Installing a two-pole oil pressure sender on a Porsche 914-4

[Windisch]

I have a '74 1.8 liter 914 that has been converted to carbs (40mmIDFs) and installed a central instrument console. In place of the clock I have a VDO 'Jet Cockpit' oil pressure gauge (0-80 PSIG). I have also installed the correctly matched oil pressure sender unit (two terminal: idiot light and pressure gauge) on the top right engine, next to the distributor (using a short steel-braded hose and an adapter fitting).

Al that remains is to connect the correct sender terminal to the oil pressure gauge on the console, and therein lies the rub. What is the accepted method of wiring up the gauge? How does one do this? Do you have to snake a dedicated wire from the sender along the car's central tunnel to the console for this purpose? Or is there an existing wire that may be used, sparing the necessity of installing that specially dedicated wire?

I am aware of the black/green wire that may be used to connect the engine oil temp gauge from its sender on the engine's 'tacoplate', but I suppose there is no such convenient wire for purposes of sending oil pressure to the console gauge, pre-installed?

If a specially installed (dedicated) oil pressure wire IS required (as I suspect it is), how easy/difficult is this task to complete? Any help answering this question would be most appreciated since I want to try to keep the wiring task as simple and 'effortless' as possible (naturally).

[Attached is a photo of my console (the oil pressure gauge is on the top...all three of the gauges are part of the original VDO 'Jet Cockpit Series', which are now difficult to source and costly to buy), merely for FYI purposes.] Thanks in advance. I was hoping there was a technical article on how to install an oil pressure gauge on the PELICAN FORUM, but an on-line search produces plenty of threads on this subject in various venues...

PS: One last word on the installation of the correct 0-80 PSIG, two-pole sender. Installing that sender in the oil-pressure idiot switch hole (right top of engine near disty) requires a 14mm line socket and for some reason a 14mm line socket of the config I prefer is hard to find. The only one I really wanted is offered by SNAP-ON TOOLS (and is, as one may imagine, quite costly). SNAP-ON wants about $75 or so for a set of two (1/2" drive, 14mm and 17mm), so I reluctantly sprang for that. BIG expense for a specialised tool like that, but as the old saying has it, "The right tool for the job...etc" The second picture here shows this SNAP-ON line-socket.]

[Dave at Pelican Parts]

There is no dedicated wire in the car for an oil-pressure gauge. It may be possible to find unused wires in the harness, but I wouldn't bet any money on that.

I ran new wires. I used a stiff piece of wire to fish the new wires through the rubber "snorkel" wiring harness protector at the firewall, and ran the new ones alongside the wiring harness in the center tunnel. I did not find it that difficult--EXCEPT: I did poke a new hole through the side of the snorkel by not being careful enough with the end of the stiff wire.

I did not have the engine out at the time, but it would be an awful lot easier to feed the wires through that snorkel with it out. I remember an awful lot of getting under the car, crawling out to mess with the top, then crawling under the car, and on and on and on. That's probably the cause of the new hole in the snorkel, frankly...

--DD

[Windisch]

3 March 2025....UPDATE Thanks for that input, Dave. I installed everything as described earlier but I seem to have a gauge that is not functioning properly. Let me explain.

When I switch the car's ignition switch on, the oil-pressure gauge's indicator needle immediately pegs out at the upper reading mark (70 PSIG) and stays there until the ignition is switched off (at which point in immediately drops back to PSIG).

At no time between switch-on and switch-off (with the engine running at idle RPMs) does the gauge give a (normal, varying) pressure reading...it just remains at 70 PSIG.

The gauge and the engine two-pole pressure switch appear (to me) to be properly installed and grounded, but other persons (and also an independent AI-prompted problem solver) suggest that grounding is still the most LIKELY cause of this anomaly.

Normal 914 oil pressure would vary (as near as I can anticipate things) from about 15-20 PSIG at idle RPMs to about 60-70 PSIG at 4500 or so RPM. But this is not being observed...just an immediate rise (from 0 PSIG at switch-on) to 70 PSIG and then back to 0 PSIG at switch off.

Any other suggestions or possibilities to consider that might resolve this frustrating glitch? Thanks.

[Dave at Pelican Parts]

The sender wire or sender itself is probably shorted to ground.

The sender is just a variable resistor. I'm pretty sure that higher resistance shows as lower pressure, and lower resistance as higher pressure. If the wire is shorted to ground, that's 0 resistance which means max deflection on the gauge.

Check the resistance from the end of the sender wire (unplugged from the gauge) to a ground. With the engine off, there should be resistance.

--DD

[914 Buzz]

Originally Posted by Windisch

3 March 2025....UPDATE Thanks for that input, Dave. I installed everything as described earlier but I seem to have a gauge that is not functioning properly. Let me explain.

When I switch the car's ignition switch on, the oil-pressure gauge's indicator needle immediately pegs out at the upper reading mark (70 PSIG) and stays there until the ignition is switched off (at which point in immediately drops back to PSIG).

At no time between switch-on and switch-off (with the engine running at idle RPMs) does the gauge give a (normal, varying) pressure reading...it just remains at 70 PSIG.

The gauge and the engine two-pole pressure switch appear (to me) to be properly installed and grounded, but other persons (and also an independent AI-prompted problem solver) suggest that grounding is still the most LIKELY cause of this anomaly.

Normal 914 oil pressure would vary (as near as I can anticipate things) from about 15-20 PSIG at idle RPMs to about 60-70 PSIG at 4500 or so RPM. But this is not being observed...just an immediate rise (from 0 PSIG at switch-on) to 70 PSIG and then back to 0 PSIG at switch off.

Any other suggestions or possibilities to consider that might resolve this frustrating glitch? Thanks.

I have the VDO dual-connector sender in my car. There were two things that were not clear when I installed it that might be your problem:

1) If you have the sender in Illustration 3 of https://vdo-instruments.com/instructions/A2C59519516%20Temperature%20and%20Press%20Senders. pdf, then the terminals are not interchangeable. One is for the idiot light, and the other for your gauge. You can't swap the connectors. If I remember, the idiot light has a "W" stamped next to the connector, and the gauge a "G".
2) The sender is a "floating ground" design, meaning you need to connect the case of the sender to your engine/chassis ground. There is no screw or terminal on the sender to do this. If your sender is directly connected to the engine this is the ground; if you used a hose then the sender is not grounded. I bought some silver-filled epoxy and attached a ground lead to the sender case; a hose clamp around the sender case with a jumper wire would let you test if this is the problem.

[Dave at Pelican Parts]

I sandwiched a bare-ended wire between the mounting clamp and the body of the sender in my car... It's hoky but it works.

--DD