MPS Inductance Curves and Tuning, Is Fuel Injection Corp USA Selling Rebuilt MPSs with Bad Transformers? Options

[Superhawk996]

During this troubleshooting I would disconnect both advance and retard on the dizzy if you haven't already. Plug the ports on the TB of course.

(IMG:style_emoticons/default/agree.gif)

Let there be static advance + centrifugal advance only without a vacuum canister compounding things based on vacuum and changes in throttle position.

[Superhawk996]

Fair enough on the resistance thing. I doubt a few dozen milliohms is the issue, but it's not much trouble to run a temporary wire to battery ground.

And the ignition theory is interesting. Although, wouldn't that be more RPM dependent than throttle position dependent? Like not enough dwell to charge the coil? I guess the advance is moving up a few degrees when manifold pressure goes to 0, so I could test with the vacuum retard hose disconnected and see if anything changes.

But otherwise, the timing is set to the notch at 3000rpm - once I got the mixture richened up on the MPS it gave me the full rev range so I was able to tune it no problem.

The thing is that your engine ground also is the path by which the coil draws current.

I’m going to use an extreme example - let’s say your initial statement of 1.2 ohms between batt negative and engine case was true.

The Bosch OEM coil has an internal primary resistance of 3-4 ohms.

If you add an additional 1.2 ohms in the ground between battery negative and the engine ground, the coil draws much less (~25% less) current and you get a much weaker spark. Could be enough spark to work at lower rpm and proper mixture but when rpm’s rise and mixture is off, you get misfire. Again not saying this is your problem, but it is why good ground matters.

Now if your coil happens to be a VW bus style mount that mounts the coil off the fan shroud housing and that has 15 ohms of resistance from shroud to battery ground, that could be a real problem. EDIT: coil has its own ground (not chassis grounded) - ground path still comes though engine case via points and dizzy to engine ground. So floating ground to shroud wouldn’t be so much of an issue for the coil.

The other thing that grounds through that fan housing is the alternator. Powder coating or painting the fan shroud without cleaning the mounting points to the engine case can cause this type of grounding issue to the fan shroud. Not sure what your situation might be causing that high of a resistance. I would want to understand this a little better to understand why that should is so far from engine ground.

Not trying to run you around endlessly on electrical hunches but they can’t be ignored when you have the unexplained issues you’re having. Even more so knowing that the car was once in a fire and knowing that fire history cars usually have unexplained wiring and ground issues.

[emerygt350]

When I was troubleshooting my 914 I ran grounds from everywhere to the battery negative. Shroud, block, transmission, etc. After everything was working great I removed them one by one.

[worn]

I have examples of FIC diaphragms in my office. They are hand made from brass sheet, soldered to trimmed used hubs. They don't appear to have good flexural properties and also look like they are difficult to seal.
My diaphragms are precise reproductions of what Bosch used, but I have to be very careful pressing the hub onto the paper thin disc or I risk stretching the center, which changes how they respond to vacuum signals.
I recently ordered more hubs but I will need to order another batch of diaphragms later this year. It takes a long time to use up one order and sales so far this year is lagging previous years. It will probably be my final order of diaphragm pressings.

Thanks Chris. You have done us a great service. I can imagine diaphragms made from roof flashing, with or without copper, being sold as replacements. You kept that from happening.

[ChrisFoley]

Here's the latest news:
I ran out of diaphragms last month. My supplier of Be-Cu stampings isn't going to make them for me anymore. It was 8 years since my last order of 250 pcs.
I may not have any new kits available for the remainder of 2024, until I work out a new plan. Right now I'm looking into an alternate material created to replace Be-Cu with a non-toxic alloy.
The material I found is also a copper alloy, with similar flexural properties after heat treat.

[emerygt350]

That is too bad. I hope you can find a new alternative.

[JeffBowlsby]

Here's the latest news:
I ran out of diaphragms last month. My supplier of Be-Cu stampings isn't going to make them for me anymore. It was 8 years since my last order of 250 pcs.
I may not have any new kits available for the remainder of 2024, until I work out a new plan. Right now I'm looking into an alternate material created to replace Be-Cu with a non-toxic alloy.
The material I found is also a copper alloy, with similar flexural properties after heat treat.

Rats on the former supplier bagging out.

Anxious to see what you come up with for a replacement.

