Interfacing with GM HEI module

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
I thought up a little project a while back... microcontroller controlled spark advance and dwell. What I need to do is use the tach signal from the HEI module to do all the calculations. What I'm finding is the module itself does not seem to put out a tach signal (at least that I can measure with my meter). I am assuming I need to use a pull up resistor on the tach line, but wanted to see if anyone knew for sure before I try anything more (I kind of cooked the project trying things already).

I had the whole setup put together and working by feeding a 5v pulse to the + side of the vr sensor on my table. It even put out it's own tach signal then, but once installed and connected to the distributor I lost the tach signal, but the engine does run.

Just to shed some light on the mechanicals it's a Durspark to HEI swap by means of chucking the Duraspark box and wiring a HEI module in it's place. I'm kind of hoping to improve on the idea a little...
 
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Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
Are you checking output on DC or AC? Pretty sure that the output is a square wave like signal but should be check on AC with a multimeter. Post #12 in this forum gives a few screen shots of the output on a scope. https://www.thirdgen.org/forums/electronics/462759-what-does-tach-signal.html
I saw those shots earlier while digging around and from what I could tell they are from the tach pin on the distributor itself which I don't want to mess with since it's just a connection to the negative side of the coil. I'm actually wanting using the signal from the 8 pin HEI module itself which to my understanding is based on a LM1815. I'm just unsure of how it is connected inside the module itself which is where I believe my problems are. It should be a sqaure wave DC output as far as I can gather from everything I've read do far... but something else seems to be going on.
 

shortbus

Joined Sep 30, 2009
8,317
Sorry you didn't state that in the first post. So thought you were using the 4 pin module. Have you looked on the Megasquirt site? lot of stuff there.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
Sorry you didn't state that in the first post. So thought you were using the 4 pin module. Have you looked on the Megasquirt site? lot of stuff there.
Actually I edited the original a few times, but still forgot that detail. The whole part of cooking the project is more or less self inflicted. The old saying haste makes waste rang loud and clear!!

I've been researching the Megasquirt off and on for some time. It's been a good source of ideas for all sorts of things not even related to it in ways. I thought I saw somewhere on the site that the factory ECU has something around 5 k - 5.6 k pullup on the tach signal. I'm more or less looking for a yay or nay to the pull up.

It still doesn't explain why it worked on the table and quit after install though. The only things I can come up with are: The PIC itself was creating enough of a pull up of sorts that it triggers the interrupt, and since the incoming power isn't as lovely as the power supply on the table it won't work real world without the pull up or whatever it needs.

Sound sensible? Time to dig around Megasquirt some more...
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
I decided to take my meter out to the garage and poke around. I did find the reference (tach) pin on the module does output a voltage. With my meter I was finding a range between .5 volts up to around 3 volts at an idle (min / max setting). I tried to get a frequency reading on both the reference signal and the vr sensor itself, but not coming up with usable numbers for either. The frequencies do match between the two points though. I'm guessing it's not putting out the perfect sqare wave I had imagined it would, but rather some sort of variation of the vr input itself.

I see the latest Megasquirt is using the MAX9924 so I think I know what needs done.
Would the NCV1124 be a good choice? It's cheaper and in a more manageable package for a home made board.

Thanks to all who took a look!!
 
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shortbus

Joined Sep 30, 2009
8,317
I can't give you an answer on the chips, I'm just a hobbyist and not into what your doing. The output from the HEI is probably a sine wave DC not square wave, so measuring on the AC scale of the meter would be the way to go.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
I can't give you an answer on the chips, I'm just a hobbyist and not into what your doing. The output from the HEI is probably a sine wave DC not square wave, so measuring on the AC scale of the meter would be the way to go.
Thanks shortbus. I too am just a hobbyist, but thought this would be a fun project. Your thought on the wave is kind of what I started to think last night (I'd love to have the extra bread laying around to blow on a scope). I guess I was kind of hoping somewhere along the line someone else played around a bit and could shed some light on how they work. It's also going to be an improvement when I finally figure it out. I've spent a good deal of time bending tabs, tweaking springs, and adjusting vacuum canisters only to come close a few times, but never seem to get the timing exactly where it wants to be... I figured a computer would take a lot of the guess work out of adjusting.
 

