Voltage running through the hatch frame?

[GreenWater]

I just found this by accident right now and I don't know what to make of it. Please note that I'm NOT an electrical genius. There seems to be some voltage running through the frame of the hatch. I opened the hatch and used a multimeter, one probe on the bare metal of one of the screws in the hatch frame. The other probe on a bolt on the frame of the car. I read .65 volts. So, I checked my other 944, that one gave me a reading of .30 volts. Why is this? Is there a short? The car is not on and the key is not in the ignition.

To make things even weirder, here is how I was able to get rid of the voltage to the frame. Disconnecting the wires that run to the bottom of the hatch struts did nothing. With both bottom ones disconnected I was still reading voltage. Once I disconnect the wires to the top of the hatch struts, then the voltage went away. I got the same results on both my 944s. Electrically this doesn't make sense to me.

What is going on?

[924RACR]

Very strange. The only two sources of juice there ought to be rear defrost and wiper.

The hatch struts are used to get the juice to and from the defrost; goes up on the LH, and grounds through the RH, IIRC. But I seem to remember there's some funky stuff (been a while since I looked at it) to maybe disconnect those circuits when the hatch is open? Switch contacts directly exposed and visible on the body of the struts at the end.

Rear defrost working properly? I'd also wonder if your ground on the LH side is good - is out of sight behind interior panels IIRC.

Either way... if you pull fuse to either rear defrost or wiper, maybe that helps isolate the source? I can't see why in either case you ought to have power there without ignition...

[rhett]

This kind of thing is quite normal in electrical circuits. The hatch frame is a conductor and probably not too well grounded through its hinges to the rest of the chassis. This means it is slightly isolated and able to 'float' to a higher potential than the rest of the chassis. If any voltage is present in nearby conductors it is possible for this to leak or induce voltage in any conductor not hard grounded. The voltage is small and the current really tiny, you can probably only measure it on your multimeter due to the very high impedance of its measuring circuit. Usually, this kind of thing is not a problem in DC environments, but does cause 'ground loops' in AC circuits where hum starts to be a problem. However, any potential can cause various galvanic effects to creep in and corrosion can be accelerated and fixings in dissimilar metals can cold weld together...but again, negligible in this case I would have thought.

Best way to reduce this effect (if you want to), is to make up a good ground strap or two and place them across the hinges. Have a good clean contact to metal on both sides and this should electrically clamp the hatch to the chassis, therefore not allowing any potential difference to occur.

Perhaps a good clean-up of all your grounding points will help to reduce this too by eliminating any stray floating potentials in the wiring, etc.

[GreenWater]

Rhett and 924Racr, thanks. I talked a little to Chicks about this since he know much more about electrical than I do, and that's pretty much what I figured out.

Rhett, that's a great explanation that even I can understand. The hatch frame not being grounded fully makes sense. I will check the ground wire for the defrost, and check to see how grounded the hatch actually is.

So could this be drawing power from my battery and slowly draining it?

[jcommin]

It should - It is bolted to car.

[rhett]

With regard to a slow battery discharge, then yes it is possible over a long period of time. However, this is probably not the only place where this sort of thing is going on and any losses would all be accumulative. Porsche are pretty good at providing good solid ground points and like a lot of forum members have mentioned many times - keeping them clean really has a benefit. Not only do the electrical items work more reliably, these slow battery drains are also reduced. If you haven't done it, break out the small wire brush, some emery-paper, the socket wrench and go for a good clean - shouldn't take too long!

Using the chassis of the vehicle as a conductor is a practical engineering solution, but it does have drawbacks. If all the individual circuits returned to the same point, these effects would be reduced, especially if all the metal structures were bonded to that point too.

Unfortunately, over time the hinges get a coating of corrosion on the bearing surfaces, wear creeps in and what might have been a good bond starts to fail and go high resistance, this is what causes the problem and it can happen on any metal structure - doors, hood, etc.

The switch contact you will see on the gas strut is actually the hatch area interior light switch. It is a simple contact to ground that is broken with a plastic collar when the hatch is closed. It isn't part of the screen heater circuit, that stays live even if the hatch is opened while it is on.

[djnolan]

Note the difference between the left and right hatch strut.

The left side of the hatch grid is fed by the positive 12v from the defroster switch thru the left hatch strut with a plastic ball and socket joint to isolate it from ground.

The right side of the grid goes to ground thru the right strut and the metal ball and socket joint to connect to ground.

The hatch light also goes to ground via the right strut when the hatch is open.

Can anyone explain why only 3 lines on my defroster grid work?

