Educate me on a starting problem please
10-29-2018, 06:50 AM
#1
Pro
Thread Starter
Educate me on a starting problem please
I have been meaning to ask this for some time but I tend to forget as it only happens at lower battery voltage which is not that common. I have also not seen this discussed.
What happens when the battery voltage drops a little is that with the key turned to the cranking position the starter engages and then disengages while at the same time the digital dash sort of resets, a bit like someone disconnected the the power from the battery momentarily and then reconnected it.
If I keep the key in the cranking position after a cycle of 3-4 on/off it will then catch properly and fire up. When it does turn over there is plenty power in the battery to turn it over, it’s just not 100% power.
I suspect some relay is failing but please feedback if you know of this problem.
What happens when the battery voltage drops a little is that with the key turned to the cranking position the starter engages and then disengages while at the same time the digital dash sort of resets, a bit like someone disconnected the the power from the battery momentarily and then reconnected it.
If I keep the key in the cranking position after a cycle of 3-4 on/off it will then catch properly and fire up. When it does turn over there is plenty power in the battery to turn it over, it’s just not 100% power.
I suspect some relay is failing but please feedback if you know of this problem.
10-29-2018, 07:39 AM
#2
Rennlist Member
As I understand the computers can have problems if the voltage is a bit on the low side. When cranking the alternator is pretty much doing squat and the battery voltage drops due to the huge current flowing through that big cable from the battery to the starter motor. Now, at the same time, the current is trying to find its way to the main bus in central electrics via the 16mm2 cable that goes from the starter motor, to the alternator then out to the hot post and then through the two 10mm2 cables into the central electrics bus.
If those 16mm2 cables are degraded then voltage will suffer ever more and if that happens starting can be tedious. The obvious question being what is the condition of the 16mm2 cables- are they still original?.If so, there is a good chance the engine harness is shot- my memory tells me you might have replaced this but...?
Whether or not your issue is connected to this remains to be seen.
If those 16mm2 cables are degraded then voltage will suffer ever more and if that happens starting can be tedious. The obvious question being what is the condition of the 16mm2 cables- are they still original?.If so, there is a good chance the engine harness is shot- my memory tells me you might have replaced this but...?
Whether or not your issue is connected to this remains to be seen.
10-29-2018, 10:13 AM
#3
Pro
Thread Starter
As I understand the computers can have problems if the voltage is a bit on the low side. When cranking the alternator is pretty much doing squat and the battery voltage drops due to the huge current flowing through that big cable from the battery to the starter motor. Now, at the same time, the current is trying to find its way to the main bus in central electrics via the 16mm2 cable that goes from the starter motor, to the alternator then out to the hot post and then through the two 10mm2 cables into the central electrics bus.
If those 16mm2 cables are degraded then voltage will suffer ever more and if that happens starting can be tedious. The obvious question being what is the condition of the 16mm2 cables- are they still original?.If so, there is a good chance the engine harness is shot- my memory tells me you might have replaced this but...?
Whether or not your issue is connected to this remains to be seen.
If those 16mm2 cables are degraded then voltage will suffer ever more and if that happens starting can be tedious. The obvious question being what is the condition of the 16mm2 cables- are they still original?.If so, there is a good chance the engine harness is shot- my memory tells me you might have replaced this but...?
Whether or not your issue is connected to this remains to be seen.
I have had a change of starter and it does the same thing. The cutting out is also coupled with the digital dash cutting out which is why I was thinking relay?
I assume when a battery runs low on power in a 928 that it just results in the engine turning over slower?
10-29-2018, 10:49 AM
#4
Rennlist Member
Marti,
None of my cars have exhibited this symptom when being started on low batteries. You are correct - they just turn over slower. Since you have changed out the starter and ruled out those connectors, I think you should check all of the grounds between the starter and the brain first. Scotland is a fairly damp environment and this may be condensation in a relay as well. Perhaps a de-humidifier in the garage or car itself might help?
Does the digital dash give any clue in the display? Perhaps "Low Voltage"?
None of my cars have exhibited this symptom when being started on low batteries. You are correct - they just turn over slower. Since you have changed out the starter and ruled out those connectors, I think you should check all of the grounds between the starter and the brain first. Scotland is a fairly damp environment and this may be condensation in a relay as well. Perhaps a de-humidifier in the garage or car itself might help?
Does the digital dash give any clue in the display? Perhaps "Low Voltage"?
10-29-2018, 11:33 AM
#5
Rennlist Member
When I changed my engine I got the opportunity to see the wiring loom and it looked fine. It is not that the drop in power from sitting for a couple of weeks is enough to not turn over the engine sufficiently, it is more about the switching mechanism to connect the power to the starter.
I have had a change of starter and it does the same thing. The cutting out is also coupled with the digital dash cutting out which is why I was thinking relay?
I assume when a battery runs low on power in a 928 that it just results in the engine turning over slower?
It is hard to visualise an original harness in a UK based 87 is going to be completely OK but I dare say stranger things have happened. Try taking a voltage measurement at the hot post during cranking and see what it drops to.
I presume you will know about and have eliminated the earth strap issues. The one for the negative terminal to the chassis is infamous for having issues under the insulation at both ends, the engine earth strap less so but still important to have good integrity/conductance.
Terminal 1 [4mm2 yellow cable] on terminal 1 of the 14 pin connector supplies power to the starter solenoid- that needs to be in good shape. The wiring harness, and specifically the 16mm2 cable in it tends to be degraded worst inside the sleeving around the alternator area. Since I rebuilt my harness the car starts like Usain Bolt and that on top of the improvements I got after changing the earth straps.
10-29-2018, 11:49 AM
#6
Nordschleife Master
...I presume you will know about and have eliminated the earth strap issues. The one for the negative terminal to the chassis is infamous for having issues under the insulation at both ends, the engine earth strap less so but still important to have good integrity/conductance...
The ground strap between the battery and the body of the car is notorious for corroding under the insulation.
It is intact enough that it shows continuity if you check it with an ohm meter or test light, but not intact enough to allow full current when trying to crank the starter.
Easiest way to check is to hook up one jumper cable from the battery to a good, solid ground on the car (mimic the ground strap).
If this solves the problem, then replace the ground strap.
10-29-2018, 12:26 PM
#7
Pro
Thread Starter
While 'long distance diagnostics' are often fool's errands, the above is what jumps out at me.
The ground strap between the battery and the body of the car is notorious for corroding under the insulation.
It is intact enough that it shows continuity if you check it with an ohm meter or test light, but not intact enough to allow full current when trying to crank the starter.
Easiest way to check is to hook up one jumper cable from the battery to a good, solid ground on the car (mimic the ground strap).
If this solves the problem, then replace the ground strap.
The ground strap between the battery and the body of the car is notorious for corroding under the insulation.
It is intact enough that it shows continuity if you check it with an ohm meter or test light, but not intact enough to allow full current when trying to crank the starter.
Easiest way to check is to hook up one jumper cable from the battery to a good, solid ground on the car (mimic the ground strap).
If this solves the problem, then replace the ground strap.
I can say that if I use pin 14 on the engine wiring harnesses straight on the jump post it does not display this behaviour, it just turns over slower as the battery looses power.
I guess that points to the starter relay and or the ignition switch?
It is such distinctive behaviour that I thought it might pinpoint a known issue.
FYI, the dash does not display any low voltage warnings