Electrical Gremlins or something else : oil light, brake lights and tach

[eflight]

Was out driving and notice a few weird things happening with the instruments on the car.

Car is a 1984 targa with about 58k miles.

Oil pressure warning light would come on when the engine dropped down to idle. Stop at a stop light, push in the clutch and the light would go on. Pressure at idle was about 0.8 - 0.9 bars. When running pressure went up as normal and light would go off. Checked oil level and its full. Oil level gauge works fine and shows about 3/4 which is normal for the car when filled per the dipstick.

When oil pressure light goes on the brake warning light and wear light both come on on. Brakes are fine, fluid is topped off. Light would go off once oil light went off. light would sometimes flicker when driving

Tachometer was working fine, I noticed that while driving with the rpm at a steady 3k the needle on the tach would blip about 200 rpm every once in while. Then the tach just stopped working.

Digital volt meter installed shows 13.9 volts. No voltage spokes noted but not sure that would show on a cheap digital gauge.

Other than those issues car runs great.

So are these all related or separate issues?

I was thinking some kind of grounding issue or alternator going bad?

any ideas? What should I look for?

thanks

[wazzz]

I sure can't explain everything and tell whether these issues are all related, but I know one thing: The brake warning lights go on whenever the oil pressure light goes on and that is normal. Kind of weird but that's the way it works. By design, the oil pressure switch is used as a test feature for brake warning lights in the seatbelt/brake combination instrument when ignition is on but engine is off.

Now, either you have a real low pressure situation correctly reported by the oil pressure switch, or the oil pressure switch is dying and shorting sooner than it should.

[Bill Douglas]

Umm, I had various weird electrical things (gremlins) in the SC. I cleaned and re-seated all the grounds/earths , about five locations, and everything came right. So it's a good place to start.

[eflight]

thanks guys, I took a look at the grounds. They all look original so I'll start the replacement process.

[eflight]

I cleaned the grounds in the engine bay, G402 and G407. Also cleaned G104 in the frunk. That looks like it helped a few things. The oil pressure is about 1 bar now at idle so the low pressure light does not come on and the brake warning stays off as well. The Tach is also working now, though it still bounces a bit.

I'll order new ground straps for the other spots and install those next.

Thanks for the help folks

[Bill Douglas]

'That's good news!

I ran an extra ground strap from the engine to the chassis where the rear engine mounts are. I just thought the flexi wire looking one from the transmission to the body looked a bit jaded.

[Kraftwerk]

Clean grounds are MOST important. The older the car- the more these things need to be attended to. Is there a quick schematic of where ALL 5 grounds are on the SC ? Maybe I will pull down the Bentley-guide for reference... I know some of them. In the past I have had "no-start" issues with a '71 914 due to poor ground and an '86 VW, also due to poor ground. It's why i always carry a wire brush and a scrap of sandpaper in my tool-kit.

[eflight]

Welp the lights are back.

I cleaned up the battery ground and replaced the negative battery cable and took it out for a quick spin and noticed the oil pressure light and brake light are back.

Good news is that the tach was not spiking and was working well

I'm in the process of addressing all the ground points. the transmission ground and alternator ground straps are next.

After that I'll see about replacing the oil pressure switch.

[wazzz]

Cleaning grounds is always a good idea, but I don't see how it could fix a situation where indicators are lighting up while they shouldn't. Since these indicators need a good ground to light up, they obviously get a good ground.
The oil pressure switch drives both the oil pressure warning light and the brake warning light (as a test feature), so unless you are facing a real low oil pressure issue, which you probably have already ruled out, the oil pressure switch seems to be the culprit. Disconnect the wire from this switch at the top of the engine case and see if both warning indicator lights now go off when the engine is on. If they are off you know the switch is bad.

[EC900]

Are your at 950 rpm at idle ?
To eliminate possibility, check your belt tension for too much slack or slap if the ac clicks on.

