Oil pressure gauge erratic.

[rswaffie]

Hi,
Having sorted out the fan belt rubbing on the bottom hose problem, I’ve been putting on some more miles this week.
There now seems to be a problem with the oil pressure gauge that started today. For the last 2000 miles since the rebuild, the gauge (electrical) normally registers 60 at cold startup, moving down to around 45 when warm and around 25 at hot idle. Travelling at 55-60mph when hot would register a steady 40, moving up to 45/50 when above 70.
Today, I noticed that it started at the far right of the gauge, moved down to 40, went back up to 60 as it warmed up, back down to 40, then down to 20, back up to 40, 60 even during a hot steady run at 50mph. It idled at 20, the dropped to 0 until I pulled away and it kept repeating the fluctuation once back up to speed.
I checked the oil earlier in the week and it was between the hatching and the ‘W’ on the dipstick so topped up with a litre and a half. The level is now just into the hatching on the stick.
Could this be a failing sender?

[MarekH]

The vast majority* of these senders work in the same way as the bimetallic voltage regulators which control the dash gauge voltage down to 10volts. Essentially these turn on and off in rapid succession so at 14volts, they are on for 10/14ths of the time, so generate an average 10volts over time. The oil gauge also has an extra dimension, in that the bimetallic mechanism sits on a hinge, such that it is pushed up by a plug if at higher oil pressure, so the "constant" average 10volts now becomes another number. The remaining voltage is what appears on the gauge. That is why the oil gauge runs off of full voltage, not from the 10volt supply.

Accordingly, there are a number of ways to get a fluctuating result.

1/ The input voltage varies - meaning your alternator isn't regulating properly. This ought not to be possible, as the sender should design this away.
2/ The electrical connection is erratic. When this happens, the gauge will drop to zero if totally disconnected, or an intermediate value if it keeps connecting and disconnecting just like the bimetallic strip does.
3/ The electrical connection is poor. Added resistance means a voltage drop, so the gauge reads low.
4/ There is some debris interfering with oil to the sender mechanism.
5/ Finally - there may be some dodgy electrical connection issue inside the gauge which is doing 1/ to 4/.
6/ Or - the oil pressure genuinely changes!

The oil light switch ought to come on somewhere at or below 3psi, so there is a double check for genuinely having zero oil pressure.

If you have access to a bicycle track pump and can plug the sender connection, you ought to be able to jury rig the dash gauge to run with a known test pressure by using some standard 1/4"BSP fittings at the sender. You can do that safely at different gauge temperatures and at different car voltages and at different pressures, knowing that the indicated track pump pressure is accurate.

As a minimum, I would start testing integrity of the wiring and connections in the gauge/sender/earth path.

Do remember that the gauge also has a bimetallic strip mechanism to drive its needle, so shares some of the same failure mechanisms as the sender.

kind regards
Marek

*I have dismantled a least one sender which looked similar externally, but had a rheostat and wiper (just like in the fuel tank) so worked on a different basis. You can test what type of sender it is by connecting an LED in series - it will strobe if it is a bimetallic type of sender.

[abowie]

Marek's advice to ensure you have good electrical connections is good.

The one worrying thing would be the gauge reading to the far right. This indicates that there is no resistance, and might indicate a failing sender unit. Everything else could be explained by a poor connection somewhere.

Is the sender new?

[rswaffie]

Hi
Thank for the replies.
I have checked all the wiring connections and can’t see anything amiss. The sender was new when the car was restored 3 years ago and has covered about 2000 miles.
To swap it out, do I need to drain the oil?

[mgcjag]

No need to drain the sender is on top of the houseing however a friend recently had eratic pressure so cleaned the bypass /pressure release valved..all was then ok....senders are a bit hit and mis...why not just fit a wet guage....Steve

[malcolm]

I fitted the SNG one. Compared the results with a professional unit strapped to the car when driving, and readings identical. Also much better and more consistent than the sender had indicated even when it was working "well"

[rswaffie]

I know the mechanical kit is a change that a few owners have made, but at £250 it seems expensive compared to £70 for a new sender (if that’s the problem). If it turns out that a replacement sender fails again, then I may consider this mechanical option.
If I want to check the pressure release valve, I guess that I will need to drain the sump as it is under the housing? Or could I just shove a rubber bung in when it’s removed?

[mgcjag]

Oil sits in the sump....the filter houseing can be removed ..if you want to know how you pressure is performing get a wet guage.....an electrical will never give you a correct reading......it just a guide at best.....how many times have we talked about bad senders on the forum..even new ones....Steve

[Bill Michigan]

Richard-

To prevent oil from draining from the sump, don't remove the 3-nut fitting that connects the bottom of the pressure relief drain hose to the sump. For extra assurance, leave the hose connected to that bottom fitting. (loosen both hose clamps and slide the hose downward to remove it from the relief valve outlet fitting).

Bill

[caveman]

Another vote here for a wet gauge Richard. Takes out the worry of electrical gremlins. I had an electrical short on my fan override circuit recently and all central gauges were lost bar my oil gauge until I reached my hotel and traced the fault and replaced a number of fuses.

[rswaffie]

Update on the erratic oil pressure readings:
I rang up my local JLR spares yesterday and ordered a replacement sender, which I fitted today. It seems to have sorted out the problem as the readings are now steady, albeit about 10psi higher than before. 60 at cold start, 50 on a hot run and about 38 at hot idle. This compared to 60,40 and 25 before the previous sender started playing up. I’ll get hold of a wet oil pressure gauge at some point and see what the ‘true’ reading is, but I’m ok with it for now.
If the sender plays up again, I’ll probably go for the wet setup as per suggestions above.
Thanks all.