Slow cranking

[Duffman502]

Hi all ive checked my battery and the charge is good as are the connections to the starter motor.
Is there anything else that would cause slow cranking its only happened in the past couple of weeks before that it started like a dream.
Regards joe

 

[zcacogp]

Joe,

When you say "The battery is good", what do you mean and how did you test it?

There are other connections in the circuit which will affect it as well; battery terminals, battery earth strap and engine earth strap. Try looking at these too.


Oli.

 

[Duffman502]

Cheers oli where is the earth strap on the engine i couldnt see it? But the battery is reasonably new and shows 12.4v off and around 13 at idle.
The rest have been removed and wire brushed.
Thanks again
joe

 

[Lowtimer]

Voltage in a no-load or light-load condition is not much of an indicator of a battery's ability to start a car - what counts is the voltage it can maintain when under heavy load. the two things are not necessarily related.

The voltage when on charge is no help at all, I'm afraid.

So what I'd be doing, having checked and cleaned all the connections, is putting a second battery across it in parallel to see if that cheers things up - if it does, then your battery is suspect. If it doesn't, and all the wiring connections are good, then it's probably the starter itself. Is it the original one? if so it's probably full of the crap of ages and may well have dubious brushes. Most times an old starter like that can be brought very satisfactorily back to life by simply dismantling it, giving it all a good clean out and reassembling with new brushes.

 

[frankydoom]

Hi

Were would you get those bushes from?? - I believe there are two types of starter motor trough the 944 range. I would love to rebuild my spare starter motor.

Cheers
Frank

s2 cab 89

 

[924nutter]

You need to get the serial or series number from your motor to find which brush set is compatible then type in bosch starter motor brushes into your search engine and see what comes up. My first look yesterday gave a site selling brush sets for £7.50 ex, but it looks like you need to be able to silver solder the braid tags on to the carrier plate, as the GA picture did not reveal the usual screw tag; then bed them in with a peice of emery wrapped arond the commutator. MY 85.5 could have either the 1.1kw (1.5 hp) or the 1.44 kw (2.1 hp) motor so as I said get the motor serial from the case. You will not get away with soft solder on the tags because it will melt. 1.1kw at 12v theoretical, is roughly 93 amps, 1.4 is 120amps. In practise the voltage will drop below 12v so the current will be higher, though I couldn't say for certain whether these are no load ratings.

 

[Veerzigzag]

the voltage will drop below 12v so the current will be higher

Surely lower current with lower voltage?
The current you calculate is at the full load although voltage drop at the motor down to 10 volts is not uncommon.
Power output is a square law variable so when the voltage drops, the amperage drops, and the power double-drops.
But 944s crank slowly anyway: "they all do that, sir". Big old high-compression lump to turn over.

 

[zcacogp]

ORIGINAL: Veerzigzag

the voltage will drop below 12v so the current will be higher

Surely lower current with lower voltage?

Nope.

Watts = Amps x Volts.

For a given Wattage, if the volts drop then the amps need to increase to keep up.


Oli.

 

[924nutter]

ORIGINAL: Veerzigzag

the voltage will drop below 12v so the current will be higher

Surely lower current with lower voltage?
The current you calculate is at the full load although voltage drop at the motor down to 10 volts is not uncommon.
Power output is a square law variable so when the voltage drops, the amperage drops, and the power double-drops.
But 944s crank slowly anyway: "they all do that, sir". Big old high-compression lump to turn over.

You raise an interesting point "Veer", but in electric mototrs there is such a thing as back emf which,as I understand it, and again, simply put, opposes the voltage and lowers the available voltage applied, which seen another way in essence adds to the resistance of a circuit. as V drops back emf drops lowering the resistance of the circuit. More or less but I don't want to turn this into a phyics lesson
It is of course this voltage drop that makes a solenoid chatter on a car with a flatish battery;
You energise the solenoid by turning the key, pull in windings pull the pinion into engagement and close the internal switch for the hold in windings which are effectively in series with the brush circuit. Voltage drops across the battery under load, and the hold in windings cannot hold the pinion in against the return spring in the solenoind, and the solenid pulls out, then the battery with no load on it pulls the solenoid in and the whol ecycle starts again.

 

[JM1962]

I think Mike is right.