Really hot weather can cause low oil pressure light at a stop as oil thins out, maybe just a false positive. Just to be clear, checking oil when engine is hot, level and running, with the full line being between the 2 dots maybe a tad closer to the top.

Might be a stretch since car runs fine otherwise…. maybe the coil could be malfunctioning, also due to heat both ambient outdoor temp and in engine compartment temp.

[eflight]

So wife and I went for a ride last week. Low oil pressure warning light comes on and stays on, even at 3000 rpm, oil pressure at 4 bars, just cruising along. Hit the clutch, shift gears, oil pressure drops, light goes out, oil pressure back up, then a few seconds latter the oil pressure light is back on.

My guess is that the oil low pressure switch has gone bad. I ordered a cheap borescope so I can take a look back there without taking the air filter and other stuff out.

Since I had already ordered the transmission ground strap I figured today would be a good day to change that. Its a pain to get a socket in there to take the nut off but I was able to do it. Much harder than it needs to be.

I watched videos where it looked pretty simple, but those cars had a much different transmission mounting bracket than mine did. Amazing how one slightly different part can make it much more difficult for you to do than the guy that says it was easy.

What I did notice was that my ground strap was not up against the transmission housing. It was placed on the outside of the nut and then held in place with washers and another nut. None of the photos I saw on pelican looked like that and the PET does not show multiple washers and nuts. So I took off the nut on the case, placed the strap against the transmission case and then washer and nut. I did have to trim the ends of the strap to fit against the transmission.

Went for a ride after the new strap was installed. Car started quicker than it has been doing and the oil pressure light did not come on. I'll have to keep an eye on it when I take it for a longer ride.

Once the borescope comes in I'll take a look at the alternator ground as well.

[Fab64]

Very cool! Sounds like you're on the right track.

Unfortunately, this stuff happens as these cars get older. My '84 developed an intermittent miss 2-1/2 years ago. After pulling and cleaning all the sensor and fuel injector connectors, it went away and has not returned.

Good luck!

[eflight]

So the oil pressure light is back. Looks like it takes about a half hour drive and then it shows up.


The borescope gets delivered tomorrow so I'll use it to take a look at the pressure switch and surrounding area.

[spoke]

Originally Posted by eflight

Low oil pressure warning light comes on and stays on, even at 3000 rpm, oil pressure at 4 bars

It seems you have a bad oil sender switch as mentioned before. You have 2 sensors, the sender switch and pressure gauge. Pressure gauge seems to be working normally yet the switch closes.

[wazzz]

That's what I keep saying. You don't want the light to come on but it is coming on, that means it is already served by a good ground. How could cleaning grounds improve things in this situation?

Replace that oil pressure switch.

[eflight]

My plan is to inspect the switch. If its leaking then replace it. If its not, then I will likely just disconnect it.

[eflight]

Got the borescope today. Just a cheap $35 unit, but it does what I needed it to do.

Looks like the switch has been leaking for a while. Time to order a new one and read about how to get in there and replace it.

I'm not all that worried about getting it out, its the getting it back in there that worries me. Well there is only one way to find out I guess.

Looks like an acorn has founds its way in there as well

[EC900]

As for the walking acorn…rodent(s) brought in overwintering provisions.
Check under the fan shroud with that new borescope for nesting materials.

[eflight]

Originally Posted by EC900

As for the walking acorn…rodent(s) brought in overwintering provisions.
Check under the fan shroud with that new borescope for nesting materials.

I found the nest this spring in the shift linkage tunnel right by the aft coupler.

[eflight]

Out with the old, in with the new.

Replaced the old low pressure warning light switch with a new one. I did the epoxy ring around the top of the new switch with J-B weld.

That old one was in there very tight. Took a good bit of torgue to get it out.

Put in the new one with 19Nm torque.

I think it took longer to get those blind bolts installed in the back of the air box than anything else.

Will have to take it for a test drive later this week to see how everything works out.