[John V.]

Decades long resto just completed, first drive uncovered some issues including Oil Pressure idiot light not lighting with Ignition On, engine off. Hoping the electrical gurus here can help me figure this out.

Might be a simple burned out bulb but trying to understand the circuit. Don't want to blow a bunch of bulbs trying to figure it out.

I would like to test the circuit but don't understand what voltage I should see and where to test it.

Bulb socket has 2 wires, Pink and Dark Blue.

A Service Craftsman News issued in '64 corrected the IGN #1 and IGN #2 wire color coding found at the Shop Manual wiring diagram. The SCN provided a complete wiring diagram.

The wire color code revision is clear. But there is another change as compared to the Shop Manual wiring diagram that relates to the Oil Pressure idiot light. No indication in the SCN that this change was intentional.

Shop Manual shows the Pink bulb wire spliced to the wire from IGN #1 along with Pink wires from the Battery and Temperature idiot lights. This wire then continues thru the firewall connector and attaches to the positive post of the Coil.

The SCN however shows the Dark Blue Oil Pressure bulb wire spliced to the IGN #1 wire. My gut says this is an error in the SCN diagram. The Battery and Temperature bulbs both have their respective Pink wires spliced to the IGN #1 wire. Without unwrapping the underdash wire harness, no way to be sure.

The 2nd bulb wire goes thru the firewall connector and then connects as a Dark Blue wire to the Oil Pressure sender/switch. The Shop Manual indicates the Dark Blue bulb wire all the way to the switch. The SCN revision shows the Pink bulb wire transitions to the engine side Dark Blue wire at the firewall connector.

With the Ignition On, I tried a jumper wire from the signal wire to ground to see if the bulb would light. It did not.

I then attempted to measure the voltage between the signal wire and ground expecting to see 12V. But auto-ranging multi-meter bounced all around, momentarily will read in mV but nothing consistent.

Seems to me, 12V is supplied from the positive post of the Coil, to the Ignition switch IGN #1 terminal and with the switch On, then to the underdash wire where each of the 3 idiot lights are spliced to it.

Then the 2nd bulb light wire goes from the bulb socket all the way to the Oil Pressure sender/switch where it will make ground. The switch would be normally closed, so the bulb should light with the Ignition On, then the switch would open once Oil Pressure reached the minimum, perhaps 5 psi from what I remember of old school criticism of idiot lights.

Not to worry, I have a separate set of AutoMeter gauges, the Oil Pressure gauge has its own sender.

But I want to have the idiot light backing up the gauge.

According to the Wiring Diagrams, the underdash wire where the bulb wires are spliced to it is identified as a Resistance Wire. I have no idea why.

With the Ignition On, what voltage should I expect at the bulb wire?

If the wiring is correct per the wring diagram, shouldn't I get the same voltage at the signal wire terminal at the sender/switch?

What effect does a Resistance Wire have on the voltage at the bulbs?

The Battery idiot light does function normally, turns on with the Ignition On, engine off. The AutoMeter Voltmeter gauge also reads voltage.

The Temperature idiot light is not hooked up, replaced by the AutoMeter gauge. No sender available for the idiot light.

But as I wrote this out, I realized I could check voltage at the unconnected factory Temperature light signal wire. Just did that and found 12.3V between the wire terminal and ground with the Ignition On. I didn't check to see if the Temp bulb lights but I assume it would with a jumper wire touching ground.

Will a burned out bulb affect the voltage in the Oil Pressure idiot light circuit?

Can I check for continuity to confirm a good splice from one of the bulb socket wires (either the Pink or Dark Blue) to the wire going to the positive post of the Coil? I think I can probe between the wire at the bulb socket and the wire to the Coil at the engine side of the firewall connector.

I'm no wiz with the multi-meter. If I check for continuity, do I need to disconnect the wire from the Coil positive post or can I do this check simply with the Ignition Off?

And what reading will confirm continuity?

[jeff chmura]

John,
Going off memory here , the pink wire is hot.
The blue wire going to the sender on the engine is the ground.
The oil pressure switch on the engine is a normally closed pressure switch.
What happens is when you turn the key to the on position, you power the pink, the blue wire is the ground in the circuit, the light lights due to no oil pressure.
When you start the engine and build oil pressure, the switch opens,breaking the ground and the light goes out.
Key on, check and see if you have power at the pink wire.
You can check continuity on the blue wire.
To check the pressure switch, check continuity between the terminal and the case of the switch to see if the switch is closed.
Hope that helps
Jeff

["QUICK-SILVER"]

Check the bulb...

IGN-1/Pink is power to the bulbs you mentioned and the resistance wire splices out after the bulb feed.

The resistance wire cuts amps to coil (+) to keep points from burning up while engine is running. IGN-2 is full battery voltage for cranking.

Temp and oil use the same bulb so voltage on the blue sender wire should be the same as the temp sender wire.

Sender wires straight to ground should turn their respective bulb on.

While testing:
Make sure points are open
OR take the coil (+) wire off
OR stick something between the point contacts to keep them from touching.

Clay

[John V.]

Thanks for the replies guys!

It takes me awhile to understand any electric circuit but after I wrote up my post, I started to piece some more things together.

First, I have a few questions.

Clay, what happens if I'm testing with the points closed? Will that damage the coil or points? Or the multimeter? I have checked for voltage in multiple places, never occurred to me that I had to open the points. This is what I worry about when testing since I'm not very good with a multimeter. Or is the reason for opening the points to kill the power only when checking for continuity?

I understand when you say the Resistance Wire cuts amps to the Coil to keep the points from burning up with the engine running.

But does it also reduce the voltage on the idiot light bulb circuits?

