Repetitive application of my head to brickwork.

Technical advice Q&A

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rswaffie
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#1 Repetitive application of my head to brickwork.

Post by rswaffie » Tue Feb 05, 2019 12:50 pm

The electrical element of my restoration has been by far the most frustrating in terms of time-consuming issues.
The most recent of which involved the failure of the power supply boards in both the 123 dizzy and the Bluetooth receiver/amp. So before putting the replacements back in, I thought I’d check on the respective earth connections to make sure they were sound. Switching on the ignition to check on the voltage at the input wires for the dizzy, after about 10 seconds I noticed a puff of smoke drifting up from the air filter area of the engine bay. Switched of ignition, disconnected battery and investigated.
I have an ITG filter and the painted trumpet from the filter housing to the rear carb was touching the brake pipe from the rear system master cylinder. The pipe was red hot and burning the paint on the trumpet, causing the smoke.
How the heck would the brake pipe be getting that hot unless there was a current being passed through it and how could this be possible? I have nothing earthed to any of the fasteners related to the master cylinder or bellows assembly. The only electrical component that is attached is the brake light switch at the union on the picture frame.
Any ideas ?
TIA
Richard
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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david muir
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#2 Re: Repetitive application of my head to brickwork.

Post by david muir » Tue Feb 05, 2019 1:34 pm

check main earth twixt engine, battery and body.
1964 3.8 FHC

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christopher storey
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#3 Re: Repetitive application of my head to brickwork.

Post by christopher storey » Tue Feb 05, 2019 2:58 pm

Richard : this is a phenomenon known as sneak currents, and it is a real fire hazard. As has already been said, check both your body and your engine earths, because what it means is that when you operate electrical apparatus, the return current is passing not through the very low resistance large diameter cables / braided earthing strap as it should do, but through whatever small diameter ( = high resistance ) cables it can find which are providing a satisfactory earth. The throttle and even more so the choke cables are prime candidates for this, and with a heavy current will almost immediately become red hot, which sounds to be what has happened in your case, although in this case it is the brake pipe which has been the return feed - if this is copper it will be lower resistance than steel and this has probably saved you from a fire

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rswaffie
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#4 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Tue Feb 05, 2019 3:44 pm

Thanks for the insight chaps.
With regard to earth straps, I have one from the +ve battery post to the firewall - via a bolt in the toeboard panel. This appears to be a good connection (I removed the paint around the hole and smeared Vaseline around the connection after the cable was connected).
I don’t recall fitting any other straps going to the engine block though, although I had the engine refitted by a garage. Is there a standard connection point for this on a 3.8? I’ll have a root around if I know where to look.
Cheers
Richard
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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ETTony
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#5 Re: Repetitive application of my head to brickwork.

Post by ETTony » Tue Feb 05, 2019 3:58 pm

A couple other Eathstraps to check, front left of engine to engine frame, rear left engine frame to body (at reaction plate). sounds scary :wow:
Tony in Devon
1967 Series 1, 4.2, OTS, RHD, Black.

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JerryL770
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#6 Re: Repetitive application of my head to brickwork.

Post by JerryL770 » Wed Feb 06, 2019 6:10 am

Richard,
Unless you were turning over the engine at the time, there should be no reason for sufficient current to heat a brake pipe unless something is short circuiting.

As the engine was run satisfactorily before (and possibly after) you got the car home, it would seem something is not installed correctly, causing a short circuit as soon as you turn on the ignition. Best check your installations since getting your car back.
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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rswaffie
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#7 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Wed Feb 06, 2019 7:10 am

Hi Jerry,
I’ve looked back at what has changed since then, and there are a couple of possible culprits. One being the ignition relay for the powerlite dynator, the other being the Bluetooth amp. There’s also the engine earth straps that I need to check on as I don’t remember seeing any after it came back.
Cheers
Richard
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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christopher storey
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#8 Re: Repetitive application of my head to brickwork.

Post by christopher storey » Wed Feb 06, 2019 11:08 am

There should be an earth strap from a bellhousing bolt to the adjacent body, usually close to the reaction plate. If there is nothing securing the engine etc to the body, then this explains why current is seeking the alternate pathway. The usual straps are braided copper/nickel, but starter cable with suitable ring ends is just as good

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malcolm
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#9 Re: Repetitive application of my head to brickwork.

Post by malcolm » Wed Feb 06, 2019 2:19 pm

Silly question, but I assume you're leaving the battery disconnected until you've sorted it out? Waking up to a fire in the garage would be a bit heart rending.
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
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#10 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Wed Feb 06, 2019 6:44 pm

Absolutely !
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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Series1 Stu
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#11 Re: Repetitive application of my head to brickwork.

Post by Series1 Stu » Wed Feb 06, 2019 9:51 pm

There should be a braided earth strap from the bellhousing to the reaction plate mount on the left hand side. These are quite fragile, especially after 50+ years and you may find that quite a few strands are broken. This can seriously increase the resistance in the earth path between engine and chassis.

I don't recall any other earth straps between engine and frame but that may have changed over the years.

I suspect the brake pipe was being heated by the current flow from the engine (air filter) to frame, rather than the other way around. Check that your brake fluid hasn't vaporised giving you gases in the system. You may need to bleed the brakes.

