Positive ground dwell

[BOBBYR]

Hi Guys
I was wondering if anyone has run into the same problem I am having. The points in the distributor are easy to set with a feeler gauge, but I would like to set them with a dwell meter because I feel this is more accurate. Being a positive ground car does not allow the common dwell meter to read dwell. Is there a way to hook up a meter without shorting out the points? As always I hope you all are enjoying your Healeys
Bobby R

 

[Randy Forbes]

This might be overly simplified, so please correct me if I'm wrong.

For the sake of setting the dwell, couldn't you just reverse the polarity on the battery, then switch it back when you're finished?

This is assuming you don't have any dedicated, positive-ground accessories fitted to your car, like transistorized ignition, radio, etc.

For the test period, the generator isn't going to care, and if your car has a mechanical (cable-driven) tachometer, you're not in any trouble there.

My own car has been negative-ground for so long (20+ years) that I've forgotten if there're any other ramifications. But rest assured, if I'm wrong, twenty (20) people that've never posted before will come out and correct me!

 

[IanIrving]

Fuel Pump?

Ian

 

[Bob_Spidell]

Hi Guys
I was wondering if anyone has run into the same problem I am having. The points in the distributor are easy to set with a feeler gauge, but I would like to set them with a dwell meter because I feel this is more accurate. Being a positive ground car does not allow the common dwell meter to read dwell. Is there a way to hook up a meter without shorting out the points? As always I hope you all are enjoying your Healeys
Bobby R

I have a dwell/rpm/voltage meter from Sears that reads the dwell on our two positive-ground cars. Connect the positive--usually they're red, mine is green--lead to the chassis, and connect the other--should be black--to one of the terminals on the coil (don't remember which--probably 'CB'--just try them both) with the engine running. My timing light works similarly.

There is no 'ground' on a car--the term is technically incorrect and misleading; the car is isolated from 'ground' by the tires (it is correct when referring to house current that is actually 'grounded' into the Earth). There is 'common;' i.e. the chassis. Electricity flows in a circuit, from the battery, through various components and back to the battery through the chassis; negative common cars connect the negative terminal (cathode) on the battery to the chassis, positive common cars connect the positive terminal (anode). That's the only difference except, of course, all the components have to be connected accordingly--if any have diodes to protect from reverse-bias connections the component won't work or if they contain electrolytic capacitors they can be destroyed if connected improperly; e.g. some of our fuel pumps (and almost any modern, digital device). To convince yourself use a voltmeter: connect the negative lead to your positive common chassis and the positive lead to a power source (usually a solid brown wire on a Big Healey). You will read a little over -12V. Then, switch the leads--you will read a little over +12V. Also, have you ever had to buy a special, 'positive ground' battery--maybe I'm missing a business opportunity here--or do you just go to your favorite parts house, buy a battery, and connect the positive terminal to the chassis instead of the negative?

Any 'negative ground' device can be connected to work in a 'positive ground' car. I installed a power connector for a friend's GPS; just wired it 'backward' from how you'd wire it in a negative common car. You just have to be careful to keep it isolated, since a positive common chassis is essentially a 'hot lead' for a negative common device.

WARNING: Some devices--possibly your dwell-tach meter--have metal cases that are internally connected to common (usually negative)--if they touch your positive common chassis you will get a spark. Also, I'm assuming your dwell-tach is the analog variety--i.e. it has a moving pointer--and not a digital one. Although a digital one should work fine, I don't have personal experience with one.

 

[BOBBYR]

Hi Guys,
I have tried hooking into this in reverse and the meter still does not work in dwell mode. It is a snap on and the tach side works well as long as I put the pos lead to the block and the neg lead to the hot side of the start solenoid terminal. Oh well, I guess I'll have to stick to the feeler gauge for now.Thanks for the response.
Bobby R

 

[Ed_K]

Hi Bobby,
If the tach function of your meter is working when attached to your car, but the dwell is not working, the dwell meter is broken. Your points are at least " in the ball park " or your car would not run at all.
There is nothing wrong with setting the points with a feeler gauge if done correctly.
Ed

 

[Deleted member 8987]

Actually, on a new, tight distributor, setting gap with a feeler is okay.

