[John V.]

Assembly was finally completed, GTO delivered to me late January. Some things were left undone at my request. But discovered other issues that need to be dealt with. First was a leaking left front wheel cylinder. Rebuild was poorly done so I got it sleeved. I put a(nother) new rebuild kit in it and all seems okay now. Other 3 were probably done the same way so won't be a surprise if one or more start leaking in the future.

Yesterday I put the front brake back together and then took my wife out for the first drive. Figured if the brakes failed, I didn't want to die alone!

Saw that my AutoMeter Custom Shop gauges were lookin' good but the Oil Pressure was pegged at 100 psi. So this morning I spoke to Doug at AutoMeter. He helped me figure out that the ground for that gauge was not working. I made up what i think is a pretty sweet wire harness using Packard 56 connectors. The spade on the back of the OP gauge had come loose and wasn't making contact. Got that sorted out and works correctly now.

The Voltmeter works. And is "backed up" by the original "Battery" idiot light which also senses Voltage and lights up when key switched to On.

The Temp Gauge works from the AutoMeter sender. The "Temperature" idiot light is not hooked up, to my knowledge there is no way to have a sender for the idiot light in combination with the AutoMeter sender.

The AutoMeter Oil Pressure sender is set up with a tee fitting so that the original sender for the Oil Pressure idiot light is also installed. So the OP idiot light "backs up" the AutoMeter OP gauge.

However, my Oil Pressure idiot light does NOT light when the key is switched to On.

My understanding is that the idiot light sender is simply a normally closed On or Off switch. With the engine not running, the sender switch closes, with the ignition switch turned to On (engine off), the circuit is powered and the idiot light should light.

When the engine is started, the pressure rises and the sender switch opens, turning the light off (I believe the required pressure to open the switch is about 5 psi).

I am looking for help to understand how I can test the circuit at the sender.

I haven't looked, but I figure accessibility to the bulb is difficult so before I try to access the bulb end, I figure I can confirm a good sender first.

Just not sure how to do that. Help appreciated.

If sender checks out, I'll check for a burned out bulb.

[Dick Boneske]

Ground the oil pressure wire at the sensor on the oil filter. If the dash light comes on, the sender is defective..

If the dash light does not light when you ground the lead, the bulb is the likely suspect. It can be accessed from under the dash(by feel only).

My bet is it's the bulb.

[johnta1]

Your idiot light sender also needs a ground it sounds like?


The sender should ground if it has no pressure, then opens, cutting ground circuit off (making it open). The wire from idiot light circuit to sender should have 12 volts?
(I believe?)

[John V.]

The idiot light sender/switch is grounded by way of the same pipe/tee as the AutoMeter sender (which screws in to the tee also). Pix worth a 1000 words, attached shows the set up. One with the idiot light signal wire attached and one with it disconnected. The "can" to the rear is the AutoMeter sender.

I jumpered a wire from the disconnected idiot light signal wire to various ground points, ignition switch On, still no OP light at the dash.

So I checked for 12V at the signal wire with the ignition switch still On, the meter bounces all over the place. If I get any reading momentarily, it seems to show in mV. My multi-meter is auto-ranging. I'm no wiz with the meter so I checked from the battery positive post to a nearby valve cover bolt and get a steady 12.5 V. So I figure I'm using the meter correctly.

My engine side wiring is all new, including the connector. The underdash wiring was a cleaned up original.

Am I looking for a wiring issue or a bad bulb?

Checking the wiring diagram on the dash side, I see a Pink wire common to all 3 idiot lights. It is a "Resistance" wire, whatever that means. Since the "Battery" light works, I'm thinking the Pink wire is good.

The other wire to the OP idiot light connects to pin location 7 on the connector, a Dark Blue wire on both sides of the firewall.

Since there are 2 wires to the bulb, is that why I don't get 12V at the signal wire? What is the purpose of the common Pink resistance wire?

[John V.]

BTW, I also checked resistance at the sender/switch between the signal wire terminal post and ground, both the body of the sender/switch and also at the pipe to the tee.

In either case, I get 17,000 ohms. I have no idea what that is telling me but for the moment, I'm thinking the sender/switch is not the problem.

Dick, if you are seeing this, I am still looking for help with the measurements of the vac secondary parts. I wonder if my email went to your junk folder? I will post here after I figure out this idiot light issue but figure you are the best source for what I need so if you have a chance to check, I'm still needing help with that.

[johnta1]

I jumpered a wire from the disconnected idiot light signal wire to various ground points, ignition switch On, still no OP light at the dash.

With the key on, engine not running, there should be 12volts going to the oil sender unit.
The pink wire goes from the light in dash to the power wires. (under dash?)
There should be a dark blue from that light also that goes to firewall then to the oil sender connection.
If the voltage is jumping around, possibly the connection of that pink wire to the main harness is bad?
(bad solder maybe?)

[johnta1]

Looked at another wiring diagram and it shows the dark blue inside from lamp goes to the main power connection in loom. The pink goes to the firewall connector which turns into dark blue on engine side of firewall?

Seems backwards?

But anyway one of the wires from lamp should go to the main wire harness power (which goes to ignition #1 on switch)

And the other wire from lamp goes to firewall then oil sending unit.

Might check the firewall connections also?

