Oil level gauge troubleshooting

dicklague · dicklague's Garage

The oil level gauge on my 1973 has been funky lately. It will sit up at MAX up at the top and then when I hit 3-4000 RPM it will start fluttering violently.....then settle down again at idle. I checked the oil level hot and I was down a quart and a half. I also found the wire connectors a little loose on the spade lugs...fixed that too, and still had the flutterring.

I did a bit of research and found some really confusing threads. Lots of misunderstanding and lack of electrical knowledge and theory.

The circuit is simple. The level sensor is just a float that is hooked to a resistor. The range of the resistor is 0 to 230 OHMS. The sender has a ground [brown wire] and a white or yellow wire for the signal.

The other end is really just a volt meter that reads what OHMS the sender it currently reading and turns it into a meter reading......horizontal is approximately full.

I suspected the sender was sending out errating reistance readings due to a bad wiper or bad wires on the resistor. I reasoned if I unhooked the send and substituted known steady ohms, getting a steady reading at the gauge would mean the gauge is OK.

I have a resistor box like this one from eBay

https://www.ebay.com/itm/0-9999-ohm-Precision-Variable-Resistor-Resistance-Decade-Box-Teach-Instrument-US/153504980791?hash=item23bd9c1f37:g:ckoAAOSwcMhc7NN z

I unhooked the wires to the sender and hooked up the box set for 100 ohms. The meter read almost horizontally and was steady. I found that about 80 ohms was horizontal on the gauge. By changing the resistance on the box within the range of 0 to 230 ohms I could move the gauge needle to wherever I wanted.

That proved that the gauge was OK and the sender is bad.

I will try to post some pictures later today. That little $16 resistance box is a great trouble shooter for senders that just change their ohm readings as a signal. Could be used for fuel tank sender, or temp, and any analog resistance sender.

I ordered a new sender and left the wires disconnected. I hate to see that poor gauge flopping around......can't be good on the mechnism.

I topped up the oil and will chnage the sensor and oil next week.

Cory M · 02-28-2020, 02:04 PM

I have the same issue with my oil level gauge Dick. I assumed it's the sender that's bad and planned to replace at the next oil change. I just ordered the resister box to check it out, great idea and I'm sure I'll find a few other opportunities to use it.

dicklague · dicklague's Garage

Quote:

Originally Posted by Cory M

I have the same issue with my oil level gauge Dick. I assumed it's the sender that's bad and planned to replace at the next oil change. I just ordered the resister box to check it out, great idea and I'm sure I'll find a few other opportunities to use it.

Cory
Good for you. I am glad my experience can help.

I was really surprised that the information was so scattered. It is a basic circuit. It took time to find the range of OHMs for a good sender. I get the feeling that the sender is usually the cause, and the gauge is pretty reliable.

My frustration was that the threads I found were not good at troubleshooting the gauge, wiring, and sender seperately.

By the way twisting the knobs on the resistance box and watching the gauge move to nice steady settings is fun.

MichaelSJackson · 02-28-2020, 03:19 PM

The oil level in that tank does change as you drive. Only read it when hot and stopped on the level. It's more of a fancy idiot light. The dipstick is how to check your oil level.

"Fluttering"? Does it look electrical or could it just be oil surging in and out of the tank?

I never top it off. Settle for 3/4ths even 1/2. If I don't park on the level a little oil gets past her valve guides and it will produce blue smoke at start up.

You probably knew all that. Sorry.

dicklague · dicklague's Garage

Quote:

Originally Posted by MichaelSJackson

The oil level in that tank does change as you drive. Only read it when hot and stopped on the level. It's more of a fancy idiot light. The dipstick is how to check your oil level.

"Fluttering"? Does it look electrical or could it just be oil surging in and out of the tank?

I never top it off. Settle for 3/4ths even 1/2. If I don't park on the level a little oil gets past her valve guides and it will produce blue smoke at start up.

You probably knew all that. Sorry.

It does look electrical. You can even hear it banging around it is pretty violent. Caused by the wiper intermittently contacting the resistor coils.

Yes, I do know how to check the oil, I have had this 911 for over 20 years.

Cory M · 02-28-2020, 07:54 PM

The needle on my gauge would flick loudly from empty to full at random. It was really annoying. I unplugged it at the sender and now it mostly stays pegged at full. I rely on the dipstick to check the level, but still want a functional gauge.

smadsen · 02-28-2020, 08:04 PM

When you throttle up, the oil level drops slightly in the tank. At anything other than idle, it splorts back into the top of the tank intermittently, probably disrupting the gauge. That's why you only check the oil on the gauge at hot idle, after the level normalizes.

