Chassis Ground Fail

[PaulTWinterton]

25 minutes ago, Simeon said:

has a hole with one of the large self tappers (similar to the radiator bolts) through one of its rear edges.

 

Hmmm...I'll have to go look.   I don't recall ever seeing that.

[Chris_B]

The fix? After installing this cable in a convenient hole in the left inner front fender (not sure what its original purpose was), using a stout bolt and cleaning the paint from an area under the washer and a star washer under that and the whole thing torqued down to the hairy edge, I got a good clean system voltage reading between the ground point I have been using for the 123 dizzy (upper screw holding coil bracket in place) and the battery. This was about +13.5V (new alternator; seems a bit low to me).  I also cleaned up the area under the upper bolt in the coil bracket.

 

Now, with the headlights on the system maintains ~+13.5V as measured at the battery or the alternator +B terminal, and the ground point (coil bracket). The 123 dashboard supports this reading; however, when I turn the blower motor up to the highest setting, the 123 voltmeter shows the voltage dropping to ~12.5V, which is not reflected in the external voltmeter reading (between battery and coil bracket).

 

Okay, so my first thought was that the 123 dashboard voltmeter was not accurate. But, just on a hunch I measured the voltage between the + input to the 123 (the green wire from the ignition switch) and the ground point. +12.5V- WTF?

 

So, now I have concluded that (1) the 123 dashboard is pretty accurate, and (2) I am losing power between the battery and the green wire at the dizzy. This could be caused by a number of things, all relating to excessive resistance in the circuit somewhere (ignition switch is prime suspect). Rather than try and troubleshoot this I am going to install a relay connected directly to the battery and using the "green wire" as the switch for the relay. Any reason not to do this?

[Healey3000]

Hi,

 

Does the 123 draw much current (is that green wire feeding the ignition coil as well through the 123)?  If it is, then your hunch about the ignition switch contact may be correct.

 

However, what does the blower have to do with it?  Is there a shared power feed?  The blower draws a lot more than the ignition coil, averaged.  It is possible that the blower, when turned on, is generating all kinds of electrical junk (highly technical term) and this noise is causing the 123 to read something other than 13.5 volts.  Do you see a similar voltage drop when you turn on your headlights (which on high beams represent a similar load)?  Do you have a 'scope you can verify with?

[Chris_B]

I have no clue how much current the 123 dizzy draws, but my guess is not much. What the blower has to do with it is adding load to the system. This isn't about noise, given that my voltmeter sees the voltage drop at the dizzy and substantiates the reading on the 123 dizzy dashboard.. I think it is about the accumulation of load on the system. When I turn the headlights on, not that much happens at the dizzy but when I add the blower motor to the mix (the best source of load besides the headlights), the reading on the dizzy dashboard sinks to ~+12.5V, and this is substantiated by my voltmeter when testing at the "green wire."

 

I will do more testing (e.g., turn the blower motor on first and then the headlights). However,  think this may be a moot point if I bypass the ignition switch as the source for power with a relay directly to the battery.

[Healey3000]

31 minutes ago, Chris_B said:

 

Hi,

 

If the green wire that feeds the 123 is not also providing current to something else, then turning on another load ought not to cause a drop at the 123.

 

While the 123 and a DVM show the same drop, they may both be subject to the same root cause.  As averaging meters, they will both be similarly affected.  An interesting problem!

[Chris_B]

I don't understand what you are saying. The electrical system in the 02 is coherent, in that any load on the system will affect the entire system. For example, if you turn on the headlamps and measure the voltage at the farthest reaches of the system (e.g., at the taillights), there will be a commensurate drop in voltage. The "green" wire only feeds the ignition circuit, but it is part of the system.

[Healey3000]

Hi,

 

In a distributed power system, where multiples wires come together to a single node, such as the ignition switch (or battery), each branch may flow a different amount of current.  This current will result in some amount of I*R drop in voltage.  Take the extreme example of a really skinny wire feeding a load that draws a very high current.  The resulting voltage at the load will be noticeably lower.  On the other hand, a beefy wire would not cause as much of a drop.

