1976 Triumph Spitfire electrical no start

I'm trying to sort out the electrical system of this car which has a 12v negative ground electrical system. The fuse box holds 3 glass tube fuses. With fuses removed and the 12v battery disconnected, 5 of the 6 fuse end clips have continuity to ground. The car has no computer and I'm thinking these fuse terminals should not be grounded. Am I correct and what's the best way to trace these faults? Also, the ignition switch has 5 wires leading to it and these also have continuity to ground, regardless of key position. Any help is appreciated.

This is the diagram for the 1975 version, yours should be the same or at least similar:

It appears that, depending on combinations of door switches, ignition points, light switches etc., just about any connection could have continuity to chassis.

I'd use a spare 12V panel light bulb or similar connected from the battery positive as "ground detector", not a multimeter; the normal resistances of many items will be quite low.

When you find a ground that lights the bulb, trace it on the diagram and try temporarily disconnecting whatever actual load items there may be on that circuit - remove bulbs etc.

What is the overall fault you are trying to diagnose? Not turning over, no spark??

The primary issue is no start due to ignition failure. I've sprayed starting fluid directly into the intake manifold and have verified good compression. The car did not start when i acquired it and I put in a new ignition coil which the prior owner had bought. I have not been able to see a spark in a plug or with a spark tester. My assumption is that a short to ground may be reducing the battery (12.5v charged) impulse below what is needed to produce a spark.

I did order a diagnostic tool that detects voltage which will arrive tomorrow. Also, since the car is 44 years old, I've found some weathered wire insulation and some worn insulation where copper wiring was visible.

The starter motor solenoid is also a coil connected to ground, so without 12v supply will also be negative potential..

Most older cars have the "non reliable rotor arm" Here is where I would start... A tiny spring and a small piece of carbon... Then to the "points" unless this is fitted with an electronic ignition ( doubtful ).. If the points are closed it'll never start.. Older cars are extremely easy to diagnose and fix...
Points gap should be about 1mm...

Ian Rogers said:

The starter motor solenoid is also a coil connected to ground, so without 12v supply will also be negative potential..

Most older cars have the "non reliable rotor arm" Here is where I would start... A tiny spring and a small piece of carbon... Then to the "points" unless this is fitted with an electronic ignition ( doubtful ).. If the points are closed it'll never start.. Older cars are extremely easy to diagnose and fix...
Points gap should be about 1mm...

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Wasn't it usually 15 thou?, or something like that?, and were the plugs 25 thou? - I can't remember the last time I ever adjusted a set of points?.

I did check the points and they are .015 which is what YouTube says is correct for the TR4 which is probably close enough. I did some wiring repairs and replacements where I could see insulation defects. Not sure which issue it was but it fired up this morning. Sometimes it bothers when I fix something but can't say exactly what the problem was.

Yes! Nigel is correct... 0.4mm (15thou)...

If its a wet damp morning and the plug leads are worn out, ie insulation is poor.. Then a drenching of WD40 will get you going... It seems that the plug leads are tired....

Ian Rogers said:

Yes! Nigel is correct... 0.4mm (15thou)...

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No idea why it's stuck in my mind - other than an embarrassing incident 40+ years ago - continued below!.

If its a wet damp morning and the plug leads are worn out, ie insulation is poor.. Then a drenching of WD40 will get you going... It seems that the plug leads are tired....

Click to expand...

Damp and wet - I was out in the car with my girl friend, and we came to a ford (there aren't many round here, but I'd been through it many times - including on push bikes). The water looked fairly high, and there was a car sat waiting - too scared to go in - so I went past him and cautiously entered the water. We got half way across and the engine stopped dead - and wouldn't attempt to restart.

'Luckily' we'd been out walking, and had wellington boots with us - 'unluckily' we'd placed the boots in the boot (where else would you put boots?). Anyway, I'd just got out and was wading through the water when a Subaru pickup comes flying down the hill, he spun round, reversed towards me, and threw me a tow rope - I tied it to the car and he pulled me out. He then proceeded to drive through the ford at about 40mph, causing a massive wave - no one likes a show off!

I opened the bonnet, and tried to dry the electrics as well as I could, but it didn't look wet at all, and it made no difference - now if you remember the start of this story, the guy too scared to go in? - he came across with a can of duck oil (like WD40 but in a pump spray can), we gave it a couple of sprays and it burst in to life - missing a bit at first, but soon settled down to run normally.

Needless to say, I chickened out, and didn't attempt the ford again - so turned round and went back the long way - driving with no shoes, socks, or trousers. We got back to my parents house, walked in (minus trousers) and sat down on the sofa watching TV - it was ten minutes before anyone noticed I wasn't wearing any trousers!.

