Grease on wiring bulkhead
 

I searched the forum and found some older threads on dielectric grease being used for wiring harnesses where they plug into the body. I have a 69 Grand Prix and the oil pressure gauge does not seem to work, so I am trying to figure out what is going on. Anyway, while investigating that (it does not move connected or not, so it seems to be grounded out) I took a look at the connections back from the connector. The bulkhead connectors have grease, but it is not like any dielectric grease I have used. The grease I have applied in the past is clear and not sticky at all, but this stuff is brown and very sticky.

So is this stuff good to go or should I clean it off and apply the clear stuff I have?

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I should add - the grease I am familiar with is the Permatex Dielectric Tune-Up Grease

What I am seeing from your photos look like the original GM applied corrosion and moisture barrier tar from 50 years ago.


That stuff lasts a good long time. I don't think modern Dielectric Tune-Up Grease would be any improvement over the original GM tar.

In regards to your oil pressure gauge, the circuit for that goes thru a dark blue wire, from the plug-in on the back of the printed circuit out thru the fire-wall plug and onto the engine compartment wiring harness. It plugs onto a variable resistor which at zero PSI would be .5 or one ohm worth of resistance. at 80 PSI oil pressure the sender is showing the blue wire 90 ohms of resistance. (The sender grounds to the engine block via it's threads.)

GM in-dash gauges often fail after 50 years. I have also seen many old senders leak oil (with the engine running) that or they will go bad electrically.

One thing I would check is the printed circuit on the back of the gauge cluster. Those will peel and fall apart from heat and cold over many seasons. New printed circuits are available to buy however I have noticed that the copper those are made from is less than 1/2 the thickness of the original GM printed circuits from when the car was new. A good rule for protection would be to remove the "gauges" fuse and note the amps rating. Replace that fuse with a new fuse that has a 50% fewer amps rating. This will help prevent your new printed circuit from turning into a $90 "fuse."

Thanks Peter.

I hooked the harness back up and checked out things a bit more for the oil pressure. I get 8.0 volts on the blue wire with the ignition on. I measured the resistance of the sender unit and get 68 ohms with the blue wire disconnected and one lead on the sender plug and one to ground (not sure I am measuring that correctly) with the engine off.

Disconnecting the blue wire with the ignition on causes the oil pressure gauge to peg out past 80. With everything hooked up and ignition on, it measures like 70psi at the gauge.

So maybe a bad sender unit?

Here is what I have - not sure if it is the right one (69 GP, 428HO 4spd)

Jeff

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According to your post it sounds like a bad sending unit to me. The sender I see is very old, and most likely it is worn out. Be cautioned, the market is flooded with cheap smaller sized imported electrical parts, especially service parts for a 50 year old GM car. The new parts are not made to the same quality standards as the original GM parts nor are they tested to perform for more than 3 to 6 months. And those places do not have access to a shop manual (one that is translated into their language) for your car. Most of what is sold now is made 1/2 around the world by people who don't have the car (like yours) & probably never even seen one.
Unfortunately the quality of new parts is not good. From what I have seen in the past 5 to 10 years they no longer make parts that will last; but they will make as many new ones as people will buy. I would recommend seeking out an older NOS, sender. One that was made in the USA.