shortbus

Joined Sep 30, 2009
8,317
Pretty sure to do something this complex a scope would be a minimum tool to figure out what's actually happening. Don't know if the DUI (Davis unified ignition) site would have something helpful. They had a product for Ford distributors that used a HEI module.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
Pretty sure to do something this complex a scope would be a minimum tool to figure out what's actually happening. Don't know if the DUI (Davis unified ignition) site would have something helpful. They had a product for Ford distributors that used a HEI module.
But it's soooo much more fun guessing and pulling my hair out!! I have a few MAX9925 chips to play with. I tried one circuit, but after looking around Megasquirt some more and seeing their circuit and values used with the 9924 chips they use I found I might have used the wrong resistors in mine (more or less the same, but I bought the complicated version). I just want a working tach to start with until it proves reliable.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
I believe I will do that in the end... as for how detailed it gets I'm not sure.

So far I started to pull things apart to see what survived the accident and found I didn't loose as much as I thought. So far with the testing I've done it seems all inputs and outputs work like they should. I can just move some programming around for now and have it up and running again in a couple hours... then swap out some resistors on another board and cross my fingers.
 

dbc105

Joined Oct 25, 2017
5
Any progress made on this project? I have put a 4 pin HEI module triggered by a Ford pickup with a reluctor with only 2 points still on it and firing a Dynatek .5 ohm dual fire coil on my 1972 Suzuki T500 vintage road race bike. I would like to use a 7 pin and a micro controller to control the timing curve. Right now it is running at fixed timing.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
Any progress made on this project? I have put a 4 pin HEI module triggered by a Ford pickup with a reluctor with only 2 points still on it and firing a Dynatek .5 ohm dual fire coil on my 1972 Suzuki T500 vintage road race bike. I would like to use a 7 pin and a micro controller to control the timing curve. Right now it is running at fixed timing.
Just found your message... Unfortunately I haven't worked with it since I last posted. Currently I'm having issues with noise triggering the vr circuit. I also have plans for a speedo and have the board connected to that sensor at the moment with the same results. I have plans and parts for another try at it. I know it will work if I can cut out the noise... it worked beautifully sitting on the table with a clean power supply!!
 
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dbc105

Joined Oct 25, 2017
5
Just found your message... Unfortunately I haven't worked with it since I last posted. Currently I'm having issues with noise triggering the vr circuit. I have plans and parts for another try at it. I know it will work if I can cut out the noise... it worked beautifully sitting on the table with a clean power supply!! Something tells me the tach signal from the HEI would work if I can clean up the noise.
Cool, I don't know how I can help but I might can clone what you are doing and maybe help in testing or something.

By all means, keep me posted.

Thanks,
DC
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
Cool, I don't know how I can help but I might can clone what you are doing and maybe help in testing or something.

By all means, keep me posted.

Thanks,
DC
I'll see if I can find a little time this weekend to draw up what I have so far. I'm thinking the problem has to do with the incoming power and converter I'm using to knock it down to five volts, or possibly the fet used to switch the whole thing off and on. I have a couple chokes to try, and a small relay to replace the fet... just have to get the motivation to do it.
 