[GreenWater]

This is interesting and a little fun. I'm not sure about the wiring djnolan, but you are right. The strut on the left isn't grounded (plastic bottom end) and the strut on the right is grounded to the car (metal end). So that should mean that the hatch frame is grounded to the body by both the hinges and the right strut. So I used my multimeter to see if the hatch frame is actually grounding to the car body. Here is what I found. Testing resistance on the left side of the hatch frame to the body of the car I get a reading of 100% resistance (not connected/not grounded at all). If I check the right side of the hatch frame to the body of the car I get a resistance reading of 900 to 1100. So if I'm correct, basically my hatch frame isn't being grounded at all.

This is something that I will need to fix obviously. Somehow.

[rhett]

Originally Posted by djnolan

Can anyone explain why only 3 lines on my defroster grid work?

This is probably due to damage of the conductor elements. A tiny crack or tear will cause an open circuit, then that entire wire element will not work. I seem to recall that the elements are on the surface of the glass and so can easily be damaged. There have been a few repair kits around that use something like solder paste to repair the wires, but I have no direct experience of those. Perhaps someone else has?

[tamathumper]

I suppose if someone used a generous amount of conductive grease or a copper-containing Neva-Seize product in the ball joints on the struts, it could have leached out enough to be carrying current where it shouldn't.

[rhett]

Originally Posted by GreenWater

This is interesting and a little fun. I'm not sure about the wiring djnolan, but you are right. The strut on the left isn't grounded (plastic bottom end) and the strut on the right is grounded to the car (metal end). So that should mean that the hatch frame is grounded to the body by both the hinges and the right strut. So I used my multimeter to see if the hatch frame is actually grounding to the car body. Here is what I found. Testing resistance on the left side of the hatch frame to the body of the car I get a reading of 100% resistance (not connected/not grounded at all). If I check the right side of the hatch frame to the body of the car I get a resistance reading of 900 to 1100. So if I'm correct, basically my hatch frame isn't being grounded at all.

This is something that I will need to fix obviously. Somehow.

I am a little surprised at the readings you are getting as the hatch frame is one continuous metal part. Also, the frame is coated in some form of anodising and they can be very high resistance. In order to get a good reading you really need to dig through the surface coating into the metal below, then you should get an equal reading basically wherever you take it on the frame. Not sure what you meant by 100% resistance, so I can only imagine the MM was flashing at you or showing ---- to indicate an open circuit?

The hatch frame is not actually designed to be grounded apart from through the hinges. However, the heater element does go through the strut to the metal ball and socket at the end and then to ground. The other end of the strut is actually isolated as the housing is plastic.

There have been a few threads on hatch struts in the past: Here is one with some excellent photos by jcommin that show the strut ends, etc:
Rear cargo/interior light

To be honest, I think the original observation is a bit of a non-issue and apart from ensuring that all the contacts for the heater grid and wiper are good, and giving the strut metal ball a good clean - I would not be too bothered about it.

[djnolan]

Originally Posted by rhett

This is probably due to damage of the conductor elements. A tiny crack or tear will cause an open circuit, then that entire wire element will not work. I seem to recall that the elements are on the surface of the glass and so can easily be damaged. There have been a few repair kits around that use something like solder paste to repair the wires, but I have no direct experience of those. Perhaps someone else has?

Well it is a used replacement hatch I sourced due to a separated frame for $25. The seal is good on it but it had window tint film on it. Maybe when I removed the window tint I damaged the conductors. The funny thing is I can't detect any breaks or damage to the conductors visually.

[924RACR]

Thanks for the clarification on the light contacts, Rhett!

Re: testing/fixing the defrost... my understanding (based on a Chrysler service manual) is that the surface of the grid lines is conductive; therefore you should be able to use an ohmmeter directly on the lines to find the break. Definitely probably that the window tint removal hurt them.

You should be able to buy a repair kit at any parts store (or possibly also Radio Shack) which has a conductive strip you can glue on to recomplete the circuit.

Hmmm, I recall I have a couple of dead lines on mine, maybe I should get off my butt and fix them... Oh, once I've got that half-foot of snow off the car... LOL

[djnolan]

Now I recall I also had to repair the left side electrical connector on the hatch as it had broken off. I used silver solder to resolder it and I did it very quickly so as to not risk cracking the glass.

I just noticed the grid wires that are working are all on the lower side. Also the new terminal was not soldered very well on the upper side. So I added a dab more of silver solder. Maybe this will allow more current to flow and get more of the grid working.

Plenty of cold weather coming, so we shall see...

[Gordon Rankin]

Interesting discussion! With my ignition off and the key out.. I can open my back hatch and the light to the left of me will come on as well as the dome light up front! That would indicate to me (I am ONLY an innocent bystander here and not an electrics person) that there would be electricity running in the frame.. Anyway, makes sense to me! : )