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nelmo

Battery power

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Is it possible for a battery to have enough power to turn the starter motor but not enough to provide a good enough current to the coil pack?

I'm having starting problems recently where I have had to spend quite a while with the starter motor going before the engine finally kicks into life - and it's quite sudden, as if I've suddenly hit a level whereby the coil pack gets a big enough spark to ignite some fuel.

All this churning away is possibly killing my battery and this morning, it wouldn't start at all. A voltmeter showed my battery had 12.4 volts and the starter motor churned away but the engine wouldn't kick in. Battery had dropped down to 11.9V  when I gave up.

I've already changed all the HT leads, spark plugs and coil pack and re-wired the plug to the coil pack, so I was wondering if my battery is shot? But 12.4V sounds ok? (Once started, it runs fine, so I think fuel supply is fine).

 

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It's not really battery power as such, but you can get sufficient voltage drop that the motor is sapping so much that the voltage at the coil pack isn't sufficient to create a consistent, powerful spark. It's often not really the battery's fault, but poor earth/poor cabling to starter, bad starter or knackered coil pack. I'd start by checking all the cabling and by measuring the voltage at the coil pack supply *during cranking*... the minimum value would really depend on the coil but I'd say (finger-in-the-airometer) less than 10v wouldn't be ideal :)

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Newer engine management regimes stop the starter from working if battery is not up to the task, yours is the other way round so I'd be surprised if the battery is the problem. However if the battery is only holding charge for a few turns it may be time to buy a new one.

If you have fuel injection the pump should pressure up the system to the injectors immediately you turn on the ignition, so the engine should start within a few turns. If it takes a lot of turning it could be insufficient fuel pressure. Not a hard job to test with a pressure gauge readily and cheaply available on-line. My regulator was shot and the gauge showed this up immediately. If you are running a higher compression engine this will affect the starting crank speed of the starter and maybe a higher ampere rating battery is required.

If you have carbs the engine may require more turning due to getting fuel into the carb(s) but not excessively, I would suggest a pressure test to ensure fuel line is working correctly.

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1 hour ago, brumster said:

It's not really battery power as such, but you can get sufficient voltage drop that the motor is sapping so much that the voltage at the coil pack isn't sufficient to create a consistent, powerful spark. It's often not really the battery's fault, but poor earth/poor cabling to starter, bad starter or knackered coil pack. I'd start by checking all the cabling and by measuring the voltage at the coil pack supply *during cranking*... the minimum value would really depend on the coil but I'd say (finger-in-the-airometer) less than 10v wouldn't be ideal :)

Actually, yes, the problem has been since I got the new starter motor. I did check the voltage at the coil pack plug but not while cranking, just with the ignition on. I vaguely recall thinking it was a bit low as well (around 9v) but at the time, I was just checking I was getting something and I wasn't concerned about the numbers - I'll check them again.

Problem is, if that is the cause, what to do about it?

@MrToad, it is FI and if fuel pressure was a problem, wouldn't it not run once started? Runs fine when it does eventually start.

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Is the cable to the starter as short as possible, direct from battery to starter (or solenoid) and "suitably thick" (thicker the better really)?

Is the engine well earthed? Check the earth strap(s), check they're not rusty/well grounded against the metal?

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The cables are the proper battery kit from GBS and only 2 years old. For the engine earth, I filed off the powder coat on the chassis before fitting and then painted over with Hammerite - looks fine to me? It's the same thickness as the battery wires (10mm OD?).

Another interesting change is that the engine seems to be running about 5-7 degC hotter than before and this started at the same time - could a weak spark cause that?

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Ok, sounds like cables and connections are fine (you could probably test resistance but I doubt that's an issue; should be only a few ohms I would think)... 10mm OD is a good size!

I'm thinking this might be something else, maybe fueling related. Trying to think logically about a weak spark - if that were the case, the flame front in the combustion chamber would be slow and you'd maybe get unburnt fuel left over after the cycle which would then likely be in your exhaust and popping/banging in there. Or lack of ignition on one revolution, and firing on a subsequent revolution, with way too much fuel now in place and... more popping/banging/misfires :)

 

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So I had the charger on all day, came back and the battery voltage was 12.8v - fine. 

Ignition on - 12.3v at the coil pack. 

Starter motor churning: 11.5v at coil pack plug - seems OK? 

Significantly, the car did start, although took a while. 