The reason for this rambling story, and perhaps why I remember the 15 thou, is that the problem that caused my engine to stop was the points - the gap was too small, and the the spark too weak, so a little dampness stopped it working at all.

For the OP's problems - the assumed 'shorts' to ground are red herrings, it's simply low resistance electrical items across the supply, perfectly normal in a car. My main concern would be the worn wires, and particularly where some are showing bare copper - it sounds like the wiring loom may need changing?, or at least parts of it. A quick google shows that wiring looms are easily available.

Also check the condencer which is across the points, Can be short circuit or non existence capacitance. Either will give no spark or verry poor spark.. With ignition on there should be voltage on one side of the coil primary.

I still need to finish the fuel system but the car now fired with starting fluid. Best I can figure out is several wires had shorts/insulation faults. I don't think the coil was getting enough electrical input?

If the fuses were not blown, a short is most likely not the problem.
Best guess would be poor contacts and/or high resistance, which when disturbed results in this:

Donzoh1 said:

Sometimes it bothers when I fix something but can't say exactly what the problem was.

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To try and pinpoint a fault when the above situation occurs, a 'wiggle-test' can help.
Wiggle may be a tad tame of a term, give the harness a good bit of manipulation.
It's better to have the root cause reproduced when the vehicle is stationary in the driveway, rather that at speed on the motorway.

I did replace a number of wires or wire segments that had insulation defects, including one with exposed copper that was resting on or near the clutch master cylinder. After I get it running completely , I will do some more wire bundle movement though to see if the problem can be duplicated.

The only place I can think of that a short to chassis would prevent starting and not blow a fuse is the wire from the distributor points to the coil.

That's grounded anyway when the points are closed; a short would just warm up the coil and ballast resistor.
[Puns resisted, with difficulty].

One problem I had that caused intermittent ignition problems was the flexible bonding wire in the distributor that bonded the plate that was moved by the vacuum advance to ground. The first few times the car stopped the fault went away while I looking for the cause. I eventually traced the fault to the broken bonding wire. (Most of the time there was a good enough connection to ground without the bonding wire.) This was on an Austin Maxi in the early 1970's.
Have you tried connecting a meter between ground and the connection on the side of the distributor that goes to the ignition coil and turning the engine slowly with a spanner on the bolt at the front end of the crankshaft. (Or pushing the car with it in a high gear.) You should see the reading change between about zero and 12 volts as the points open and close.

Les.

rjenkinsgb said:

The only place I can think of that a short to chassis would prevent starting and not blow a fuse is the wire from the distributor points to the coil.

That's grounded anyway when the points are closed; a short would just warm up the coil and ballast resistor.
[Puns resisted, with difficulty].

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With a ballast resistor, a short on the power side of the coil wouldn't blow a fuse. The ballast resistor might overheat but it would take time.

Cars from the 1970s had a lot of circuits not protected by fuses. The circuit that rjenkinsgb posted shows three fuses. One for the lights, one for the stuff that is only available when the ignition is on, and one for the stuff that is available all the time. The ignition circuit and the headlights have no overcurrent protection.

This car does have a significant number of wires coming off of the positive battery terminal. All fuses we re intact but I'm thinking at least one of these may have been shorted. Then again, I couldn't see any spark between cables and battery terminals when attaching or detaching that might be expected with a live circuit or short to chassis. This distributor short mentioned above may need further consideration. Do not know much about the Austin Maxi but I'm guessing it too was British.

Donzoh1 said:

All fuses we re intact but I'm thinking at least one of these may have been shorted.

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I hope so.... Fuses are a short.. Or else they wouldn't blow when excess current goes through..

I expect you mean to say one had blown!

After looking at the schematic again I think the distributor probably does not use a normal mechanical set of points. The reason for thinking that this is the case is the item "drive resistor" that is connected to the distributor. A short to ground or an open circuit on the wire between the coil negative and the distributor would prevent the car from starting. I suggest taking a more logical approach to tracing the problem than just looking at fuses and damaged wires at random.
Start by checking that you have a voltage of a bit less than 12 volts at the coil positive terminal with the ignition switch on. (The reason for it being less than 12 volts is due to the ballast wire which is behaving as a resistor. This is shorted out when the starter motor is operating so the coil receives the full battery. This is to compensate for the battery voltage dropping due to the very high current taken by the starter motor.) As at this stage I am just guessing that the distributor does not use mechanical points can you post a picture of the distributor with the distributor cap removed so we can see if it uses mechanical points. )One test you could do is to connect a single spark plug directly to the high voltage connection on the coil (Using ignition wire as normal wire insulation will break down with the high voltage.). and test if you get a spark when turning the engine over with the starter. Post the picture and the test results and we can then suggest the way to proceed.

Les.