Bouncy oil pressure needle - is there actually a solve?

[69fi]

Tom, do you also see those same RFI spikes and short-pulses on the Bat wire going to the gauge or is it somehow being introduced by the interplay between the gauge and the oil pressure switch itself? I’m just wondering if the electrical system itself is that noisey.

[Tom]

I didn't specifically scope the power wires going to the cluster, but I'd be surprised if there were any unshielded wires in the harness that don't pick up the ignition spikes. The cause of the gauge jitter does not seem to be the RFI though, but rather the micro-pulsing of the signal. You can see it in the video below -- maybe 5 or more tiny pulses per second, presumably from sender picking up the low RPM pulses and the gauge then exaggerating them by bouncing.



Edit: also, looking at the pulsing on that scope, I'm starting to wonder how much of the gauge bounce is just a reflection of that pulsing. Back of the envelope math and eyeball measurements on the scope would lead me to think the needle bounce may just the gauge showing what's on the signal (or mostly so anyway). The voltage seems to 'bounce' maybe .8 volts peak to peak with a mean of 4.9v, which equates to a swing of about 16 ohms (108 to 125) by my math. And there are maybe 4 or 5 such bounces per second. If you look at the sensor specs, 108 to 125 is a little over 3 Bar and a little under 4 -- pretty much exactly the speed and magnitude of the needle's bounce. Now what I don't know is whether that pulsing is actual oil pressure changes or some artifact from the sender. Can't help but notice the bouncing reduces when you put the clutch in at idle. All that brings be back to George's first comment -- checking with a mechanical gauge would shed a lot of light....

[sefeing]

i’d definitely be interested in a PCB, love the idea of anything non-destructive to make the fix, and this is the only actual solve i’ve ever seen to this issue.

but that said, may order a few capacitors in the meantime - what three values did you use on your board? funny enough, my needle bounce varies, so wonder if somewhere between max and min makes most sense.
pulling the cluster isn’t awful, just slightly more of a headache with the air bag wheel.

blown away by the quick detective work here!

[Tom]

i’d definitely be interested in a PCB, love the idea of anything non-destructive to make the fix, and this is the only actual solve i’ve ever seen to this issue.

but that said, may order a few capacitors in the meantime - what three values did you use on your board? funny enough, my needle bounce varies, so wonder if somewhere between max and min makes most sense.
pulling the cluster isn’t awful, just slightly more of a headache with the air bag wheel.

blown away by the quick detective work here!

I have a kit of capacitors from amazon that goes up to 2000uF I think. (For a more permanent install, when I pull the cluster, I'll be sure to use low-ESR automotive capacitors though... ) To hone in on the best value, I left the sender wires attached to the sender, and ran another wire from the sender blue/white side to the cabin (via the passenger window ) and sat in the car idling until the needle was good and bouncy -- then just tried various size capacitors until I found the right range (wire from sender to positive lead, wire from grounded door latch to negative lead). I was looking for the lowest value that effectively eliminated the bounce. Give it a try and let us know what works best for you -- it will help inform what I should put on the pcb.

On that little board, I have three 470uF capacitors. When you run them in parallel you can just add the values, so that set up gives the option for 470, 940, or 1410uF. I have a 1500uF right now (in the engine compartment) and used the car yesterday to run errands, etc. and the gauge did great. 1500uF might be overkill --1000-1200uF seemed about the same -- but it's still responsive either way and I had a 1500uF cap. The RC time constant is around 150ms at 3 BAR with a 1500uF cap, so the needle still reacts to change without any noticeable delay or sluggishness. It's just twitchy enough to seems like a real-time gauge, without being bouncy, and without being overly medicated like most modern cars.

[Drscottsmith]

All -

I have developed a similar problem with a bit of a twist…I am getting the twitch on my fuel, coolant and volt gauges - but the pulse is timed with the cycling of the speedometer sensor in the tranny - so the slower I am going the slower the “pulse” on the gauge.

This is a 928 by the way but I am pretty sure the sensor in the transmission is the same for both.

Can I put a capacitor on the line from the speed sensor and eliminate the pulse or am I looking at that wrong. On the wiring diagram there is a ground that goes directly from the Speedo sensor in the tranny to the IP and to the ground line for those gauges.

I get no pulse in the Speedo itself.

Thanks all!

[Tom]

All -

I have developed a similar problem with a bit of a twist…I am getting the twitch on my fuel, coolant and volt gauges - but the pulse is timed with the cycling of the speedometer sensor in the tranny - so the slower I am going the slower the “pulse” on the gauge.

This is a 928 by the way but I am pretty sure the sensor in the transmission is the same for both.

Can I put a capacitor on the line from the speed sensor and eliminate the pulse or am I looking at that wrong. On the wiring diagram there is a ground that goes directly from the Speedo sensor in the tranny to the IP and to the ground line for those gauges.

I get no pulse in the Speedo itself.

Thanks all!

That sounds like a very different issue. The speedometer signal is a ground pulse. If all the gauges are 'pulsing' in time with the speedometer signal, that would make me suspect bad grounds to the cluster. Sounds like the cluster has dirty ground points, and the speedometer signal is giving the cluster a slightly better ground when pulsing. A capacitor would only be masking a more fundamental problem. What happens if you unplug the speedometer signal at the transmission -- do the other gauges all smooth out? @dr bob is our resident 928 expert-- I'd be curious if this is a common failure mode in that car.

[Drscottsmith]

Thanks Tom -

Unplugged the connector in the spare tire well (that is where the sensor plugs into the rest of the harness).

Of course the Speedo and Odo stopped working, but all other gauges are as still as they can be.

Not sure what that means?

[Tom]

Thanks Tom -

Unplugged the connector in the spare tire well (that is where the sensor plugs into the rest of the harness).

Of course the Speedo and Odo stopped working, but all other gauges are as still as they can be.

Not sure what that means?

I would start by cleaning all the grounds. If the 928 cluster has the 'football' connectors behind the smaller gauges (like the 944) I would clean those too. If that doesn't help, then I'd suspect something going on with the cluster or speedometer (e.g., burned out capacitor, drawing excess current, etc.).

p.s., might be worth starting a separate thread in the 928 forum where it might attract attention from other 928 owners. It's kind of hiding here, and my 928 knowledge is limited to whatever the 928 shares with the 944....

[dr bob]

Depending on the year of the 928, I's start by looking at the constant voltage control in the gauge cluster. I'd have to dig through some old docs, but the idea is that there's a 5V regulator. With the year I could look a bit harder at the cluster circuitry.

A capacitor on the line from the speedo 'sensor' (a reed switch closed by a magnet in the differential carrier) just reshapes the pulses detected. It's normally a pretty low-current loop, and won't take a lot to kill speedo functions I suspect.

[Rveague]

A tip for anyone using the "Carpoke Capacitor" approach. I installed this on my oil pressure gauge to address a bouncy needle, while I had the dash cluster out (for other reasons).

I obtained a suitable capacitor, soldered ring terminal leads, and installed it behind the "football" as suggested. I rigged up some test wires and verified it worked, and I was seeing appropriate needle movement based on different resistors I tested with. All good. When I installed the dash in the car, no needle movement at all. The pressure warning light went out as expected, so I knew I had oil pressure.

I pulled the dash cluster again, and realize the two ring terminals (that lead to C8 and C10), caused the football to lift just enough to lose contact on C5, 12V in. So, no power to the gauge. I put a washer of the same thickness as the ring terminals on the 12V in post (the left pin, facing the back of the cluster), and verified I had good contact. Reinstalled the dash, and all good.

Hope this helps!