To check the gauge, read the dipstick. The two should correlate closely, high, middle & low. If not, my guess would be the 47 year-old sending unit.

HarryD · 02-29-2020, 04:22 PM

Dick,

My original level sender started acting all kinds of crazy. I finally replaced it with a new unit.

When I first checked it against the dip stick, it was reading high. I pulled it back out and compared the bend of the arm in the old one against the new one when at full droop. I needed to bend the new arm slightly to get the same geometry. All is well now.

FWIW, you do NOT need to drain the oil to change the sender.

dicklague · dicklague's Garage

Quote:

Originally Posted by HarryD

Dick,

My original level sender started acting all kinds of crazy. I finally replaced it with a new unit.

When I first checked it against the dip stick, it was reading high. I pulled it back out and compared the bend of the arm in the old one against the new one when at full droop. I needed to bend the new arm slightly to get the same geometry. All is well now.

FWIW, you do NOT need to drain the oil to change the sender.

Good advice. I will check that on my install.

I know I don't have to change the oil for install. this time I just need to do both as the oil change is over due.

proporsche · proporsche's Garage

Quote:

Originally Posted by HarryD

Dick,

My original level sender started acting all kinds of crazy. I finally replaced it with a new unit.

When I first checked it against the dip stick, it was reading high. I pulled it back out and compared the bend of the arm in the old one against the new one when at full droop. I needed to bend the new arm slightly to get the same geometry. All is well now.

FWIW, you do NOT need to drain the oil to change the sender.

yup, that is how to synchronize the interior gauge with the oil level sending unit;-)

good add Harry

Ivan

VFR750 · 03-01-2020, 01:43 PM

The sender is a large reostat made from a large, wire wrapped board. The wiper arm slowly wears the wire down and you start to get thin spots that will lead to an open circuit. I had to solder the wires to get continuity.

911T70 · 911T70's Garage

I'm getting quite frustrated with mine. I've removed it from the tank twice to tested it and it works perfectly but when I place it back in, it doesn't really move while I can see the level going down on the dipstick over the course of time. I'm thinking that maybe the float gets stuck against the inside wall of the tank but I don't know how to verify that. Maybe I can check through the drain hole with a boroscope now that the tank is drained for the winter.

Has anyone else had this problem? Any suggestions on how to troubleshoot this?

shadowjack1 · shadowjack1's Garage

Check and see if someone at onetime dropped a dipstick into the tank. Happens all the time. This dipstick could be your problem.

911T70 · 911T70's Garage

Quote:

Originally Posted by shadowjack1

Check and see if someone at onetime dropped a dipstick into the tank. Happens all the time. This dipstick could be your problem.

Thanks, but it's unlikely. I had the tank out last winter and didn't notice anything rattling in there. I'll have a look through the drain hole with a borescope to see if there's anything abnormal or if the float is binding somewhere.

Nditiz1 · 03-02-2020, 07:49 AM

Is it still doing the "fluttering"?

This is how mine works:

If Oil level on dipstick is at the 1/2 mark (a mark I made on the dipstick between hi/low) the gauge will read close to the bottom. It will not move a lick until the car is warmed up unless I accelerate it may drop to empty.

Once the engine is warm and the temp kicks the oil cooling circuit (~174) the oil level rises up to max as long as I am sitting idle or cruising at a constant speed. Any other acceleration/deceleration pegs/lowers the reading. This makes sense as oil inside the tank is sloshing around.

This all seems normal to me.

manbridge 74 · 03-02-2020, 08:24 AM

Done a lot of these over the years. Common to find the arm/float part bent wrong from earlier repair attempts. Install with attention paid to arm angle, shouldn’t have to force it.

l_turn9 · l_turn9's Garage

I removed my sender after connecting the 2 wires hooked to it and saw the Needle go the top of the gauge. Now, I have the Sender disassembled on my bench and want to test the Ohms. I'm not electrically inclined but this is what I think I am supposed to do: Connect the + probe to the Center connector and the - probe to the mounting plate. As I move the float I can see 12. +/- at one extreme and around 108 +/- at the other. Is this telling me the Sender is working as it is supposed to? (BTW, before all of this, when I turned the ignition ON, the needle would rise about 1/4".) Now, I fear I removed it for no reason and merely need to wait for the front oil cooler T'Stat to open to get a good reading ;-)
Thanks -