I've digging around in my 1990 Convertible and have come across a large (looks like ground) eyelet that is bolted into a threaded hole on the RH rear top of my intake manifold. It's always been this way.

When I unbolt and remove the eyelet while the car is running I SWEAR I notice a change in idle for the better. When I put a DMM on the free eyelet I get 12+v battery voltage, it's weird. When I go to reconnect, I sometimes see the faintest spark.

The associated wiring (unless I messed up when tapping up wiring looms) isn't long enough to reach the fire wall, or another good ground point. And there are 3 or individual wires crimped onto this eyelet. Definitely OEM.

What the heck is it? And what the hell is going on?

Center. Bolt with 3 black earth wires to intake - which can't be the best earth point.

 

Hmmmm.

My [1989] S57 Wiring Guide shows 3 grounds wires for the oxygen sensor circuit but doesn't state the exact location. It just says "Engine Ground". These would be for the O2 sensor heaters and the shielded wiring.

Cheers
DD

 

Not exactly clear what you mean JJJ. Do you mean the P clip holding the blue thing?
If so that is the full load enrichment vacuum switch fixing.

If you are getting 12 volts off the manifold it is because your engine ground is not working. Fit a ground strap from a bolt on the engine to shiny cleaned metal somewhere on the body ASAP.

If you mean the hardly visible eyelet under the bolt to the right of the jubilee clip, that will be a factory ground, and assuming it is grounding something to do with the engine, if you disconnect it and put a meter on the disconnected end of the wire you WILL get volts from it because the ground is not grounding, so it is grounding theough the meter! Also no doubt this WILL mean something to do with the engine is NOT grounding that needs to, so no doubt the engine note will change.

 

Jay,

When you put one probe of the meter on that bolt head, where are you putting the other probe to complete the circuit? Im struggling to see how you'd have a live 12v at an inlet manifold bolt?

Paul

 

Oh please, ignore the blue thing (I wish I could although I believe it's functioning). I'll repost with circle.

It's an OE eyelet. Can get a good photo because of the sun and it's location, but will remove it and try. I can tell its OE because of the loom sleeve/cover.

I'll see if I can trace back the general direction of my O2 sensor wires. Seems like a long run for the LH side sensor. Also, since we are talking about O2 sensors,,, when I removed and refit my RH downpipe the other week, the damn sensor ended up in a bit of a different place and NOW the damn sensor comes up against the body of the car I at it's home in the wheel well. Maybe it's damaged and,,, this is a whole DIFFERENT story... I all of a sudden have TOO MUCH going on! Fans and other things.


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Red probe on ring of eyelet, black probe on chassis ground. Should I EXPECT to see 12volts is I do that? When the elelet is free (unbolted) and bolted down? Let me go run the measurement again... One sec...

 

JJ, see above:
"If you mean the hardly visible eyelet under the bolt to the right of the jubilee clip, that will be a factory ground, and assuming it is grounding something to do with the engine, if you disconnect it and put a meter on the disconnected end of the wire you WILL get volts from it because the ground is not grounding, so it is grounding through the meter! Also no doubt this WILL mean something to do with the engine is NOT grounding that needs to, so no doubt the engine note will change. "

 

"+" voltage to ground is how the heaters work; like a rear window defroster. It's an controlled, intentionally created short-circuit-to-ground.

I'm not sure that you should being seeing a full 12 volts, tho, as the voltage must pass thru the heating elements inside the O2 sensors. Are you using a test light or a voltmeter?

Cheers
DD

 

Doug
Isn't how everything works, lights etc? Essentially by removing the ground aren't you in effect switching off the device? In other words it is only the resistance in the device being activated that prevents a dead short in any circuit?
My money is on your 02 sensor diagnosis, if they are switched off I suppose the engine management system would do something or stop doing something, wouldn't it ?

 

Yup, that eyelet, Greg.

And Doug, I'm not sure where the O2 sensors are grounded. Perhaps this is them. I was testing with a DMM. And shouldn't the heating element be off after a short time - as controlled by ECU?

If I had more energy, maybe tomorrow, I would like to extend these wires and go to the bulkhead ground on the RH (two beautiful unused, until today) posts. Still covered with the OE caps and paint. Puuurty.

I was really just stumped,,, unbelievable that I never noticed the ground before, what they might be a part of, and if they were even "right".

Bulk head must be better than the aluminum intakes, thru head connection with intake and head gaskets in between.

 

Jay,

I admit I'm confused (although it doesn't take much!). Where was the "chassis ground" that you touched the other probe?

