Rebuilding the 308 electronic ignition (SM805A distributor, Marelli AEI200 Ignition)

OMG, LCS, you have no idea how much I appreciate a response! This distributor has been sitting in the corner of my office (and the car crippled in the garage) for 1-1/2 years now because I could not for the life of me figure out how to test or properly gap the VRs. THANK YOU !!!

OK, separately:

1. NO ONE should copy my moronic test setup. I have the distributor driven by an electric drill(!) which conveniently runs at 600rpm. I'm testing, trying to use an oscilloscope for the first time in my 76 years of being alive, using a $50 pocket device I got off EBay because I now live in a small redneck Texas town ... which I love but which has no local professionals of any kind.

2. The 2 images are from 2 separate tests using the same probe; not simultaneous. So, I certainly might be using this all wrong (the little instruction booklet is in Chinese-English and pretty useless). But the test setup is the same in both.

3. Yes, I see the twitch in "rear VR" as it crosses zero but have no clue if it's meaningful.

4. HERE IS THE BIG ONE:
How the heck are you even measuring (physically) the gap???? THIS is why I've been stopped for 18 months. There isn't a way to get a feeler gauge inside the distributor with the top & bottom plates in place! The shaft is captive on the lower assembly, and using just one of the 2 bearings yields a shaft with too much play to measure anything (can easily push the shaft to actually come into contact with one VR and then push in the other direction and do the same in the other direction). No way to measure anything! But, apparently, you were able to measure the gap and know it was 0.2 or 0.4 ... so you figured out how to do so. Would you mind sharing, because I've been stumped on this simple issue for a very long time.

 

@308inTex In reply to your post:
1/ I don't think there's anything wrong in the drill approach, crude as it maybe. There was another thread on someone doing something similar a while back - it gives you a repeatable means of testing/setting up and perhaps easier/quicker than working on the car.
2/ yeah, you don't want to connect both pickups at once, even if you did have a dual channel 'scope - the pickup outputs/module inputs need to "float" and if you did connect on the same device there would be a common path between the ground connections.
3/ Pretty sure you need to track down what's causing the glitch - in the extreme, the module could erroneously energise the coil briefly, then energise again proper. Swapping the modules/coil packs is an easy first step to narrow things down. You should be able to check what's happening by just scoping on the coil negative - beware though that your scope probe should be a x10 type as you'll be looking at potentially a few hundred volts and this could otherwise overload your scope input on a normal x1 probe. Comparing each bank may well show a difference - you only want to see the signal go down to ground once per cycle as the coil energises and then a subsequent single large spike as the coil switches off for the spark.
4/ ahh, good point - I'm actually one of those rogue FIAT-Lancia twin-cam people who happen to have the same pickup and coil pack as you guys, but without the double bank requirement My distributor is a mere SM801 and you can remove the rotor to get decent access to measure - the technique of just feeling the pickup snap is the way to go.

 

Man, again, THANKS for taking the time to reply and to offer up your experiences. I REALLY appreciate it.
The traces you saw on mine have nothing to do with the ignition modules. I have the distributor away from the car, attached to a wood plank along with the drill. The trace is directly from the VR lead. The ignition module/coil was sitting out in the garage at the time; not involved.

Ahhhh - so with your distributor you can get a feeler gauge into the thing while it's assembled and check the gap. So THAT'S the explanation. On my distributor (see photo) there are 3 main subassemblies: (1) the bottom plate, which when removed from the distributor has the captive shaft and the centrifugal advance mechanism; (2) the hollow body, into which #1 fits; (3) the top plate which fits into the body from the other end and which, on it's UNDERSIDE holds the VRs (in other words, when the top plate is inside the body you can't see or gain access to the VR's to check the gap. I tried just fitting the top & bottom subassemblies (putting the shaft of the bottom plate thru the bearing of the top plate without the body. Then of course you have access to stick a feeler gauge in and check the gap. BUT - without the body to keep the top & bottom plates perfectly parallel there is far to much "slop" and with very little finger pressure you can make the gap be anything you want simply by tilting the top assembly in relation to the bottom assembly. Easy to even make a VR hit against the reluctor lobe, then tilt the other way and make the other VR hit. I just don't see how on earth to measure anything or adjust anything. I'm not a stupid person, but I sure as heck feel stupid with this. And this is why the distributor (and car) have been sitting for the last 1-1/2 years!
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Been chasing a misfire for a similar period off and on too, and in between other projects. Regarding the air gap between stator and VR sensors, remove the name plate from the disi side (two small screws) and you’ll see a slot machined allowing you to insert feeler blades to set the gap.