If the battery is down on volts the current will be less as at start up the resistance of the circuit is fixed. I=V/R, until the starter spins this is the only thing that counts in a DC starter circuit. If the starter spins a back emf is generated which opposes the applied voltage and reduces the starter motor current.

The maximum torque of a DC series motor is delivered at the instant current is applied to the armature and is proportional to the product of current and field strength. It is inversly proportional to speed so as the speed rises the back emf builds and the current and torque falls.

If the battery is faulty it will provide less current at start up, therefore less torque which may be insufficient to turn the engine over, hence only the click of the starter solenoid.

 

[924nutter]

Incorrect my friend. The pull-in windings are separate from the hold-in windings in the solenoid, but both are energised by the key from terminal 50. The internal switch applies voltage to the brushes, (terminal 30) and when the 92 amp demand is made upon a flat battery it cannot supply enough current to maintain the magnetic flux in the hold-in windings, so they release the iron core of the solenoid, which turns off the draw, and with the key held over, the pull in windings, requiring only something over 20 amps are energised via the key position. When the solenoid kicks, the pull in windings are rendered useless by the current draw as the motor, as you correctly state, delivers it full torque, which is why there are hold-in windings in the first place. If you look closely at the theoretical circuit in the manual you will see two diagonal lines in the icon for the solenoid. One current path earths in the normal way, the other completes its earth retrun via the brush circuit. The motor is energised from termial 30 and the other set of contacts is 15a, a redundant connection for systems with points which uses ballast resistors of two different values to supply voltage to the coil. Something like 0.5 ohms and 0.6 ohms. 15a shunts out the 0.5 ohm so the coil can get full voltage because of the voltage drop across the starter motor. Ohms law should truly be exprerssed as
R= V/I because it is resistance calculation, although of course this does transpose to V=IR. Thinking about it I might have talked some nonsense about currrent and mixed my ac up with my dc and I may have mistakenly believed that the current will increase, because there is no inductive resistance in a dc circuit, but it has been an interesting discussion, and I am not too big to concede a point.

 

[Veerzigzag]

Wow, some brain-power here. I think the bottom line is that IF the starter motor is at 100% performance and IF the battery is ditto and IF the earth contacts are clean and good THEN it will spin the engine "like a normal car" but any slight reduction on any of those and you have a big old heavy load to schlep..

 

[924nutter]

Im only doing it to up my number of posts, like this one.

 

[924nutter]

No not really. The 944 crank is really heavy so has lot of inertia; like diesels, but to a lesser extent, once a full cycle of cranking has taken place the cylinders which are "pumped up" but not yet having fired release their potential energy and assist on what would be the power stroke, which is why you get the characteristic neeerrhhhh duckduckduck when you try to start. A 3 litre engine with only four pots with an absurdly high CR of 10.7 :1 is going to proportionatley more difficult to crank than a 3 litre split six ways. The ECU won't even switch on fuel pumps or spark until the engine reaches 200 rpm, so as you say everything needs to be in first class order to be effective,

 

[Veerzigzag]

neeerrhhhh duckduckduck

that is onomatopoeia at its finest level (am just trying to maintain the fine scholarly excellence of this thread.....)

 

[tref]

Except in this case it is a 2.5, not the 3l 16v "they all do that sir"... The 2.5s should normally spin over reasonably well... if the battery is good, my first look see would be the engine earth strap. As part of the diagnosis I would try jump starting it from another car. If it spins over fast jump started from the battery, then it is the battery or it's connections at fault. If it is still slow then I would try the positive jump lead to the battery, the negative directly to the engine, and if that spins it over faster, it is the engine earth lead.

 

[tref]

ORIGINAL: Veerzigzag

neeerrhhhh duckduckduck

that is onomatopoeia at its finest level (am just trying to maintain the fine scholarly excellence of this thread.....)

I was trying to think if the starter clicking sounded a bit like Quackquackquack... but it doesn't!

 

[924nutter]

Fair cop on the 3.0 v 2.5 but it is all relative, a 2.5 4 will be more difficult to crank than a 2.5 six, but didn't the 2.5 have the weedy 1.1 kw motor initially so even Porsche must have had concerns about slow cranking to fit the ~1.5kw post 1985

 

[924nutter]

Surely it is more of a clack-uck-clack

 

[zcacogp]

ORIGINAL: 924nutter

Im only doing it to up my number of posts, like this one.

Yeah. So am I.

(And I'm winning!)


Oli.