[rjames]

Here's the latest news:
I ran out of diaphragms last month. My supplier of Be-Cu stampings isn't going to make them for me anymore. It was 8 years since my last order of 250 pcs.
I may not have any new kits available for the remainder of 2024, until I work out a new plan. Right now I'm looking into an alternate material created to replace Be-Cu with a non-toxic alloy.
The material I found is also a copper alloy, with similar flexural properties after heat treat.

Bummer. I really appreciate you making these in the first place. I have 2 MPSs rebuilt with your diaphragms- one in the car, and one as a spare. Even though I don't need another diaphragm, I'd happily preorder one or two if it helps make a new batch happen.

[davep]

Chris, I hope that you can get a new supply of diaphragms made to proper specification.
To OP, You may need to either get new meters or your current meter calibrated. There is little more frustrating than trusting a device that gives erroneous readings. You also need to make a list of your assumptions, and verify that they are true. Best is to have someone else do the verification; nothing worse than making the same mistake over and over, and I have learned that fresh eyes do make a big difference.

So, I did just read through this from start to finish. Your ground readings are a particular concern. You need to short the probes and get a perfect zero reading before starting. If you are tracing grounds all over the car, then you may need to have a 15 foot test lead of 18 AWG wire with a plug end suitable for your meter, and a ring connector for the other that you can clamp onto the battery post connector. I have found that I cannot trust the ground stud on the engine compartment wall above the battery. While a meter test may indicate a low resistance, when starting the car and 200 to 300 amps are flowing through that ground stud the true resistance may be much higher! Remember that the high current paths are the most important; from alternator to starter solenoid to battery positive on the positive side, and alternator through fan shroud, through engine case, through transmission case, through trans ground strap and body stud, through the body and body stud, and through the battery ground cable to the battery. This ground path is complicated, and every transition from one conducting part to the next connecting part has contact resistance in addition to the bulk resistance of each conductor. I rarely find an alternator harness that is healthy; I replace them all. I found on one car that the battery wall stud was not healthy so I connected a new ground cable to the fan shroud where a suitable 8mm hole was located; problem solved with a shorter and more robust ground path. Don't forget the ground stud near the relay board on the left side of the engine compartment; up to 8 ground wires depend on this point. The grounds and positive connections may have nothing to do with your problem, but it may not hurt to eliminate these potential problems to create a proper baseline. Also, try to compare your meter with those of friends. Find some test parts (inductors say, in the range you are trying to measure) and use them to help calibrate your meter.

[914_teener]

Chris, I hope that you can get a new supply of diaphragms made to proper specification.
To OP, You may need to either get new meters or your current meter calibrated. There is little more frustrating than trusting a device that gives erroneous readings. You also need to make a list of your assumptions, and verify that they are true. Best is to have someone else do the verification; nothing worse than making the same mistake over and over, and I have learned that fresh eyes do make a big difference.

So, I did just read through this from start to finish. Your ground readings are a particular concern. You need to short the probes and get a perfect zero reading before starting. If you are tracing grounds all over the car, then you may need to have a 15 foot test lead of 18 AWG wire with a plug end suitable for your meter, and a ring connector for the other that you can clamp onto the battery post connector. I have found that I cannot trust the ground stud on the engine compartment wall above the battery. While a meter test may indicate a low resistance, when starting the car and 200 to 300 amps are flowing through that ground stud the true resistance may be much higher! Remember that the high current paths are the most important; from alternator to starter solenoid to battery positive on the positive side, and alternator through fan shroud, through engine case, through transmission case, through trans ground strap and body stud, through the body and body stud, and through the battery ground cable to the battery. This ground path is complicated, and every transition from one conducting part to the next connecting part has contact resistance in addition to the bulk resistance of each conductor. I rarely find an alternator harness that is healthy; I replace them all. I found on one car that the battery wall stud was not healthy so I connected a new ground cable to the fan shroud where a suitable 8mm hole was located; problem solved with a shorter and more robust ground path. Don't forget the ground stud near the relay board on the left side of the engine compartment; up to 8 ground wires depend on this point. The grounds and positive connections may have nothing to do with your problem, but it may not hurt to eliminate these potential problems to create a proper baseline. Also, try to compare your meter with those of friends. Find some test parts (inductors say, in the range you are trying to measure) and use them to help calibrate your meter.

(IMG:style_emoticons/default/agree.gif)

Great post.

I'd add;

Resistance to ground on all grounding points should be 0 ohms. Those grounding points are on the factory wiring diagrams. Harness condition is key to a well functioning EFI system whether it's D-jet or other EFI systems.

The post demonstrates the value of a good testing procedure(s) to avoid chasing your tail.