Payner44

I've searched and not found an answer specific to my issue, so here I am looking for help.

I have a '70 Cutlass Supreme. Recently the oil light started coming on after driving about 5 mins and then it stays on. I have disconnected the sending unit to make sure it's not causing this, which it doesn't, light still comes on as described. Also, if I turn the car off and then turn the key back to "on" the light stays on and will do so until the car has cooled down. Of note: when the car is used from a cold state the oil light does not come on during the bulb check procedure.

I hope that this is enough information to come up with a fix. I'm thinking there's a short happening somewhere...perhaps the ignition switch due to the time lag when the light comes on from a cold start and the check bulb function not working? Once current it going through the switch the warmth is just enough to expand things and cause a ground??

joe_padavano

First, I hope you've (at least temporarily) installed an oil pressure gauge to verify that you aren't going to hurt your engine.

As for the oil light, this is just about the simplest circuit there is on the car. One side of the light gets power with the key in RUN, the other side connects to the blue wire that runs to the switch in the block. Below 4psi, the switch is closed, grounding the blue wire and completing the circuit to illuminate the light.

If the light comes on with the sender disconnected, there is obviously a short somewhere. The only way to troubleshoot this problem is brute force. Chase the wire from the sender to the bulb. The blue wire goes through the firewall connector, so look for potential shorts in this wire. Check the firewall connector for a bad terminal. Check the under dash harness. Look at the connector to the dash pod. Note that it is not uncommon for the terminals in the circuit board on the back of the dash pod to loosen up, causing problems. Finally, look for wires or solder bridges that could be shorting the circuit board itself.

oldcutlass

Are you sure you disconnected the oil pressure switch on the front top of the engine and not the temp switch? Do you think possibly the 2 sensor wires are swapped?

joe_padavano

Interesting theory. Blue wire goes to the oil switch, green to the temp switch. The oil switch is directly above the water pump. The temp switch is next to the t-stat housing.

Payner44

Eric,

Brilliant thinking. I just bought this car and didn't known the color scheme for those sensor wires until now. Turns out they were swapped to the wrong sensor. Switched them back and BINGO! The bulb check now works for the oil light, as well.

Many thanks, guys!!

joe_padavano

Kudos to Eric on this one. I never would have even thought of that!

oldcutlass

Glad you got it fixed. It was just another one of those things that popped up years ago with a similar issue a friend had with an engine swap.

Payner44

Now I hope that the temp sensor wasn't turning the oil light on due to a hot condition. I suppose not, since I drove the car about 40 miles yesterday with zero indication of overheating. And, the temp sensor is new, because I had to replaced it after a friend leaned into the engine bay to look at something and broke the terminal on the old one.

joe_padavano

Assuming you have a switch for a TEMP light and not a sender for a gauge, that switch is designed to close (and thus turn on the light) at about 258 deg. If your engine really isn't getting that hot, then either the switch is bad or there's still a short elsewhere.

Payner44

So, if there's no short in the temp wire I should be able to ground it and see the HOT light come on, correct?

joe_padavano

Correct, with the key in RUN, of course.

oldcutlass

I prefer a set of after market mechanical gauges in lieu of the idiot lights. It takes all the guess work out of oil pressure, engine temp, or alternator issues. They are easy to install and now might be a good time to consider this option.