Testing the VB921

[mario_82_ZXT]

We just finished wiring a friend's car yesterday, and it's my first MS-I v3 install with the VB921.

 

During cranking we see RPM and both LEDs flash (and so does the spark output box in MegaTune) yet we are getting no spark. It is wired up like the DIYAutotune schematic as we are using the VB921 to directly drive the coil.

 

I have triple checked the wiring, spark inversion, dwell, everything, and we still have no spark. Is there a way to test the VB921? The only thing I have gathered so far is to get a voltmeter and flick the ecu on and make sure it's reading above 0.5v.

 

Is there any other tests? (Like the gate test for the HEI module?) If we can't get this to work I'm installing the spark through FIDLE and using an HEI module like the v2.2 install.

 

Thanks,

Mario

[mario_82_ZXT]

Oh yea the box was a premade MS230-C from DIYAutotune, and we had to reflash it (has 029v on it now) before it would communicate with my laptop.

 

Mario

[naviathan]

Use a stim. That's how I knew my VB was bad. Well that and the smell. It wasn't until I pulled it off that I saw the cracks in it.

[Careless]

i used artic silver thermal paste to get the most out of the cooling fin at the top of the case/board. I also noticed that on the case i have, the cooling channel does not butt against the cooling bar on the board, thus not allowing proper heat dissipation.

 

possibly one thing to help you along your way.

[mario_82_ZXT]

Hooked up an o-scope to the Pin 36 cable disconnected from the coil (this is the wire that is supposed to trigger the coil) and I get a square wave with the magnitude of 3.5v. So I think the VB921 is working right.

 

When it is hooked up to the coil I get 12v with it in the "on" position and a drop to about 9.5v when cranking and no wave. Is it dropping down too much? Still no spark, I'm going to try hooking up the FIDLE circuit to an HEI module and see if we can get it to run like that.

 

Mario

[mario_82_ZXT]

Also, what fixed duty should the dwell be set to when using the GM HEI module?

 

Mario

[mario_82_ZXT]

Found it, 50% fixed duty to run the HEI.

 

Mario

[z-ya]

You don't want to run in fixed duty cycle mode with the VB921. The input pulse width is the dwell, so if you are at 50% duty cycle, the VB921 will be conducting current through the coil half of the engine cycle. At idle, it is going to get really hot.

 

If you have a scope, the input to the VB921 should be a positive 5V pulse with the width being close to the dwell you have set in MT. On the output of the VB921 you should see a negative pulse, again close to the dwell time you set in MT. It should go from 12V to close to zero for the dwell time.

[mario_82_ZXT]

You don't want to run in fixed duty cycle mode with the VB921. The input pulse width is the dwell, so if you are at 50% duty cycle, the VB921 will be conducting current through the coil half of the engine cycle. At idle, it is going to get really hot.

 

If you have a scope, the input to the VB921 should be a positive 5V pulse with the width being close to the dwell you have set in MT. On the output of the VB921 you should see a negative pulse, again close to the dwell time you set in MT. It should go from 12V to close to zero for the dwell time.

 

The fixed duty cycle was for running a 4 pin HEI module, not the VB921.

 

By input and output of the VB921, do you mean inside the case? What would pin 36 be considered (this is what is plugged into the neg side of the coil and is supposed to control it). Like I said the o-scope was showing a square wave magnitude 3.5v when cranking with the coil disconnected. With the coil connected I couldn't see anything, just a drop from 12v to 9.5v, but the automotive o-scope doesn't have as high of resolution as a "real" one.

 

Anyways, I wired the HEI module using the FIDLE circuit (like oldschool ) and car now fires up and is timed. Now for tuning...

 

The HEI seems to get quite warm though, I need to get my Z running and compare it.

Mario