How do I check the spark at the plug these days? I was planning on just pulling it out, connected to the HT lead, hold near to engine block and turn the ignition (saw a mechanic do this once years ago). But reading online, seen a lot of comments saying this isn't a good idea as it might fry the ECU. Opinions on that? Any other way to test the spark?  Edit: just ordered HT lead testers - bit of a rip at £14 but need this sorted. 

Edited by nelmo

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Sparking with the plug grounded to the engine block/head bolt/etc doesn't sound like a problem to me, it's only what it essentially happening inside the engine normally?

Yes voltage at coil pack sounds fine so at least you've ruled that out, mind you check it when the car *isn't* starting :)

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Quote

battery voltage was 12.8v

Ignition on - 12.3v at the coil pack. 

Just switching on the ignition knocked 0.5v off! That sounds like a pretty gutless battery to me (or poor wiring to the coil pack). And why did it need charging in the first place?

Rig up booster cables from your tin top to the hood battery and repeat.

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Ah, that's a good idea for a test - I'll try it...

I think my battery may have got in this state because my alternator circuit may not be right. I believe the battery warning light is what triggers the alternator charging circuit - is that right? My battery warning light is an LED, not a bulb. GBS tell me that they have some resistance built into their loom so this shouldn't be a problem but I'm not convinced my alternator is charging the battery.

With the engine idling, I'm only getting 13.4v across the battery. Even if I rev the engine, it never goes over 13.9v. According to some 'expert' on the web, I should be getting between 13.8v and 15.3v across the battery when revving the engine (or maybe even idling) - is this correct? So, it's possible my alternator is not charging the battery enough.

I charged it yesterday because, after trying for ages to start the car, the battery voltage had dropped to 11.9v.

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Charging voltage varies but 14.2 plus or minus a couple of points seems about right to me. 13.9 should be OK. Over 15 is too high! My thinking was your battery has little reserve if switching on just ignition which can only be running coil, ECU, dash and fuel pump(s) only leaves 12.3 at the coil. Maybe you should check the wires, supply from battery to ign switch or ignition relay to the coil to make sure you have no high resistance joints all the way.

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On 7/24/2019 at 12:07 PM, nelmo said:

Actually, yes, the problem has been since I got the new starter motor. I did check the voltage at the coil pack plug but not while cranking, just with the ignition on. I vaguely recall thinking it was a bit low as well (around 9v) but at the time, I was just checking I was getting something and I wasn't concerned about the numbers - I'll check them again.

Problem is, if that is the cause, what to do about it?

@MrToad, it is FI and if fuel pressure was a problem, wouldn't it not run once started? Runs fine when it does eventually start.

The injectors need a constant pressure to allow controlled fuel flow by the ecu, if that pressure is low the spray pattern will be affected meaning that petrol will only dribble into the cylinder and be harder for the spark plug to ignite. With this the engine will have to churn over more to get fuel to be vapourised enough to start, also on start an engine needs the fuel air ratio to be over rich for initial start, ecu's now control this evident by faster running when cold etc. Good old carbs used to need a choke control or holding the accelerator pedal down among other techniques.

With the engine running ok when started the fuel pressure may be enough to maintain operation but not for start up, as suggested a simple fuel pressure test will either prove or eliminate this function.

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Most aftermarket ECU's have a battery voltage compensation table, able to make appropriate injector duration changes dependant strangely enough on battery voltage. Your fuel pump should have a wide excess supply margin and be working well within your engines needs even at 9v so I doubt its poor fuel supply if the engine is able to crank.

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Aha - possible fix on the way...

I ordered some HT signal testers (you plug them into the HT lead and then onto the spark plug) which flash if there is a good signal to the plug and cranked the engine - only 3 of them flashed!! No flash on cylinder 1, the one with the longest cable. As I cranked, eventually it started flashing and the engine started immediately.

So it looks like the engine has been trying to start on 3 cylinders only.

I swapped out the coil pack and it did the same thing with that one. Checked the HT lead, resistance of 2.5kOhms, same as the others and much better than the old HT lead I took off - that had a 7.5kOhms resistance and with that in place, I didn't get a flash from the tester at all and the engine wouldn't start.

Which, as Longboarder said, points the finger at the battery. My wife has inconsiderately taken the tin-top for a trip to the in-laws for the weekend, so I'll do a test with a neighbours car tomorrow (jump leads to my battery) and see what happens. If it starts immediately, it's definitely the battery and a nice simple (sadly not cheap) fix.

Edited by nelmo
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