Electrical issue?

[Riknos]

Hi guys,

Having a few problems with my MX5:

I fitted a second hand starter motor, new Westco battery, and battery terminals. Car starts fine most of the time, but occasionally wont. When it wont start, it wont start until I put the battery on the Optimate and charge it for a few days, then it starts fine a few times, until I have to do the same again.

Examples: Charge battery to 13.2v, car starts fine. Charge battery to 12.7v, car does not start.

I'm 99% sure it's an electrical issue, but not sure what it could be? I'm slowly replacing earths, but it's cold outside and I don't have a drive so barely get access to do such a job.

When the car does die, bump starting it down my hill gets it working.. Why does that work then?

Cheers.

[The Wookie]

I'd be looking at engine earth straps and starter connections before anything else.

[Riknos]

The Wookie said:

I'd be looking at engine earth straps and starter connections before anything else.

Engine earth straps are all fine. Connection to the starter motor snapped when fitting new (used) one (the wire is almost the width of a hair!) but a new connection was fitted on it.

Also replaced one of the earth straps in the boot, and this got it working again, for a while..

[The Wookie]

Riknos said:

Engine earth straps are all fine. Connection to the starter motor snapped when fitting new (used) one (the wire is almost the width of a hair!) but a new connection was fitted on it.

Also replaced one of the earth straps in the boot, and this got it working again, for a while..

Hmmm, usually the wiring for the starter is fairly hefty but if you've had two parts that are the same then it must be right.

I'd start looking towards the starter itself if you're confident that the battery is healthy.

[Ozzie Osmond]

Get a proper high current drain test done on the battery. Without that done first you could be wasting a lot of time and effort over what is probably a failed battery.

Also, check the voltage at the battery terminals with the engine running. The alternator should be charging at about 14.6 volts.

[Riknos]

Ozzie Osmond said:

Get a proper high current drain test done on the battery. Without that done first you could be wasting a lot of time and effort over what is probably a failed battery.

Also, check the voltage at the battery terminals with the engine running. The alternator should be charging at about 14.6 volts.

The battery was sent back to MX5parts.co.uk who supplied the battery. They said they bench tested it and found no issue with it.

I can't remember what the voltage on the battery was the other day when I tested it whilst the engine was on. Think it was around about 13 ish? I don't have the car with me so can't test it unfortunately

If it is say the alternator, surely the car should still be able to start under 13volts? Or could it be another issue also?

[Ozzie Osmond]

Hmmmm

Here's a home battery check for you. Get the battery out of the car into the house and charge it using a charger. Then leave it to stand indoors overnight. This matters because the test which follows is temperature sensitive and needs to be done at room temperature. also the battery must be left to stand unmolested for several hours before attempting the reading.

In the morning, use a good voltmeter and test the voltage across the battery terminals. If it's anything less than 12.4 volts the battery IS knackered, despite what they said to you. (A 100% healthy battery will show at least 12.7 volts. 12.4v is about 75% healthy and anything less means knackered)

If the battery is OK and if there is 14.6 volts at the terminals when it's in the car with the engine running then you are back to looking at earth straps and other possible poor connections.

[Riknos]

Ozzie Osmond said:

Hmmmm

Here's a home battery check for you. Get the battery out of the car into the house and charge it using a charger. Then leave it to stand indoors overnight. This matters because the test which follows is temperature sensitive and needs to be done at room temperature. also the battery must be left to stand unmolested for several hours before attempting the reading.

In the morning, use a good voltmeter and test the voltage across the battery terminals. If it's anything less than 12.4 volts the battery IS knackered, despite what they said to you. (A 100% healthy battery will show at least 12.7 volts. 12.4v is about 75% healthy and anything less means knackered)

If the battery is OK and if there is 14.6 volts at the terminals when it's in the car with the engine running then you are back to looking at earth straps and other possible poor connections.

Thanks, I've actually done this several times over the last few weeks / couple of months. The battery has been taken into the house, charged on the Optimate for several days, and then placed back into the car. The last time I charged it, I only left it on for a day and a half and it got to 12.8, but once placed in the car it wouldn't start. I then took it back in, charged for another 2 days, then put in the car at 13.2 and it started fine. I then drove for about half an hour and measured the volts on the terminals whilst the engine was on, and it was reading 12.86?

Tests on previous weeks revealed higher volt readings on the battery whilst engine running. Always in the 13s though I think. Could it be the alternator is on it's way out and only intermittently charging the battery?

Thanks for all the advice so far by the way!

[Nick1point9]

Am I missing something? Charge battery, it works fine for a while, then stops working.

So, either the battery doesn't hold charge properly or the alternator is on its way out....

[1point7bar]

I have had alternators show charge across the regulator (no dash warning light) but the windings not delivering the amps.
I had the units tested (& repaired) by an auto-electrical specialist company.

[Andyjc86]

When you say it won't start, how do you mean? Does it turn over slowly, click, turn half a turn or nothing at all?

[Ozzie Osmond]

Nick1point9 said:

So, either the battery doesn't hold charge properly or the alternator is on its way out....

That's the nub of it.

[Riknos]

1point7bar said:

I have had alternators show charge across the regulator (no dash warning light) but the windings not delivering the amps.
I had the units tested (& repaired) by an auto-electrical specialist company.

This could be it then.

Nick1point9 said:

Am I missing something? Charge battery, it works fine for a while, then stops working.

So, either the battery doesn't hold charge properly or the alternator is on its way out....

I originally thought it could be the alternator, but I put a multi-meter on it and it showed the alternator was charging fine!

So it could be the alternator is intermittently working, or there is high resistance on the wiring in the car, which would cause a low charge from the alternator also. Battery is almost new and has been bench checked by the supplier so is not faulty.

A lot of mkiii Supra owners report high resistances due to the age and thinner Japanese wiring, so they fit relays for the starter, which I might try.

Andyjc86 said:

When you say it won't start, how do you mean? Does it turn over slowly, click, turn half a turn or nothing at all?

Get a series of clicking but it doesn't turn over at all. It's either all or nothing.

[TallPaul]

Could the second hand starter motor be little better than the one you took off?

[Andyjc86]

If it is clicking as opposed to turning then I would say it's your starter not engaging. It could be down to a poor connection, but I would be looking at the actual unit itself.

[littleredrooster]

Riknos said:

I then drove for about half an hour and measured the volts on the terminals whilst the engine was on, and it was reading 12.86?

Your alternator is goosed. 12.86 is well below the necessary 14.2-14.5v that it needs.

[muley]

Ozzie Osmond said:

Get a proper high current drain test done on the battery. Without that done first you could be wasting a lot of time and effort over what is probably a failed battery.

Also, check the voltage at the battery terminals with the engine running. The alternator should be charging at about 14.6 volts.

This. Had identical problems on my Elan last year - it was the battery.

[jet_noise]

Dear R,

from the history above and by elimination (assuming the new bits are good of course) what have you not replaced?

The alternator. [/Sherlock]

This is supported by the low charging voltage.
Are there any other MX5 owners or a specialist MX5 forum who might measure their charging voltage to confirm what a good one should be?
Different systems can have 13.5-14.5V typically and dependent on load and engine revs, but <13V is definitely an issue,

regards,
Jet

[E30M3SE]

Before you jump in and change the alternator, measure the voltage at the back of the alternator with 2500rpm on the engine.

What is the reading?

[Smiler.]

OP, I have one of those battery condition testers & if local enough to you, you're welcome to pop around & borrow it.