mobythevan Posted July 23, 2007 Share Posted July 23, 2007 Have you ever heard of a VB921 actually blowing? What reason could it have done that? This seems to be the easiest thing to blow up on a megasquirt. Dwell settings and spark output inversion settings are the two major reasons for blowing these up. I didn't see dwell settings in the pictures above. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2007 Author Share Posted July 23, 2007 Max dwell is set at 3.6, I think maybe I need to bump that down some. I got the settings from someone elses files. I've been talking to Matt through email and I think I might have this figured out. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 23, 2007 Share Posted July 23, 2007 Max dwell is set at 3.6, I think maybe I need to bump that down some. I got the settings from someone elses files. I've been talking to Matt through email and I think I might have this figured out. 3.1 MAX!! Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2007 Author Share Posted July 23, 2007 Yeah, I'd say that's probably what blew it then. Thanks cygnus, I'll drop that down tonight and hopefully i'll have my new VB by Thursday at the latest. Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 23, 2007 Share Posted July 23, 2007 The difference between 3.1ms and 3.6ms will not blow the VB921, especially if you are just idling the motor. You need high RPMs (duty cycle) to get the VB921 hot if all settings are close (I've run a VB921 on the test bench overnight at 7000RPM, and it got quite hot, but never burned up). Chances are, the output is not inverted, so the VB921 is sourcing current through the coil most all of the time. Check your settings and wiring again. If I had an MS-II, I would hook it up in my lab and confirm that the output is inverted with a scope, but all I got is MS-Is. If you turn on the ignition, and don't start the engine, the VB921 should never get hot. If it gets hot when the engine is not running, the output is not inverted. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2007 Author Share Posted July 23, 2007 I need an O-Scope so I can check that. If it's not th dwell that overheated the VB then what the hell could it have been? I've considered maybe the jump box did it. It didn't have any problems until we hooked that up. Think that's possible? Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 23, 2007 Share Posted July 23, 2007 In your msq that I looked at above, the setting was Going High, Inverted which is correct for the car to run....AFAIK. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 23, 2007 Share Posted July 23, 2007 Since I know how much everyone loves eye candy, here's some window shots for you to peruse... Yes it is set to inverted...I remember specifically looking for that because it's all over the Megamanual in !!!BOLD!!!. Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 24, 2007 Share Posted July 24, 2007 If in fact the output is inverted, then it must be in your wiring of the coil circuit. Hear is the MS-II manual page on wiring your coil: http://www.megamanual.com/ms2/vb921.htm Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 24, 2007 Author Share Posted July 24, 2007 I know, I haven't changed that setting which is what confuses me about the VB burning up. The dwel wasn't that high and the spark is inverted. The only other thing I can think of it being is the jump box. I have a pull up resistor on the VR trigger and it might have over loaded with the pull up. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 24, 2007 Author Share Posted July 24, 2007 My IAC stepper motor has a spring on the pintle. I can easily turn it in but when I let go it springs back out. Which stepper motor setting would this be? By the time this thing is running I should be a MS expert... Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 24, 2007 Share Posted July 24, 2007 Take the IAC stepper motor off, and just adjust the idle screw so that the engine gets enough air to idle. After you get all the bugs worked out, you can revisit the IAC control. Too many variables in an equation makes it very difficult to solve. Hear is what I would do: - Make sure ignition coil is wired correctly to VB921 - Get a stable and reliable spark - Verify crank timing with timing light - Start engine. - Adjust idle screw on TB so that it idles - Adjust required fuel to get smooth idle - Verify that timing in the timing map matches the actual timing (with timing light). Change trim angle in spark settings so that actual timing matches the timing in the timing map - Put the IAC back on and debug that. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 24, 2007 Author Share Posted July 24, 2007 That's good advice. Get it to idle with the throttle plate cracked then work on getting it to idle with the stepper. Thanks for that tid bit. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 25, 2007 Author Share Posted July 25, 2007 I know now what blew my VB921. The secondary coil windings on my coil (negative to center) ohmed out at 17.2k, 5k more than the max recommended. I guess all the cranking pushed the old coil too far. I have a good one in from a parts car. Tested out good by the FSM specs. Once I get my new VB I should be good to go. Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 25, 2007 Share Posted July 25, 2007 A higher resistance coil will draw less current and there fore is less likely to damage the VB921. Unless you damaged the coil when blowing the VB921. Double check your coil wiring before you put anew coil and VB921 in there. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 25, 2007 Author Share Posted July 25, 2007 Coil wiring is fine. Pin 36 and stock tach wire on negative and the stock ignition power wire is on positive. Should be good. With the nature of the VB though, if it's having to push the charge because of too much resistance then it might overheat that way. Just a theory. Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 25, 2007 Share Posted July 25, 2007 Coil wiring is fine. Pin 36 and stock tach wire on negative and the stock ignition power wire is on positive. Should be good. With the nature of the VB though, if it's having to push the charge because of too much resistance then it might overheat that way. Just a theory. Actually, it won't. I = V*V/R The higher the resistance, the lower the current, the less power. My point is that the damage may have been done to the coil by the VB921 before the VB921 blew. Disconnect pin 36 from the coil when you first power up the MS with the new VB921. Measure the voltage on pin 36, it should show a positive voltage. If it is very close to zero volts, you have a problem, and you will most likely burn out another coil and VB921. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 25, 2007 Author Share Posted July 25, 2007 Will do, thanks for that info and everything else as well. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 26, 2007 Author Share Posted July 26, 2007 Actually, it won't. I = V*V/R The higher the resistance, the lower the current, the less power. My point is that the damage may have been done to the coil by the VB921 before the VB921 blew. Disconnect pin 36 from the coil when you first power up the MS with the new VB921. Measure the voltage on pin 36, it should show a positive voltage. If it is very close to zero volts, you have a problem, and you will most likely burn out another coil and VB921. So does ~3.3V count as a positive voltage that's not too low? I checked it on the stim (which is fed by a 12V power supply) and that's what I got. Quote Link to comment Share on other sites More sharing options...
z-ya Posted July 26, 2007 Share Posted July 26, 2007 Yes, that is fine. As long is it is not below 0.5V or so. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.