 

It therefore depends on where in the circuit you are measuring the voltage.  Of course, if you are measuring it at the B+ terminal, then you are correct - the entire system is loaded down uniformly.  However, I think you are measuring it at the load point from your description and if that is so, the voltage there can be lower than at the B+ terminal.

 

The other issue that I was referring to is that measuring devices can be affected by the quality of the signal itself.  If the voltages are steady, clean DC, then a multimeter can be trusted.  If the signal has AC noise, the meter can be fooled.  A blower is a really inductive load and can spew lots of crud into the local circuit.  A headlight is a nice, clean resistive load and if the behavior is different than with the blower, it might be  clue.

 

Do keep us posted on what you find.

[Harvanj]

I am also looking to improve the battery to chassis ground by installing a nut and bolt for a more secure connecting point.  Is using a stainless nut bolts and washer the best, is stainless a good conductor for this application?

 

o =OO= o

['76mintgrün'02]

The bolt is really just a clamp, making contact between the body and the end of the ground strap.  Stainless should be fine, I'd think.

 

Stainless is a poor conductor though, of both heat and electricity.  That makes it fun to weld, since the puddle appears right away.  As for the electrical physics involved, there's this:

 

"The chromium atoms disrupt the regular iron lattice and increase the chances of inelastic collisions with moving electrons. "

 

https://physics.stackexchange.com/questions/92960/why-is-stainless-steel-a-poor-conductor-of-electricity

[jimk]

2 hours ago, '76mintgrün'02 said:

Stainless is a poor conductor though, of both heat and electricity.

Stainless steel is used in heavy duty commercial terminals so I wouldn't say it is poor, maybe not good for cable.

['76mintgrün'02]

3 minutes ago, jimk said:

I wouldn't say it is poor

 

I'm not really trying to argue, but here's a clip from the link above

 

You are correct, stainless steel is a really poor conductor compared to most metals. This source lists it as 7.496×10−7Ω⋅m which is more than 40 times worse than copper. 

 

At least I did not say really poor  

 

It is the quickest metal I've welded, when it comes to puddling up.  

 

It makes a much stronger cable than copper or aluminum, that's for sure.

[Chris_B]

21 hours ago, Healey3000 said:

Do keep us posted on what you find.

 

It turns out that all of my electrical system problem(s) were related to poor chassis ground. The voltage drop (about -1V) at the dizzy is probably not that unusual, considering the resistance from insertion and contact loss in the circuit between the battery and the dizzy. This didn't seem to affect my factory Bosch dizzy. The 123 specs say "voltage:4,0-15,0 Volts. I would say, not. The dizzy craps out when voltage drop below about ~12V. With the poor ground, my system voltage at the dizzy was falling below +12V with any major load on the system.

 

After I fixed the chassis ground, the voltage at the dizzy was much more stable, but still dropped to +12.5V when I loaded up the system. Today, I installed a relay drawing power directly from the battery, using the "green" wire from the ignition switch as the trigger for the relay. As I had hoped, this provides ~+13.5% at the dizzy regardless of load on the system, so I am a happy camper.

[Healey3000]

Excellent!!

[NYTransplant]

You may want to consider the galvanic reaction between stainless and carbon steel.  You could end up causing corrosion that would not happen if you use plain carbon steel hardware.  If you add anti-seize compound to the threads of a carbon steel nut & bolt, it might be your best bet.

[jp5Touring]

Just redid my engine to chassis main negative ground. I have a trunk mount  battery.

Fired it up to take it for a test drive, the engine idled better when cold and my gas gauge went from less than a quarter tank to a half with a very solid needle.   Could be wishful thinking but felt like it ran better.I can't believe how well everything worked with all my average at best grounds.  I need to go through and check every ground. Thanks to this thread. ?

This was the old ground mounting point.