dbc105

Joined Oct 25, 2017
5
I'll see if I can find a little time this weekend to draw up what I have so far. I'm thinking the problem has to do with the incoming power and converter I'm using to knock it down to five volts, or possibly the fet used to switch the whole thing off and on. I have a couple chokes to try, and a small relay to replace the fet... just have to get the motivation to do it.
LOL, I can give that. I would like to have something on my '68 Plymouth RoadRunner. I have Chrysler electronic on it now but I'm stuck with the mechanical curve. My race bikes have no timing curve right now. I raced this weekend and the HEI ignition sure works good.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
594
LOL, I can give that. I would like to have something on my '68 Plymouth RoadRunner. I have Chrysler electronic on it now but I'm stuck with the mechanical curve. My race bikes have no timing curve right now. I raced this weekend and the HEI ignition sure works good.
Kind of got a chuckle... back in my younger days our neighbor had a RoadRunner that was pretty quick. There used to be a wrinkled patch of asphalt in the road in front of the house that I thought was like that from too many times of being lit up. My dad has a 69 Dart that I have many memories of as well. He could jam gears as fast as an automatic if not quicker. I've heard he was a tough one to beat, but I'm sure it did happen time to time (I just never witnessed it). It took me several years to understand the bridge to the next side road was a quarter mile where several races took place (I finally figured it out when I got my license).

At any rate... how is your electronic knowledge? How about programming? Are you looking for something that works, or do you have the tools and such to troubleshoot? If you have a points type distributor you could lock out the advance and toss it in that would be the easiest in my opinion. I'm sure what I have put together so far would work properly with a little fine tuning and points to trigger it. I wanted to try that myself, but having an old Ford FE engine doesn't leave me with a lot of performance options and I'm not buying a new reman distributor after reading the crappy reviews I've found. I still want to get this working even though the old truck will probably fall apart before I get it done (sooner of later the cab will end up looking like a quilt from being patched).
 

dbc105

Joined Oct 25, 2017
5
Kind of got a chuckle... back in my younger days our neighbor had a RoadRunner that was pretty quick. There used to be a wrinkled patch of asphalt in the road in front of the house that I thought was like that from too many times of being lit up. My dad has a 69 Dart that I have many memories of as well. He could jam gears as fast as an automatic if not quicker. I've heard he was a tough one to beat, but I'm sure it did happen time to time (I just never witnessed it). It took me several years to understand the bridge to the next side road was a quarter mile where several races took place (I finally figured it out when I got my license).

At any rate... how is your electronic knowledge? How about programming? Are you looking for something that works, or do you have the tools and such to troubleshoot? If you have a points type distributor you could lock out the advance and toss it in that would be the easiest in my opinion. I'm sure what I have put together so far would work properly with a little fine tuning and points to trigger it. I wanted to try that myself, but having an old Ford FE engine doesn't leave me with a lot of performance options and I'm not buying a new reman distributor after reading the crappy reviews I've found. I still want to get this working even though the old truck will probably fall apart before I get it done (sooner of later the cab will end up looking like a quilt from being patched).
I have basic knowledge I guess you can say. I have built a few simple circuits but nothing very complicated. One of the things I've been thinking about is controlling the A/C functions on my RR with maybe an Arduino. My care has factory air but the heater core is bad, the condenser doesn't look any better and for the cost of replacing them I can go to after market but I want to use my existing A?C control switch cluster on the dash. Since systems like Vintage Air now have electrics controlling the dampers like modern cars do I think this can be done but that would be another discussion. As to the ignition I have on the car it is a MOPAR Electronic. with a mid '70s dist that still has mechanical and vacuum advance. I have plans to replace the MOPAR module with a GM module and put it under the fender well so it just looks like points under the hood and also eliminate the ballast resistor. You said you can trigger off a set of points, is it possible to trigger off the tach signal post on the 7 prong HEI module? Best I understand this is a square wave and the timing function is mostly nothing more than that giving a variable band width to this signal and sending it back to the module while including dwell time in the signal. Since my car has a magnetic pickup that is what I would have to try to do. My motorcycle is the same way. I am a IBM I programmer, I've not done a microcontroller but it should not present a problem, just some studying. The part I would have the hardest time with is the circuits themselves. When/where to use capacitors and resistors and what values to use, etc..... Or even which micro would work better for which functions. I do know for this it would need to have a pretty decent clock speed.
 
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