Strange Starting Issues

Spongo · Post by **Spongo** » Mon Jul 06, 2015 3:38 pm

Ok I am running an MS2 on a stroked 383 Small Block Chevy, since putting it back together I get some strange starting issues. When left for a while and the battery is a bit low it just wont start, starter turns the engine OK ( about 150 RPM) but I am seeing lots of sync losses (1 per revolution) and it occasionally appears to semi lock on one cylinder as if its trying to fire but at the wrong time.

If I charge the battery it starts no probs, I tried to log the crank signal but it looks good and there are no losses when its running. The fault appears to be battery voltage related could it be the pots on the MS need tweaking so the crank signal is better whilst cranking?

Thoughts please as its one of those really niggly get under your skin kind of issues.

JSF55 · Post by **JSF55** » Mon Jul 06, 2015 4:23 pm

It's just not down to a poor or dodgy battery? be a lot easier to have it checked and put right than having to go through all your MS settings ?
And have you checked the engine earths !

DaveEFI · Post by **DaveEFI** » Mon Jul 06, 2015 4:40 pm

If you are having sync issues, it may well fire at the wrong time, especially when cranking. Most of MS works on a regulated 5v rail, so shouldn't be confused by fluctuating battery volts. Although the injectors and coils can be.

I like to make sure the hardware part of the VR inout is working correctly before suspecting the software. A scope looking at the input of the VR circuit (Tach select) and the output (Tsel) is IMHO the best way.

The VR circuit changes the signal from the sensor which is a varying voltage (as well as frequency) approx sine wave into a constant voltage approx square wave (or really pulses) which the processor can read reliably.

So what you're interested in is seeing a decent square wave of approx 5v peak to peak at the output, and tweaking R56 to achieve this. R56 is the pot nearest the bottom edge of the board. The other one, R52, I've never needed to adjust.

It's not a critical adjustment - usually 1/2 turn either way won't matter. I generally find about 6 turns in works for most VR sensors. But of course I've not tried them all.

This is a pic of what my scope sees on my test rig of a VR sensor and 36-1 wheel - both standard Trigger Wheel units. Might well not look quite so clean on a running engine.

The upper trace is 5v per division. The lower 1v.

The above all refers to a V3 PCB. Other versions can have different pots, or non at all.

Spongo · Post by **Spongo** » Tue Jul 07, 2015 7:53 am

Battery is good I did all the earths etc when i put it back together but maybe I didnt do them very well?

Dave I dont have access to an ocilliscope so but the tooth logger was perfect when it was running.........

Think I will go through all the earths and major connections again to make sure, as its in a Ranger Rover P38 you can bet that there are some connections that are a bit rough

Spongo · Post by **Spongo** » Wed Jul 08, 2015 10:36 am

OK poked around with the trusty voltmeter and found one of the new earth straps has a dodgy connection, so will replace and hopefully thats another issue sorted.

I wonder what the next one will be