MS1 PCB3.0 Fuel only - no RPM from stim

[toyotaspeed90]

I am not new to Megasquirt... however I haven't had an MS car in about 6 or 7 years.

I had a 1987 MR2 4agze on MS1 V2.2 over 10 years ago - it was on EDIS. That is when the stim (V2) was built. I recall there being lots of firmware issues back then...

I had another 1987 MR2 that had a variety of engines in it, MS1 V3.0 - it was also on EDIS. I recall there were many issues with firmware and I couldn't get an RPM signal, and I had to find an older firmware (even for then) to get it to work. That was 024s9a.

Here's where things get different.

I have a 1971 Landcruiser with a late 70's 350 SBC, MSD Dizzy (vacuum advance) and quadrajet. I was planning to MS the rig in stages - starting with fuel only. This is meant to be a super simple setup with a TBI unit and using the existing dizzy. (there are plans in the future to go HEI, but for now that isn't the goal, and want less down time of the vehicle).

I built my (new to me) MS1 PCB3.0 this weekend - and there didn't seem to be any issues. The preliminary tests based on the instructions guide it passes everything. I followed the "fuel only" -ve instructions to a T (the only exception being the zener diode to put in backwards was a 22V unit from DIYAutotune, and I saw in other posts this is what they've chosen instead of the 36V unit) and double checked them at least 4x. I was careful in polarity of all pieces that are polar-based.

The unit powers up, the unit registers all sensor inputs from the stim, the FP light on the stim kicks on when initially plugged in. Can fetch/burn to/from the ecu, and have changed the settings in Megatune as advised.

I can't get the older firmware to load (I get an error that states "The system cannot find the path specified" and when I click to continue, it shuts down). I can, and have, successfully loaded both the 028y4a and the high-res - which both are responsive in Megatune and the load doesn't have a problem. However, no ignition events....

I can't find anything about the stim not working to send a negative coil signal only... but I haven't really seen what it mentions. I know the stim hasn't been used in quite some time, but I can't imagine that's the problem. I do plan to go forward and start to wire the DB37 separately in the vehicle to see what I get there... but, I want to see what others have to say first.

(I haven't attached an MSQ because at this point it is literally what is pulled in with the firmware, and the only change being MSNS turned off).

**Edit** - I am using the same (read: ancient) 'laptop' that I used for all other MS projects I've had, using serial 9 directly, COM1. (If anyone cares, it's a weird old Fujitsu Sylistic 3400)

[slow_hemi6]

To test rpm with the stim you will probably need to put a temporary jumper across the zener diode and another temporary jumper across XG1 and XG2.

[toyotaspeed90]

Since there's no RPM signal there are no injector outputs...

I haven't read others having issues with fuel only setups on a stim...

[slow_hemi6]

It's in the manual so that might be why there are few posts with issues around this.

If you have no rpm, it may be because the stimulator may not have the voltage to 'overpower' the D2 diode, which must be 'jumpered' to test MegaSquirt® on the stimulator. This diode (D2) is needed if:

Your ignition system has a large offset bias - most systems do not have such a bias.
You are triggering directly off the coil negative terminal.

So, to start, you can either solder in a jumper wire in this location, or, you can install the diode D2, and then install a jumper around the two leads of the diode - in effect shorting it out. The latter will allow you to snip the jumper later on if needed, putting the diode back in circuit. (Note that D2 should not be installed for a Hall or optical sensor, it should only be installed if triggering off the coil negative terminal.)

[toyotaspeed90]

Thank you so much!

Unit is working flawlessly... just wanted to verify it worked on the bench before I started to throw it in the truck.

The build instructions could possibly use an update. I figured the zener installed backwards was being used against its purpose - which is to effectively block the signal... especially since mentions of 22v and 36v didn't matter. If the build instructions said "For testing on a stim...." that could help others.