No oil pressure after rebuild

[rcv]

I blew it out with compressed air right before I put it on, but I just ordered a cheap torch tip file that I’ll use before I button up the valve cover. 

[Captain Obvious]

8 hours ago, rcv said:

I tested the voltage on the unplugged connector.  What's weird is that I don't get a solid voltage between the unplugged connector and ground. It seems to be oscillating somewhere between 0-12 volts. I don't have an oscilloscope at home and my crappy $20 multimeter can't give me any more details but it's definitely not a solid signal.  Is this expected?

Yes, it is to be expected. That's normal for the signal going out to the sender units (Oil, fuel, and temp).  They are pulse width modulated.

I don't think it directly pertains to your issue (which sounds as if it may be a non-issue after all), but if you or someone else wants a description on how the gauges work, let me know and I'll put something together.

[Diseazd]

I’m with you Charles and Mark......fire that thing up. You’ve got oil to the valve train!

Edited September 7, 2020 by Diseazd

[heyitsrama]

I had this issue on my rebuild, although i confirmed oil was getting it the head, and the oil pump was primed. After that I just ran it, and the oil gauge showed pressure. I’m not sure is there is an official way to do it.  ?

[rcv]

@Captain Obvious I would love a rundown of how those gauges work.

[Captain Obvious]

Will do. I'll put something together.    

[rcv]

Well, I took yall's advice and started it up this afternoon.  Oil pressure jumped to the middle of the gauge (70PSI?) almost immediately.  I started another thread here if anyone's interested in following along with the fun.

FYI before I started the engine I also tried connecting a mechanical pressure gauge in place of the sender.  After cranking for a few seconds, I didn't even get a blip on the gauge.  I then just pulled out the gauge and put my thumb over the hole and saw a little squirt out where I didn't press hard enough.  That was good enough for me, and starting it up was definitely the right decision.

 

@Captain Obvious I'd still love a rundown of how the gauges work if you're up for it.

Edited September 21, 2020 by rcv

[Captain Obvious]

Oops! Sorry!  Memory like a goldfish. Well that, and I kinda wander in and out of on-line consciousness.

I'll put something together. I just took some pics.

[rcv]

Sounds great, looking forward to it

[Captain Obvious]

OK, I promised to describe how the gauges work... Not sure it should be here in this thread, or if I should start a new thread for this. I'll put this here for now, but if people think it should be separate let me know and I'll start a new specific thread about gauge theory.

So in order to understand how the gauge system works, I think it would be a little easier to first understand how they DON'T work. I think everyone already has a good handle on this simple system, but it's an important place to start.

Here's a pic of a simple (uncompensated) gauge design. Wrap a heating coil around a bimetallic strip and mechanically attach that strip to the linkage of a gauge needle. Flow current through that heating coil and to a sender unit who's resistance reflects the level of whatever you're trying to measure (temp, oil, fuel, whatever).

As the resistance of the sender unit changes, the current through the heating coil changes and that changes how hot the bi-metallic strip gets.

The lower the sender resistance, the higher the current.
The higher the current, the hotter the bimetallic strip gets
The hotter the strip, the more the needle moves.

Here's a pic:

This simple uncompensated gauge system does "work", but it is subject to a few real-world outside influences that can affect the accuracy:

First, since the whole thing works on the temperature of the bimetallic strip, the gauge will read differently on a hot day than on a cold day.

And second, since the gauges are powered by the battery system of the car, changes in that system voltage will affect the gauge readings. The gauges would read differently sitting with the engine off than they would with the engine spinning at 3000 RPM when the alternator has kicked up the voltage a bunch. The system voltage can vary from about 12V to over 14V and the gauge readings would change as the voltage varied.

So the basic gauge system above sorta works, but these two real-wold effects are undesirable. If the above basic gauge system makes sense, I'll get into how they compensate for those two real-world effects.

[rcv]

This is great so far @Captain Obvious. I’m out of TV shows to watch, so I’ll be anxiously awaiting your next installment for my new entertainment.

 

I think this probably warrants it’s own thread. It would be a shame for all of this info to get buried under this random topic. If you do start a new one can you just link it here?

 

[Captain Obvious]

LOL. I'm pretty much out of TV shows too. I get it.

There are two "stages" to the compensated gauge. One of those stages is he simple gauge we already talked about above. The other stage is the compensating or regulating stage. It's job is to compensate for changes in ambient temperature and varying system voltage.

The compensation stage consists of another heating coil wrapped around another bimetallic strip. This bimetallic strip forms an electrical switch such that when the strip heats up, it breaks contact. And when it cools down, it re-establishes contact. The trick is, that it makes and breaks contact to it's own power source and the power source for the simple gauge stage.

The result is that this second strip/heating coil combo will make and break the power source such that it will always achieve the same average temperature. Of course the temperature is rising and falling some, making and breaking connection, but the AVERAGE is always the same.

Colder day? Power to the compensation strip will have to be on longer to reach the temp that bends the strip to the point where it breaks the switch connection. Hot day? Just the opposite. Power will be on for a shorter time.

Low system voltage? Again, power will have to be connected longer in order to heat the strip to the desired temperature,. And conversely, if the voltage is higher, it'll take less time.

The result is that the compensation stage creates an ON/OFF/ON/OFF pulse train whose duty cycle and frequency will change depending on the ambient temperature and system voltage. This effectively creates a voltage source that will always supply a constant amount of POWER to the gauge system under all conditions.

Clear as mud? Connect that constant power source to the simple gauge from above, and you have this:

If you put your key in and turn it to "ON" you should see the gauges start to rise. Don't start the car. Just pick a needle and watch carefully. You'll see the needle start to rise, but probably before it reaches it's final position, it'll pause... Then start rising again. Then pause again. This will continue until it finally reaches it's final position. And even then, if you watch carefully, you'll see the needle actually wiggles a tiny bit.

This effect can most easily be seen with a needle that's moving well above minimum like a full tank of gas. That pausing and wiggling is the compensation stage opening and closing.