How to prevent resets during extreme cranking conditions?

[GintsK]

Hi!
Probably many VEMS users has faced with dropped communication during cranking. It happens because at very beginning of cranking starter motor consumes huge current and voltage drops below critical. VEMS resets.
I had few customers where small battery due weight saving or simply old battery or thin wiring is used. And then voltage can drop below critical during cranking.
Problem escalates in winter time. VEMS itself has good capabilities to manage engine start even below -25C. But it needs voltage with no even shortest drops below ~6.5V (?). While engine revs are still sufficient for sync.
One solution is good, well charged battery, excellent wiring, best oil...
But here I want to discuss how to keep steady 5V to microcontroller when supply voltage drops below sufficient.

Some external device? Modification of existing circuit?
Some special voltage regulator?
Big capacitor?

Gints

[MWfire]

But you have bigger problem. Most of injectors doesn't work below 8V. So no feul. And with 5V spark is very weak.

[dnb]

If the problem is simply one of a transient voltage drop lasting a few 10s of milliseconds causing a reset (followed by recovery and starting after extended cranking), then you might get away with one of these to smooth out the 5v power rail for a short time http://uk.farnell.com/panasonic/eecs5r5h105/capacitor-1f-5-5v/dp/9697497   

I suspect that you're far better off fixing the original problem because the ecu dropping out is the least of the problems as highlighted above.  After all, the next problem you are likely to see is that the capacitor only holds the supply up for 20 milliseconds and I need 30 to guarantee starting the engine...  (Repeat until you have 2 batteries in the car )

[GintsK]

Yes. It's  exactly transient issue. But one per rev is enough to loose sync.
My observations shows that OEMs keeps alive better in such situations. What solutions they use?
I just had customer with damaged battery bank.  - around 11.4V when fully charged. But still steady around 150rpm cranking. Motronic shows no problems to sync. VEMS does.

[VEMS]

Was the problem ecu reset as stated in topic start or no sync during low rpm cranking ? Both would require a different solution, please elaborate.

Best regards, Dave

[MWfire]

Yes. It's  exactly transient issue. But one per rev is enough to loose sync.
My observations shows that OEMs keeps alive better in such situations. What solutions they use?
I just had customer with damaged battery bank.  - around 11.4V when fully charged. But still steady around 150rpm cranking. Motronic shows no problems to sync. VEMS does.

I started car with 110rpm(bad wire for starter) without problem. Reset is less than 10ms, so that can't affect time of engine start because reset happens during start of cranking(so crank isn't moving).

Check did you put also good voltage compensation, some time injectors open time is much bigger.
Also prime pulse helps when you have very low voltage because that is only fuel.
Also with very low voltage car starts after you stop cranking. Because you have enough voltage for injectors and engine is still spinning.

[GintsK]

My exact case and topic is about too low voltage for Atmega.
That V8 since weather comes bit colder suffered from prolonged cranking. I came to record triggerlog and it was not possible at all. However rpms was high enough.

I got that engine running. warm it up to 30C. And then went from opposite side: in safe mode switched on starter and waited until communication drops. Last value in datalog was around 7.5V.  If voltage is steady I suppose voltage regulator should hold 5V output down to 6.3 or so volts. No?

Gints

[perboddum]

Hi

I had exactly the same problem on my V8. Even with a new starter and heavy duty Odessey battery the voltage drops to around 5.5V for 30-50ms on initial start cranking. This somehow confuses Vems and it takes long time to start.
We have tried with extra cables but to no avail.
The solution to me was to put a big capacitor (some 16000uF) on Vems 12v supply line and a diode so the current cannot flow back to the starter.

Per