However, once that distributor (shaft, bearings, breaker plate) is a little more "broke in", a dwell meter is really essential.

I have several dwell meters, the cheapies are two-wire, connect red to positive on the battery, black to dist, or, of + grounded, black to "hot" (negative) lead of battery and red to dist.

The other is an ancient Snap-On, if I need to use it on positive ground, or six volts, it's a crXp shoot.

 

[Keoke]

Yes TOC, I have one of the pre digital Snap On timing lights and I have no problem using it on positive ground systems. If he has a digital Snap On it will not work on the dwell setting on a positive ground car.---Fwiw-Keoke

 

[BOBBYR]

Hey Guys,
I think I know where the problem comes in. In order to get the tach side of this meter to work, I have to put the red lead to the engine block. Then I connect the black to the hot side of the starter solenoid. To complete the hookup, I place the inductive pickup to the number 1 spark plug wire. That leaves one wire left which should be connected to the negative side of the coil.The negative side of the coil also connects to the points. Now I'm wondering if because this ignition system is positive ground also, do I have the coil wiring reversed? The car runs really great but I wonder if this could be the problem? As soon as I turn the knob to the dwell position, it's seems to short out the points.I'll try to get out my wiring diagram today and see if the coil wiring is correct. Well boys, you've managed to get me to engage my brain again, so it's off to the books to see what trouble I can into. Thanks for your help.
Bobby R

 

[Bob_Spidell]

Hey Guys,
... The negative side of the coil also connects to the points. Now I'm wondering if because this ignition system is positive ground also, do I have the coil wiring reversed? The car runs really great but I wonder if this could be the problem? ...

Yes. You can think of negative as 'power' (or 'source') and positive as 'ground.' Points switch to ground; you have a positive ground car so the '+' terminal should be connected to the distributor base and through the points (some coils are marked 'SW' and 'CB' for 'switch' and 'contact breaker; i.e. points).

Coil primary and secondary windings are wired in series, so if connected properly you'll get a somewhat more powerful spark; i.e. the voltage produced by the primary winding is 'added' to the voltage produced by by the secondary winding (but the car will run either way).

 

[Johnny]

Hello Bob, I've always wondered if a GPS will work on a Pos grounded (Common?) car, but never had the courage to test it.
Can you enlighten us???

 

[Keoke]

BOb said:


Most "'negative ground' devices can be connected to work in a 'positive ground' car. I installed a power connector for a friend's GPS; just wired it 'backward' from how you'd wire it in a negative common car. You just have to be careful to keep it isolated, since a positive common chassis is essentially a 'hot lead' for a negative common device".----Keoke----

 

[Bob_Spidell]

Hello Bob, I've always wondered if a GPS will work on a Pos grounded (Common?) car, but never had the courage to test it.
Can you enlighten us???

I'll get some photos and post a description in the next day or two.

 

[Johnny]

BOb said:


Most "'negative ground' devices can be connected to work in a 'positive ground' car. I installed a power connector for a friend's GPS; just wired it 'backward' from how you'd wire it in a negative common car. You just have to be careful to keep it isolated, since a positive common chassis is essentially a 'hot lead' for a negative common device".----Keoke----

well, I thought I understood a little about Electron flow but now I'm really confused. If an electronic device is built for + HOT and - COMMON why is it possible to operate just fine with the opposite polarity in a mixed use environment? Meaning, the rest of the car is operating the opposite of the GPS.

 

[Randy Forbes]

Johnny, the device has been isolated from chassis ground (the "-" common) and the wiring was revered, presumably, at something like a cigarette lighter plug (that goes into the socket).