I'm struggling to see how that eyelet can have 12v? Doesn't that mean that the whole top end of the engine is a live 12v source? How can that be?

If you were touching the other probe to a bulkhead "ground", remember that some of the bulkhead posts are +ve posts not -ve posts.

Cheers

Paul

 

Sorry. I was tinkering with last stages of electric fan stuff when I wrote the last posts.

The eyelet is bolted to a free spot on the top rear off the RH intake. If I remove the bolt and put one probe of a DMM to the free (not touching intake anymore) eyelet and the other probe to a ground point,,, one of the ground points on the fire wall, or a bolt into the body of the car, a ground point,,, I get 12+v on the eyelet.

I didn't know that I would get a 12v reading on a wire that was serving as a ground (one of the two wires, other being positive) for an accessory... But I guess it makes sense...?

I am an idiot when it comes to electrical stuff. I just am.

 

JJJ
Read this carefully, you are confused, I feel.
If 12v is going to a device (bulb, coil, whatever) from the battery positive pole that device will not work until the return wire (ground wire) is connected to the negative pole of the battery. All a switch does is to cut this circuit, or restore it as required.
When you remove the eyelet from the manifold, you are cutting the 12v circuit because the earth wire, via the engine and chassis, is returning the circuit to the negative pole of the battery.
Therefore, and this is important to grasp, there is 12v in the circuit right through the entire circuit.
So, if you remove the eyelet from the engine, you are switching off whatever devices the wires connected to the eyelet are involved with.
If you connect a meter to the eyelet having disconnected the eyelet from the manifold, (+ on the eyelet, - onto a ground) as you described in the last post, the current flows through the meter and makes the circuit to ground and the battery negative pole.
Therefore you see 12 v on meter on the eyelet under these conditions, as all you are doing is measuring the voltage in the wrking circuit.

Next point:
The engine as a ground is perfectly good, the igniton uses it, the spark plugs use it, your alternator uses it. Just understand a 12v circuit, re-attach the eyelet and stop worrying!
Also the bulkhead is not necessarily a good earth either, and certainly not nearly as good as the engine!

 

Yup. I was a bit surprised by the voltage... Didn't expect that. I guess its one of those ideas I wrestle with when I come across a situation where I am faced with a "reading" on the ground side of a circuit. I drove a BUNCH of people crazy on the x100 forum as they tried to get me to understand (a generous word obviously) this idea with the IN and OUT wires of the fuel pump. "BOTH GIVING 12V!!! WHERE IS THE GROUND"??? lol, I went on for days.

Electricity is a very mysterious thing!

Yeah, I worry some, but it might not be as much as comes through in my posts. It's more that I have a hard time moving forward until I THINK I 'understand' - and I don't always,,, but just keep going.

I've already bolted it back down... I'll call it grounded, lol!

Thanks, Greg, Doug, Paul...
Tough cookies, y'all are. And it's good!

 

JJJ
Think of a 12v circuit rather like a circular belt (say a belt sander). The belt where the sander is applied to the wood being sanded is doing the work, but the ENTIRE belt is moving at the same speed (ie the equivalent of 12v throughout the circuit). The rest of the belt is just going round again before it engages on the wood once more. BUT if you stop the sander belt from turning by cutting the belt at any point, no work is being done (no sanding happening).
In a 12v circuit of (say) a light bulb, the bulb being illuminated is the equivalent of the part of the belt doing work and sanding, the rest of the circuit ( battery +, wire from+ to bulb, wire from bulb back to battery -) is the equivalent of the belt going round ready to start work again. If you cut this circuit at any point, it does not matter where, the bulb will go out, just like cutting the sanding belt at any point. The wire from the bulb back to the battery negative will always carry the same voltage as the wire to the bulb from the battery positive, just like the belt sander moves at the same speed regardless of its position.
When people talk about the "ground" or "earth" wire in a 12v automobile context, it is just shorthand for talking about the wire leading from the bulb back to the battery negative. The fact that this happens by sending the current back through the steel chassis is irrelevant. This is just done to make the wiring system simpler.
The circuit, if it fails does not know whether: i) you have switched it off, or ii) whether the bulb has blown, or iii) whether you have removed the earth wire, or iv) whether a fuse has blown, or v) whether you have disconnected the negative clamp from the battery!
And if it still suprises you that there is voltage on the negative wire from a device back to the battery think what would happen if you took a battery, first connected the positive pole to one end of a large piece of metal, and then secondly connected the negative to the other end!