Regards
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
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#12 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Wed Feb 06, 2019 10:17 pm

Cheers All,
From what you are all saying, I strongly suspect I am missing the engine earth strap. I can’t take a look until the weekend but from memory (not infallible) and pics I took when it came back, there is nothing in or around the near side bell housing/reaction plate area that resembles an earth strap. If I order one now, it should arrive by Saturday.

Thanks
Richard
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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#13 Re: Repetitive application of my head to brickwork.

Post by 1954Etype » Wed Feb 06, 2019 11:49 pm

On some of the later S2 cars, they fitted a woven earth strap from the frame to the fuel filter on the rh bulkhead
Angus 65 OTS 1E11497. 67 FHC 1E33656

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JerryL770
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#14 Re: Repetitive application of my head to brickwork.

Post by JerryL770 » Thu Feb 07, 2019 7:21 am

Everyone seems to be missing an important point.

If you just turn on the ignition, what is going to draw sufficient current to heat to very hot, a brake pipe possibly capable of carrying 100+ amps?!!

Something is short circuiting.
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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#15 Re: Repetitive application of my head to brickwork.

Post by christopher storey » Thu Feb 07, 2019 10:13 am

It would be a very interesting brake pipe indeed that would carry 100 amps without heating up ! In any event, only 2 or 3 amps is needed to get a thin wire such as a choke cable very hot indeed, and bearing in mind that the heat produced varies as the square of the current , if there is sufficient resistance e.g. in a small cross-section conductor , the temperature rises at an alarming rate even if the current is merely doubled, and a rise from say 1 amp to 4 amps produces 16 times the heat ( and your 100 amp example produces 10,000 times the quantity of heat )

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#16 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Thu Feb 07, 2019 12:10 pm

Now you are really scaring me😳

It sounds like I could plug the car into the national grid and get a massive rebate on my heating bill!
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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#17 Re: Repetitive application of my head to brickwork.

Post by christopher storey » Thu Feb 07, 2019 3:56 pm

Richard : no, don't get scared. In your situation, and bearing in mind what Jerry has said about a short, I would make the following tests ( quite apart from checking the earthing strap position ) :

1.With battery disconnected at earth lead , make sure that every possible switch is OFF . If you have a hand held ammeter ( or a multimeter with an amps function ) connect one end to the battery earth terminal , and then the other end to the disconnected battery to body lead. NO current should flow. If any does other than the most minuscule amount from e.g. the clock, then you have a short somewhere .

2. If you do not have an ammeter/multimeter, a rather less sensitive test is to watch carefully as you connect the earth lead to the battery . There should be NO spark/arc as the connection is made. If there is any arcing, then again you have a short.

3. If either of these tests reveal the possibility of a short, disconnect immediately, and then remove every fuse . This should stop the arcing during the test. If it does not, then the short is in an unfused circuit

4. Assuming that all unfused circuits seem OK, connect the fuses one by one and repeat the test. When you reach a fuse which creates an arc, that is your shorted circuit

Incidentally, I have always used my low amp battery charger with cutout for circuit testing with the battery completely disconnected. Although this provoked outrage from a Professor of Electronics, because the charger produces mixed AC/DC rather than a pure DC signal, the cutout prevents the possibility of fire if there is a short

Apologies if all this is teaching my grandmother etc, but I hope it will help you. Incidentally, I can recommend unhesitatingly the Coventry Auto Components circuit diagrams, which split the circuits up according to function and thus make fault tracing much easier

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rswaffie
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#18 Re: Repetitive application of my head to brickwork.

Post by rswaffie » Thu Feb 07, 2019 5:55 pm

Brilliant, thanks Christopher. You are not teaching me to suck eggs - this is just the testing info I needed. I do have a good multimeter and 2 copies of the Coventry books! I used them when I installed all the new looms and standard ancillaries.
Richard

S1 3.8 FHC Opalescent Golden Sand 889504
Undergoing a comprehensive restoration in my small, but utilitarian garage :wrench: :hammer: :fingerscrossed:

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JerryL770
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#19 Re: Repetitive application of my head to brickwork.

Post by JerryL770 » Thu Feb 07, 2019 7:32 pm

Excellent advice Christopher.

One suggestion I could make would be to substitute a headlight or 21W bulb for the ammeter as this will be easily visible and would limit any current which may otherwise blow the multimeter. Solder some decent size wires on and fit appropriate croc clips.
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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#20 Re: Repetitive application of my head to brickwork.

Post by BRM » Thu Feb 07, 2019 11:19 pm

Seems to me you might be looking for faults that aren't there.

I have no idea what goes on inside a 123 distributor but with a standard system, if the points are closed, as soon as you turn on the ignition you will get a current flowing from the battery through the coil and back to earth via the engine block. If the earth strap is missing and the only route it can find to earth is through a tiny touching contact with a brake pipe you will get smoke and heat !

I suggest you get that earth strap fitted, if it is missing, before you give yourself any more work. You just might find the problem disappears.
Brian

1969 S2 FHC 1R20267
1960 Austin Healey 3000

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