 

[Bob_Spidell]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]
well, I thought I understood a little about Electron flow but now I'm really confused. If an electronic device is built for + HOT and - COMMON why is it possible to operate just fine with the opposite polarity in a mixed use environment? Meaning, the rest of the car is operating the opposite of the GPS. [/QUOTE]

Electronic devices are designed to be part of a circuit. A DC-powered device requires one positive and one negative terminal from a power source. For a positive-ground Healey, the chassis is the positive terminal and a power lead is the negative terminal (just the opposite for most modern cars). Like Randy said, the device has to be isolated from the chassis (if it's got a plastic case like most modern gadgets it's already isolated).

Got my camera battery charged up--I'll post some pics of my rig tonight.

 

[Bob_Spidell]

Here's my installation:

I started with a socket similar to this (bought from Amazon, but I can't find the exact unit now):

https://www.amazon.com/Parts-Express-Ciga...9775&sr=1-1


I mounted it with an Adel clamp and an inline 5a fuse:

For this to work with a modern 'negative ground' device, the inner sleeve needs to be wired to a negative terminal, for my positive-ground car that's a power lead. So, I soldered the terminal for the barrel sleeve to a wire which will go to a power connector on the fuse block, and the terminal for the center terminal goes to a positive source; i.e. the chassis.

Here's the wiring on the firewall:

Notice the two blue crimp connectors: the one on the fuse block goes to the barrel shield on the power adapter (that's negative, 'ground' for a modern device). The one on the O/D relay screw goes to the center terminal on the power adapter (that's positive--'hot' for a modern device). My radio power lead is switched, and I wanted to be able to use the GPS without turning on the switch so I needed a 'full time' source of power. To do this properly, you should have a voltmeter or at least a continuity checker.

Oh, the custom dust cover? That's a cap off a can of aerosol brake cleaner--turns out the OD of the power adapter and the ID of the cuff on the brake cleaner can are both one inch, exactly!

 

[Johnny]

Thanks Bob, glad you got your camera charged. If I understand correctly (sorry to ask so many questions but I want to get it right) the barrel of the power outlet would be HOT and the center terminal COMMON?

 

[Bob_Spidell]

Thanks Bob, glad you got your camera charged. If I understand correctly (sorry to ask so many questions but I want to get it right) the barrel of the power outlet would be HOT and the center terminal COMMON?

For a positive-ground car, yes.

Keep in mind you don't have to buy a 'positive-ground' battery for a positive-ground car--'common' is the correct term, but I use 'ground' because it's the 'common' term--you just connect a different terminal to the chassis. The car is a circuit--whether it's positive- or negative-ground--you just need to connect a device to the correct terminal. For the typical electronic device (some devices are 'polarity insensitve') the negative terminal is 'ground,' so you'd connect that to a negative terminal; i.e. the 'hot' lead on a positive-ground car. For the device positive is 'hot,' so you connect it to the chassis on a positive ground car.

If you don't have a voltmeter, buy one. They're less than $10 for a cheap one at HF or Radio Shack. Use the voltmeter to convince yourself: if you touch the chassis with the positive (red) lead and a power lead with the negative (black) lead you will see approx +12V (assuming your cutoff switch is on). Reverse the leads and you'll see -12V. The device doesn't care which is 'hot' and which is 'ground,' only that it's 'seeing' the correct polarity on the correct terminal/connection.

As noted, because the device and your car have different nominal polarity, you need to keep the adapter isolated (note I used a plastic adapter--a metal one will NOT suffice). If you somehow connected the inside barrel of the adapter to your chassis you will have created a fusible link; i.e. a short circuit. Most modern gadgets are encased in plastic so it's not a problem, but if there's any exposed metal on the device it may be 'hot' to the car, and sparks may ensue.

 

[tahoe healey]

I bought a Fox Valley tach/dwell/ohm meter a few years ago at a flee market for 3 bucks. It has a knob to go from + to - and it works very well. This is an old one from the 50 with a bake-lite case. I wanted one to carry in the car. I usually use a newer one around the garage also made by Fox Valley but it does not have a + to - switch. The old one is accurate as the new one. I run neg ground so its now a problem for me, but I use it at shows for guys with troubles. These things are out there and on E-bay. They are pretty cheap as not a lot of people work on their own cars any more. So, go with the wife to those garage sales.
TH