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Discussion Starter · #21 ·
Those look OK at first glance. Check those connections and heavy duty cables. From the battery through the big fuses and to earth from the battery and the engine/gearbox.
Forgot to mention as I didn't get to the actual earth connections I did test the resistance between the neg battery terminal and the engine block and got 0.3 ohms and to the car body and got about 1.3 ohms. Is that a worthwhile test?
 

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Scenic II, 2004, K4MW761, DPO
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0.3 R to block at say 250Amp will drop 75V off a 12 V battery :eek: ................. so NO (IF the reading is accurate anyway)
 

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Discussion Starter · #23 ·
0.3 R to block at say 250Amp will drop 75V off a 12 V battery :eek: ................. so NO (IF the reading is accurate anyway)
Yes that's kinda what I thought - wondered of I was doing it right. As for accuracy - it's a cheapo DM - so who knows! I'll see if I can find those earth connections. Thanks
 

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51 plate Laguna 2 1.6
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0.3 R to block at say 250Amp will drop 75V off a 12 V battery
What? How can a 12V battery "drop" 75V?
I'm not trying to catch you out, I genuinely need an explanation.
 

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Discussion Starter · #25 ·
What? How can a 12V battery "drop" 75V?
I'm not trying to catch you out, I genuinely need an explanation.
The current would not actually be 250A in that scenario... 12/0.3=40A. So my meter could be wrong. My earth connections are dodgy? If we assume the motor has some internal resistance and the rest of circuit is zero ohms then the motor resistance would need to be 12/250=0.048 ohms which sounds plausible I think? Which makes my 0.3 ohms rather significant (if it's true).
 

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Thought to wait for LvR and his explanation, I am sure he knows math behind it. In the meantime....

Let's assume that the electric power of starter is 2400 Watts.
That means that on 12V it will pull 200 amps even though lot of starters are pulling much more.
Simple formula of Ohm's law says that resistance of starter is 0,06 Ohm.

If we add 0,3 Ohms to that, current will be cut down to only 33 Amps and power of starter is then only 400 Watts available to turn the engine. Not enough to start.

Calculation is more complicated, with internal resistance of battery, connections, starter itself.... this is simple explanation just to show the influence of unwanted resistance on grounding.
 

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Glad you like it and aprove.
We can't be far off as this is Ohm's law, no room for interpretations.

Was getting late, next time you can fill in on my post if I'm not around.
 

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@Mr
What does it label 5 and 7 as?
with your cheapo multimeter what resistance does it measure when you hold the meter leads together? Do you have any jump leads?
My guess is your measurement is way out. try bridging from batt -ve to clean metal near the starter?
Did you do a resistance measurement from battery positive to the starter?

Simple explanation of voltages and resistances with apologies to those for whom this is obvious, lets say for example the lead to chassis is 0.1 ohm (that would be high) and the starter motor is 0.05 ohm, total resistance combined is 0.15 ohm.
ohms law V=IR or I=V/R or R=V/I
Current through both, I is V/R=12/0.15 = 80A
Voltage across earth strap =IR = 80x0.1=8V (wattage dissipated in strap 640 watts)
Voltage across starter =IR = 80x 0.05=4V (wattage dissipated in starter 320 watts)
total volts =8+4=12
Does that help? (it does illustrate why the connections to earth and starter need to be very low resistance)
 

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DID you bother to check under your passenger seat?
IF yes and its dry
Take the under bonnet fuse box out, and pull the brown plug off and see if the connections inside are good
 

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Discussion Starter · #31 ·
DID you bother to check under your passenger seat?
IF yes and its dry
Take the under bonnet fuse box out, and pull the brown plug off and see if the connections inside are good
I will try to get to the fuse box this weekend. I did look under the passenger seat but so far have only found a drawer - I suppose I need to take that out do I? :) (sorry...bit slow this as I don't get much time to work on it!).

Thanks
Mike
 

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Discussion Starter · #32 ·
@Mr
What does it label 5 and 7 as?
with your cheapo multimeter what resistance does it measure when you hold the meter leads together? Do you have any jump leads?
My guess is your measurement is way out. try bridging from batt -ve to clean metal near the starter?
Did you do a resistance measurement from battery positive to the starter?

Simple explanation of voltages and resistances with apologies to those for whom this is obvious, lets say for example the lead to chassis is 0.1 ohm (that would be high) and the starter motor is 0.05 ohm, total resistance combined is 0.15 ohm.
ohms law V=IR or I=V/R or R=V/I
Current through both, I is V/R=12/0.15 = 80A
Voltage across earth strap =IR = 80x0.1=8V (wattage dissipated in strap 640 watts)
Voltage across starter =IR = 80x 0.05=4V (wattage dissipated in starter 320 watts)
total volts =8+4=12
Does that help? (it does illustrate why the connections to earth and starter need to be very low resistance)
5 is multiplex module
7 is alternator

Not sure of the circuit diagram is actually indicating that the switching circuit is fully solid state? Might there be a relay involved?

My cheapo multimeter changes it's mind depending on which part of the probes I touch together - but never zero!! :LOL: Pointless trying to measure anything below couple of ohms with it. Need to use another technique but if I an find the earth points I'll just make sure they're solid.

Getting to the starter motor is proving to be tricky. It's buried so far down behind the engine that I can barely see it let alone get to it without removing several other components. But I'll see how far I get this weekend.

Thanks
Mike
 

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Fuse box under the bonnet, what colour is it
ALL black, or Black and white
 

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Discussion Starter · #34 ·
Fuse box under the bonnet, what colour is it
ALL black, or Black and white
Fuse box is all back. I've now checked the earth to the engine block and it's solid. Also got to the rest of the fuses and they are ok too. I can't get to the starter motor from the top (I removed the air inlet hose from the air filter to the throttle body but still can't see the connections to the starter motor). I guess it has to be done from underneath.

Going back to the wiring diagram form the Haynes manual I can't actually see a black 12 pin connector with the white and purple wires from the voltage regulator anywhere on the multiplex module so maybe that diagram is not quite right for my car?

Not sure what else to try other than get it to a garage.
Thanks
Mike
 

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Discussion Starter · #36 ·
Probably a stupid question but have you tried to jump start it from a known good battery?
Not a stupid question 'cos I haven't! I have tested the existing battery but only with my vaguely antique looking battery tester (that someone kindly left on my doorstep a couple of years ago - so I don't know here it came from!!). Also the battery was new last September - so maybe I'm slight reluctant to believe it could be duff :unsure: . I'll see if the local garage will test it for me or try another battery.
Thanks
Mike
 
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