Battery voltage is about 12.8, the voltage at the terminal sender end of the Temp signal wire was 12.3 as mentioned yesterday. Doesn't seem like much of a reduction. Does the Resistance Wire cause a bigger drop as current draw increases (engine running) perhaps?

Should be clear from my questions that I know just enough about circuits to be dangerous. Which is to say, I don't know much!

Jeff, I had worked out the general operation of the Oil Pressure idiot light circuit as you described it. That prompted the checks I had already made.

Last night after I posted, I realized that I should be able to check for voltage at the bulb socket. And I also concluded that I should be able to prove which of the 2 bulb wires was spliced to the IGN #1 Pink wire, either the Pink Wire as shown in the Shop Manual wiring diagram or the Dark Blue Wire as shown in the SCN wiring diagram. Assuming I have a good splice to it.

So I pulled the bulb socket out and found 12.1 V at the bottom center of the bulb socket, this is the Pink wire terminal (so the SCN is wrong as my gut said it would be).

And the good news is, I think this check proves that I have a good splice to the IGN #1 wire, so no need to unwrap the harness to inspect it.

So that leaves the Dark Blue wire ground side.

I'll wait for Clay to help me understand if the Resistance Wire is functioning as it is supposed to be before doing any continuity checks on the Dark Blue wire.

But since I pulled the bulb out to check the voltage at the socket, I did one more check.

I looked at the bulb to see if I could detect a broken filament. If it is, I couldn't tell using a magnifying glass.

So I set the multimeter for continuity and checked the bulb between the positive base and the outside metal shell. No continuity.

Does that mean the bulb relies on intact filaments for continuity between the Pink and Dark Blue wires?

If so, a burned out bulb will cause a no voltage reading between the sender/switch terminal end of the Dark Blue wire and ground?

If that is true, I think I have been going nuts trying to check for voltage when the burned out bulb was preventing it.

Clay, if you see this, I'd still like your comments about my questions above. I'd like to know that the Resistance Wire is doing what it is supposed to be doing. My underdash wiring and the fuse block are believed to be originals. The engine side wiring including the connector is a reproduction. Wherever the Resistance Wire is supposed to be, I'm hoping it is working properly.

Thanks.

[johnta1]

The resistance wire only affects the voltage to the coil.
It's losing voltage going through the extra resistance of the coil wire.
(the splice point just provides the 12 volts, the wire degrades the voltage)


The bulb socket having 12 volts means it is working.
The bulb not having continuity means the bulb is bad.

Does that mean the bulb relies on intact filaments for continuity between the Pink and Dark Blue wires?

Yes it needs to be OK for the circuit to be complete.


Glad the pink wire is the 12 volt source, meaning the SCN was a misprint.
(don't remember if there was a correction on that or not)


A new bulb should take care of your problem?

[johnta1]

BTW, on the points thing, if the points are closed, it's completing the circuit and voltage is powering the coil. It will make it hot because points are made to open and close rapidly.


If the points are open then no voltage is flowing.

["QUICK-SILVER"]

John V.,
John W. pretty much covered everything you was asking.

Those bulbs don't ground in the dash...Power goes through them, In through pink and out through blue to the sending unit for ground.

Your resistance wire is in the dash harness. You can check coil (+) voltage with engine running to see what it does. I've seen anywhere from 3 to 6 volts lower than battery volts because of the resistance wire.

Keep us updated on this and good luck
Clay

PS I've seen plenty of bulbs that the filament was good but the little legs weren't making contact down in the base. Glass fuses are some of the worse to not be blown out but the ends not making contact. You need to continuity test new ones before you try to use them.

Clay

[John V.]

Thanks John & Clay. I'm confident now that replacing the bulb will fix this issue.

Obviously a learning experience for me.

I'm going to assume that I didn't damage the coil or points when testing. I only switched the ignition on for a couple minutes at most each time while checking for bulb circuit voltage.

And if I was lucky, the points happened to be open anyway.

Good tip on checking the new bulb also.

John, on the SCN thing, I don't recall seeing a correction. Almost too funny since the purpose of the SCN was to correct the color of the IGN #1 and IGN #2 wires. How they managed to cross up the Oil Pressure idiot light wiring is crazy.

And since it always hurts my eyes to try to follow the wire lines in the diagram, I'm guessing nobody that cared ever noticed the cross up, too hard to read it that close. And anybody who ever needed to actually use the diagram to trace a fault likely would have assumed like I did that it was an error anyway.

Blame it on the draftsman and the guy who checked his work.

The color coding change does make me wonder if the guys making up reproduction wire harnesses are aware of the SCN correction for IGN #1 and IGN #2 color codes.

[John V.]

Good news and bad news.

Since I am not using the Temperature idiot light, I decided to pull that bulb out and swap it to the Oil Pressure bulb socket.

Installed the dead bulb into the Temperature idiot light since I figured it was a bad idea to have the factory wire laying like it was on the intake manifold with constant 12V to it anyway. The dead bulb kills the wire to the temp sender wire.

Put the good bulb into the Oil Pressure bulb socket and expected to see a nice red glow when I switched on the Ignition.

But dang it, got nothing.

Pulled out the multimeter and this time with some idea of what I was doing, I checked for continuity between the sender/switch terminal and the body. Nothing.

I jumpered the wire to ground and the bulb glows red in the dash so that is good news.

Next I'll be looking to replace the sender/switch.

As far as I know, it was a brand new sender/switch. I even checked to see if the switch body is just not making good ground by jumpering the body directly to ground. Still nothing.

Not completely fixed yet but at least I'm not blindly shooting darts at the problem anymore.