Still a process of trial and error. I split the disi and ‘pinched’ the screws securing the sensors (so they would move with light pressure) with a slight interference set between stator and sensors, reassembled and turned the disi gently by hand allowing the stator to move the sensors - insert a feeler blade between the stator and sensor, through the slot underneath the name plate, then turn the disi and it sets the gap for you. Dissemble disi and tighten the sensors screws, reassemble and check. I had to do that a couple of times before it was right.

As earlier in this string, then spin the disi by hand slowly to feel the magnetic resistance - mine was pretty slight when set correctly, but nonetheless there - spin the shaft a few times and let it stop, if your gaps are correct, they need to be equal for both sensors note, the disi should stop in different places and always at a ‘magnetic’ point if you see what I mean.

Word of caution re parts. I discovered lots of niggly faults in my fuel and ignition systems built up over the years, together impacting performance but on their own not significant. But most frustrating of all was fitting new parts only to find later when my patience was almost exhausted by the misfire continuing, that they were faulty and I’d introduced new faults whilst thinking I’d eliminated components by replacing them.

I suffered the amplifiers issues, plus a new rotor arm where the insulation inside it broke down allowing the current to flow down the centre of the disi and not out to the rotor arm contact with the disi cap. I bought another and noted the manufacture of it was much improved (from the same part supplier but obviously sourced from a new supplier to them, or a change in manufacturing process).

I then had the car running perfectly and the best it ever had but only for a few miles. Then a terrible misfire, the rev counter over reading, and soon after the car stopping (after a few mins driving). Leave for a few mins and back to running great, then a repeat a few mins later. I deduced that the ignition system was over firing and sending multiple sparks hence the rev counter over reading.

I then noticed the new distributor cap did not have the three small holes in its body that the original had. Refitted the original cap and the problem is fixed, finally. Without the holes, static builds up inside the disi sending multiple signals to the sensors. I’ve since drilled the holes in the new cap with a Dremel keeping it as a spare. Note this is only needed for our electronic ignition cars, not traditional points setup.

These days we can’t always rely on new bits being correct, keep your old bits until you know what the problem was and are sure you’ve fixed it, then through the offending original bit away.

Good luck, I’d not used the 308 properly for so long, other cars had been restored and when they’re back one tends to use them first, and I’d forgotten just how super the 308, especially in this early two valve carb format, is. Persevere, you have an absolute gem of a car there.

And to the originator of this string, and all the subsequent contributors, you are legends, without this I don’t know how I’d have sorted the problem. What a brilliant forum this is!

 

Yep, BB, I so agree with your last sentence. Unbelievable that people would spend their time with someone they have never met just to help them figure things out. I mean - Adrian started this thread (I think) back 16 or 18 years ago. And here I wander in and .... well, thank you.

Anyway, if you check the 2 photos I posted (just above) the name plate is off but there is no slot to stick a feeler gauge in. Maybe my car is some sort of weird one ... European (I bought it in Milan in 1984), single disty, dual electronic modules. But the body of my disty is solid aluminum; no slots, no holes, nothing. I can just pull the top plate (the one holding the VRs) off and play around, put it back together and pray. Have thought about that so many times, but afraid the VR gap **IS** set correctly and all I'll do is make more problems. But what's that alternative? Just forget about the car? Maybe the VR is bad and I'll just have to end up fiddling anyway. But the factory HAD to have some way of actually setting the gap properly ... Maybe pull the bottom plate (with the centrif advance) and pull all of it apart, just stick the shaft back in which would give access from the bottom (inside). Thing is, when I had the 2 plates apart and set the body aside, then slid the shaft (attached to the bottom) into the top bearing ... well, great access of course without the body but also without the 2 plates firmly screwed to the body there were way too much "play" and no meaningful measurement was remotely possible. The plates have to be perfectly parallel, and for that they have to be in the body, but that precludes getting a feeler gauge in. Just flat out puzzling.

Curious you had so much trouble with your car. I always said "this 308 is like a chevy - get in, turn the key, drive ... always works fine". Until now, of course.

 

I see what you mean looking at the photos again. If you did want to set the gaps (and/or change the VR’s) then I’d say you will need the slot - a machine shop would easily machine that for you using a shell end mill (not a drill) and the dimensions are as large as possible whilst still allowing the name plate to cover the slot. Curious why you haven’t got the slot already as without it it’s impossible to set the VR sensor gap as you say…curiouser and curiouser.

