Starter motor voltage drop

[timbo]

-Smallport
-Starter relay
-New positive lead to starter
-New wiring for relay to; earth, 12v and solenoid.
-Battery is good. 12.7v 480CCA when tested and over 14v when car is running


Folks, car won't start sometimes, mostly when hot.

Crank the car over and there is a voltage drop to 9v... The drop is from + to - on the battery and from battery + to starter +. Battery + to starter casing (earth) measures normal and drops when cranking.

Is the drop in the cable or the starter? I'm thinking starter as this tends to happen when the car is hot which makes me think it is getting hot and causing a voltage drop. Have been reading some notes on this but can't work out where the exact problem is...

[Matt]

Voltage drop to around 10v is pretty normal when cranking a car as it takes a bit to turn the motor over...

Just confirming that the car cranks, but won't fire when warm or doesn't the starter motor kick over at all?

[timbo]

When cold or cool it drops to around 10v, but when warm goes to 9v or lower. When cold it takes a few turns to start the car, but when warm the starter will turn a few times but doesn't seem like there is enough guts to get the motor going and I would have thought it was a lack of volts.

[greeneyes]

That's normal I thought. Its why we use 9V coils with ballast resistors, so the coil works at full voltage when cranking.

If you're using a 12V coil it will be struggling at 9V.

[lolwat]

4 a starters dont like getting hot, get your self some heat shield or heat wrap extractors, if you search you will find there are several threads (just saying)

[macca1590]

engine warm, rings and pistons expanded, more compression??

[DX20VT]

Measure the voltage from the battery positive to the starter positive while the engine is cranking.
Expect to see around 0.5 volts

Measure from the starter motor housing to the battery negative while the engine is cranking,
and about the same 0.5 volts.

Let us know the results and it will help diagnose it better.

[timbo]

Yeah,all valid points. Didn't think about coil voltage...

As for heatshield, it has the factory piece on there now so maybe I'll try adding some ACL stuff to it. I used the same stuff for my dizzy with great results.

More compression makes sense. I've only encountered this problem since I rebuilt the motor (new rings,shaved head, thinner HG)...

[timbo]

Measure the voltage from the battery positive to the starter positive while the engine is cranking.
Expect to see around 0.5 volts

Measure from the starter motor housing to the battery negative while the engine is cranking,
and about the same 0.5 volts.

Let us know the results and it will help diagnose it better.

Will do tomorrow and let you know. If either is lower then what is the likely case?

[Skylar]

Bad connection(clean up the contacting surfaces), bad wire(unlikely since your stuff is new) or wire not big enough to handle the current.

How many k's since rebuild?

How old is starter?

[timbo]

Maybe 2500km since rebuild? Starter was brand new a year ago. I have had it out and looked at about a month ago and it was ok.

I might try another trigger wire just in case, although the + lead is nice and thick with clean new terminals.... Get back to you all this afternoon.

[LittleRedSpirit]

Bring your car to my shed on the weekend and we can go over all your wiring and look for faults. We can test a few things, fix some grounds up, check the power feeds to the key and then the trigger from the key to the starter.

[timbo]

Added some more heat shield to the existing one. Started a couple of times when hot then would not. I'm really thinking it is heat related again, the solenoid is so damn hot so I'm thinking heat from the block is transferring to the starter...

[timbo]

Bring your car to my shed on the weekend and we can go over all your wiring and look for faults. We can test a few things, fix some grounds up, check the power feeds to the key and then the trigger from the key to the starter.

Sounds like a date. How about tomorrow? Grand final today...

Also I added more heat shielding, a piece on the block that separates it from the solenoid and another curved piece under the headers. The starter is pretty much enclosed and it seems to have done the job.

[timbo]

Well back to square one. Started playing up again when the car is hot, even after heat-wrapping the exhaust (been meaning to do this for a while purely for under bonnet temps/smoother driving).

The starter seems to be getting a lot hotter than it should, is it possible to have a voltage leak that is constantly giving the starter 12v without actually running the starter (as in the pus rod)?

[DX20VT]

Are you using the smallport planetry gear reduction starter,
or the conventional type starter?

and do you have standard manifold or aftermarket headers?

We found on a rally car the with aftermarket flow and the conventional starter,
that there was no way of stopping the problem as the armature swells in the heat and polls on the field housing,
and the front bush also swells up and jams the shaft.
Even giving them both extra clearance in the lathe only made it better not fixed.

[timbo]

Smallport reduction starter.

Aftermarket headers.

I think I might pull the starter out for an inspection again but what you have said makes sense unfortunately!