.8 ohms; secondary 12K.

But when I measured the coil itself, primary resistance is 2.5 ohms. Not 1.8 ohms. Weird.

1. Would the high coil resistance, combined with the resistor itself cause the spark to be too weak to ignite the fuel?

2. How the heck could the coil resistance go *up*?

(Minor detail that I had the ignition and distributor wires to the coil backwards for the entire 7000 mile trip.)

Tom

Tom:

Most cars have the ballast resistor "switched".

In other words, during the starting, the starter switch bypasses the resistor and provided a full 12V to the coil.

Once you release the key to the "run" position, the power to the coil goes thought the resistor (to save the coil and points).

Thanks Nial. On this car, the ballast is unswitched - it's always 'in the loop'. Ran fine for 6000+ miles that way. But later in the trip, got harder to start when cold. I removed the ballast from the circuit, and starting greatly improved.

But now I've got slight hesitation on upgrades under load. Not knock/ping - more like ignition.

Edit: timing is set spot on: 8 BTDC at 800 rpm idle. Points are set correctly - 0.018". Plugs clean, good fuel flow.

Can't figure out the problem. Any ideas?

Thanks.
Tom

Tom:

So, what happened with the timing issue - from your earlier thread?

I suspect that the meter you are using is the problem. Sometimes modern digital meters don't always work well for low resistance inductive readings. Good old analog for somethings still works best. Analog meters put out much more current on lower resistance scales than digitals of today.

George - you're right. The digital I'm using isn't very accurate in that range. Flickers up and down, so I have to "average" the readings. My analog doesn't oscillate at all - but at the low scale (x10) the needle is only just over the first line between 0 and 1, despite zeroing the meter first.

But ... solved the whole problem by replacing the points. The Bosch points were blue/green and pitted, and the cam follower was only about 2/3 the length of the new one. Put in new points, re-gapped to spec for dwell (50), reset timing - and engine runs great.

Really surprised the points wore so much in only 7000 miles of driving - especially with that big ballast resistor.

Glad it turned out well (and back at home). I keep a good old simpson 260 meter around for some of the "older" problems. Now if I could only remember who I lent my dwell meter to?