The ‘in the field’ check to spin the disi shaft by hand and feel for those magnetic ‘tugs’ as the stator passes the VR sensors is pretty good I found though (mine shows only slight resistance, but noticeable nonetheless).

A thought: could it be that the factory had a jig they used when new to set the VR gaps on the plate, then assembled the disi? That’d certainly be the quickest and most accurate way, and if you’re making a few hundred of these why wouldn’t you. It might be that my slot is an after market machined slot to resolve exactly the issue you’re facing and I was just lucky.

You know through all my trials and tribulations it was actually the new rotor arm which failed causing my misfire - I lost the rear most bank to a very, very weak spark - same as you... This was only obvious when you removed the arm, turned it upside down and looked up inside the part which locates it on the disi spindle. There you could just about see evidence of arcing - sending current down the disi shaft instead of out to the contact end. Hence my message re poor quality replacement parts - as I’d replaced the rotor arm, and cap, previously I dismissed them as causes - new bits equals elimination of possible causes, right. In fact I’d introduced two faults (plus replacing the amplifiers which introduced another fault).

And the whole saga started because I just sensed the car wasn’t quite ‘on song’ from 5000rpm up. It coughed a little when cold but soon cleared, and didn’t have that noticeable push beyond 5000rpm. Changed the plugs, leads, disi cap and rotor arm, it got better, but not as good as it is now, then wallop, I lost the rear bank of cylinders.

I also took over a year to fathom it out! The bearing in the top of the disi had also just started to spin in the plate (the bearing had begun to fail and seize), which would have messed with the Vr sensor gaps. I sorted that, and found a few ‘cheap’ fixes had crept in over the years such as replacing parts of the fuel lines and using connectors to spice them into the original lines creating flow restrictions. I replaced all the fuel lines, plus cleaned out the filters to improve fuel flow.

Keep going then, you know it’s going to be something silly (they’re not complicated cars as you say).

 

Here endeth the lesson...

My assumption that Hella wouldn't change the design of the ignition modules that had been bog standard for over 40 years, sent me into a world of pain. I now wouldn't bother with anything other than the Petronix items
Distributor Caps made of cheese also didn't help.

The sensor gap. I tend to adjust them until they are just brushing the pick up and then back them off a tiny bit until they aren't. When you rotate the mechanism (at the angle/position it will sit in the car) - you should get 8 distinct 'resting points'. It's then a case of being happy that the the 2 sets of 4 positions have roughly the same 'pull'.
That will trigger the spark and the engine will run. Generally that's good enough. The proximity of the sensor to the pick up will dictate the dwell. I have a dwell meter but I've haven't used it - I will, one day.

I also rigged up a test bench and used an electric drill to test the my dizzy at the most basic level - i.e. car battery, dizzies, ignition modules and 8 spark plugs! I have a video somewhere, I'll try to remember to post it up

 

Quick/urgent question just in case someone runs across this in the next day or so
I just ordered a pair of replacement VRs from Ricambio
https://www.ricambio.co.uk/electronic-pick-up-pulse-sensor-fiat-124-131-lancia-beta-croma-regata-lancia-prisma-9936331-9937730-9939763
But they have not yet shipped. It's now the middle of the night here and I woke up in a start remembering something Adrian said:
"the VRs are 'handed'. left and right"
Oh crap! Should I try to cancel the order since I asked for 2 of the same VR's? I'm looking right now at the original (factory) sensors still mounted to the underside of the top disty plate. The appear ALMOST the same, except one corner of the mounting plate has a rounded corner while on the other the rounded corner is on the opposite side of the actual sensor. No visible part numbers on either; just the Marelli/Magneti symbol and "made in italy".

SHOULD I REAL QUICK TRY TO CANCEL THE ORDER WITH RICAMBIO?

 

Hi, I fitted these (from Superformance here in the UK), they looked identical. I needed to ‘trim’ a corner from one (in the vice with a decent file) to allow them to adjust, as standard they interfered with one another and I couldn’t get the gap correct on one. Easy and straightforward to do.

 

Thank you, 512! I see the mounting plate lacks a rounded corner - I guess what you are speaking about. So - the adjustment slot (not the pivot hole) on one does not need to be worked on?

 

Correct, no work on the slots, just trim a corner off to create space for that sensor to be adjusted. I guess you could trim a corner off both, I set one sensors gap (remember I have the slot in the disi side so could use feeler blades) then found I couldn’t close up the other one to achieve the correct gap as the corners touched. I trimmed that one only and all was well. Good luck! Paul