Can not establish gound in speedometer ingnition light

As you know there is a bulb for both the fuel and ignition in the bottom area of the speedometer. I have tested the bulbs to be in working condition. I have a good ground to the case of the speedometer as evidenced by other bulbs working in the same case. I can manually take the ground wire, turn the key on and get the ignition bulb to illuminate by touching almost the side of the bulb. I CAN NOT get the bulb to light by touching the barrel of the assembly that pushes into the back of the speedo. This barrel also has a rubber sheath on it.

I see that there is a separation between the barrel and the prongs.

I can rig a wire temporarily from the side of the bulb which is screwed in to the ground wire on the case and it will work.

I do NOT know why the prongs are not grounding to the case of the spedo which is grounded already.

I can not test the low fuel as I have a full tank of fuel.

These all worked before I removed the speedometer.

If the part closest to the bulb would somehow move closer to the case when the prongs were fully seated into the case then it would work.

Spent too many hours messing with this and did not get the tach installed yet.

Takes miniature hands to install the brackets on the back of these gauges and tighten them against the wood.

Take note of the location and LENGTH of all hold down brackets as they are different for the speedo and tach.


The ignition light is NOT supposed to be grounded. One side connects to the ignition switched 12V line and the other side to the output of the charging system.
Simply put, if the generator system is not charging and there is no voltage output, the light will illuminate because of the 12V on the ignition line.
When the system is charging, there will be the same voltage on both sides so no voltage across the bulb and hence it does not light.

I agree with your statement.

My problem is that the light will not illuminate with the engine off and the key on as it is not obtaining a ground from the casing of the speedometer. I can jump the bulb and it will light but not when pressed into the case.


It seems to me that both of these sentences can’t be simultaneously true. I suspect that we still don’t understand what you are trying to describe.


If I turn the key on then the light should glow.

If then I start the engine and the alternator provides power than the light should go out.

Also this bulb could possibly glow if one were driving and the revs went very low and low current were being produced.

This was the case before my adventures.

Now the light will not glow with the key on and the engine not running as the bulb only has a positive current going to it and not grounding with the speedo case.


Dear Gerard,

The light in the ignition circuit lights up when there is a voltage across it. It can only have a voltage across it if the circuit is complete, i.e. one side is not simply left hanging unconnected. If there is no complete circuit path then both sides of the bulb will be at whatever voltage the connected side is connected to, assuming it is connected to anything.

Conventionally, one side is connected to the battery positive via the ignition switch white wire. The other side of the bulb is connected through the diodes in the alternator which is itself earthed and thus forms the return path to the battery negative when the car engine is not running.

As you report that the bulb works on its own, then either the alternator side of the circuit is not connected or it is faulty OR the ignition switch side of the circuit to the battery is faulty when the key is on but the car is not running…

If the bulb is at 12v but not lighting then there is a path to the battery positive but no path back to the other side of the battery through the alternator.

If the bulb is at 0v and not lighting then there is no path back to the battery positive.

It never expects, nor wants, to find a return path to the battery negative through a ground connection at the speedo case. The bulb is only ever supposed to light up the difference between the two power sources, not the difference between a power source and earth.

kind regards