Trigger misses 2500 rpm / cranking LM1815

[mauser240]

Well, it has been discussed a couple of times but on this I didn't find nothing.

Here's the story: Volvo 940 turbo on MS1extra v2.2, Volvo standard VR-sensor & 60-2-flywheel , LM1815 (msextra-circuit).

In the beginning I got random trigger misses @ about 2500 rpm. With or without load. Otherwise running well.

Somebody told me to try a 10k resistor in series with the VR but the car would not start with that one mounted. I soon found that 4.7k was about the resistor to go with, except, the cranking (ignition) went nuts. The engine now (especially when cold) fights back the starter a couple of times before the car starts and runs.

Smaller resistance = better cranking = worse cruising.

Any advice what to try next? I thought of by-passing the resistor with a relay during cranking but I'd rather fix the problem without adding more moving parts.

Thanx!

[slow_hemi6]

On my ford 36-1 I had to get rid of the 300r resistor and add in a 10K series with the 18k. I run with pin 14 grounded as well. How do you have the lm1815 output connected to the MS?
You really need to scope the signals to know what is going on. The reason you add the extra series resistance is due to the VR voltage getting too high with increasing engine revs and overpowering the lm1815's input. The data sheet says it needs to be less than 3mA max IIRC. The problem with adding series resistance to reduce the peak voltage is it is still there when revs/voltage is low and then the input can fail to trigger. Non mechanical ways around the issue, I might maybe try series resistance to suit cranking with voltage clipping the negative 1/2 cycle perhaps with bias, depending on reliability of zero crossing triggering.

[mauser240]

Hello!

Now ended up with the input shunt removed, 18k + 3k3 on input, pin 14 grounded. The output goes to U4 pin 6 with 4k7 pull-up. Conditioner board in MS box.

Car now syncs faster on cranking but the trigger misses still occurs at 2200 - 2500 rpm (maybe at 5000 rpm as well but I haven't been spending much time up there). I still may be trying more resistance on the input but I'm having a hard time believing that the input amplitude rises to high as the problem only occurs at cruising revs. Seems like the conditioner "adapts" to the false trigger as I usually get several misses once they occur but it can take several seconds in the critical rpm range before they occur.

I hate to tell you but there is no scope and the last sentence went pretty much un-understood. In other words, trial and error at it's worst...

[mauser240]

Seems like I found something:

"I have read on these forums somewhere where a 100K (I think) resistor was placed between pin 7 (peak detect) and the 0.33 cap/1M resistor. The intention was to reduce the adaptive arming effect."

Will be testing soon.

[mauser240]

No success. The last trick just moved the problem all over the place...

I kind of gave up and ordered a MAX9926 unit. Hope that will be the fix.