[John V.]

John, I also checked the revised '64 Tempest wiring diagram at 64 SCN 3-11. I do not have access to my Shop Manual, so not sure what the wire diagram in it would show.

Per the SCN, there seems to have been a revision to the wire colors at the Ignition Switch, specifically relating to the "Pink" wires.

As I see it, the "Pink" wires from each of the idiot lights are joined at a splice to a wire from Ignition No. 1. It then routes to the firewall connector at Pin position 1 and from there extends to the Coil positive post.

Except that in the SCN diagram, for the OP idiot light, it is the Dark Blue wire that joins to the other Pink wires at the splice. While the Pink wire presumably is shown going to the connector at Pin position 7 (impossible for me to distinguish pin positions in the SCN wiring diagram).

I'm guessing that is an error, why would the OP light be different in regard to the Pink wire?

The other diagram I have shows all of the Pink wires spliced together.

The diagrams all show a Pink wire from Pin position 1 to the positive post at the Coil.

My reproduction engine side wiring has a sorta Lavender with Black stripe wire at the coil. Haven't traced it to Pin position 1 at the connector (which is the wrong color, black instead of tan) but I would guess that even if the wire color is wrong, functionally it would still work.

I think you are seeing the same thing and perhaps looking at the same SCN revised wire diagram.

Since the underdash wiring is an original, it seems I will need to crawl under the dash and see if I can find the wire from the OP light that should be spliced to the other two Pink wires and check that it is connected and see if I get 12V there with the ignition On.

I guess it would be easiest if I could check at the bulb socket but from what Dick indicated (plus what I'm imagining), getting to the bulb socket with a multimeter will be near impossible. I'm thinking I may need to pull the front seat out to have any chance to crawl up in there.

I think I have another underdash harness that I could study to know what I'm looking for. Curious to see where the splice is and what it looks like. But it is packed away somewhere so no help there.

If anybody has any better info, a spare underdash harness to study, or any other ideas, I'm all ears.

[Goatracer1]

You didn't use Teflon tape on your sender did you? It could insulate the sender and it won't ground.

[Andre]

I looked at your picture(s). Isn't that oil pressure sender the one for cars with factory gauges, and not the one for idiot lights?

[johnta1]

He has 2 senders there, one for gauge and one for the idiot light.

[John V.]

Correct, using a tee fitting so I can have both. And no teflon tape, the pipe assembly has a good ground connection to the oil filter adapter.

As a '64, no factory gauges. The sender can Andre saw is the AutoMeter sender used for my underdash AutoMeter Gauge set. In the pic, the blue signal wire for the gauge is connected to the far left end of the can (rearward relative to the car).

The idiot light sender/switch is screwed into the vertical screw hole of the tee fitting. The factory style signal wire for it (also blue wire) is connected at the TOP of the sender/switch in my set up.

If anybody has a '64 with a working Oil Pressure idiot light, can you confirm that there is 12V at the signal wire with the ignition On? Just disconnect the wire from the switch at the oil filter and use a multimeter to probe from the wire end to ground.

I'd like to be sure that it is supposed to be 12V there before I bite the bullet and crawl under the dash.

[John V.]

Nothing to do with the non-working Oil Pressure idiot light.

But here are pix of my Custom Shop AutoMeter Gauge set and the wiring harnesses I made up using Packard 56 connectors.

I was very pleased with my wiring set up. I made it up so I could connect to power (at the radio) and the dimmable dash light circuit (at a factory Packard 56 connector) without cutting any factory wires.

I only had single wire Packard connectors. That made it more challenging as I needed to run 2 wires to a single connector in some cases. I spoiled a couple terminals getting them crimped but my final product came out better than I could have expected.

I bought colored wires to match to factory colors except for the lighting circuit. The factory used Gray wire for the dash bulbs. I couldn't find Gray wire so I used White wire for the gauge lights.

Sorry to hijack my own thread,

Back to the OP idiot light discussion.

[John V.]

With help at the Electrical forum, I was able to trace the fault down to a burned out bulb confirmed by a continuity check of the bulb. I'm a slow learner and it took me awhile to understand that a dead bulb would cause a "no voltage" condition at the terminal end of the Dark Blue wire out at the sender/switch.

Then decided to kill 2 birds with one stone. I pulled the good bulb from the Temperature idiot light socket and replaced it with the dead bulb from the Oil Pressure idiot light socket.

With all my testing, I discovered that the unused Temperature idiot light wire from the factory type engine side harness was left laying on the intake. I figured this wasn't a great thing since that wire carries 12V whenever the Ignition is On.

So the dead bulb kills that wire. I could have just left the bulb out I guess.

Next I plugged the good bulb into the Oil Pressure idiot light socket. Switched the Ignition On and expected to see the Oil Pressure light glowing red.

But I got nothing!

I then checked the sender/switch for continuity between its terminal and the switch body and got nothing.

Turns out the sender/switch is also bad. I'll look to replace that next.

I jumpered between the terminal end of the wire and ground and did get the idiot light to glow red.

Just for yucks, I jumpered the switch body to ground just to check that it wasn't just a bad ground at the switch body. But still no light doing that.

Don't know how a new sender/switch could have gone bad. But at least now I know I have a working light in need of a working sender/switch.

On to the next issue.