Unclear on how to check for distributer pulse
 

First I apologize - this is another newbie question from me but I have checked all through the threads here and I cannot find this very specific detail for checking the distributer pulse and I am hoping someone can guide me through this. (I have posted this question on a thread relating to this but have yet to receive a reply so trying here).

When checking for distributor pulse following the instructions I have found on threads here do I probe 31D to 7 with my multimeter (VAC) (ie. 1 probe from multimeter on 7 and the other on 31d) or do I attach my multimeter negative to ground and then positive probe 31D and 7 separately?

When attaching probes to 31D and 7 I get no pulse which makes sense right, because no power is being delivered to the distributor? Is that correct?

But, when I then ground the multimeter and probe 31D I get pulse. I then did the same to 7 and have pulse.

Am I doing this correctly? Thanks everyone!! Appreciate any advice you can give.

Thanks as always!

Dan

First why are you checking for a pulse? Is the engine not starting? From your signature I assume you have a 79SC; I have a 78SC and when it didn't have a spark it turned out to be the green wire. I checked the pulse from the distributor by removing and spinning it by hand with a meter across contacts where the green wire plugs in and got a small pulse so replaced the green wire and fixed the problem.

Your correct I have no spark. I have tracked it to a possible faulty coil but while I wait for a new one to arrive I am checking everything else. I’ve followed a lot of threads on how to check the green wire which is what I am doing above. I’m just unsure if I am doing it correctly and hoping someone can clarify how to perform the test above properly. I don’t want to remove the dizzy and then have to reset the timing.

If your car is the same as mine there is a connector from the green wire that goes into the wiring harness (and eventually to the ignition box) somewhere near the left hand chain case. If you disconnect at that point you can check for a pulse between the two wires while cranking the engine. It's where you would disconnect the green wire if you were replacing it. If it's the original green wire then it could well be your problem.

Yes that’s what I am doing but I am not sure if I am doing it correctly. If you wouldn’t mind re-reading my original post I have detailed how I am doing it but not sure which of the 2 methods I have tried is correct.

From your original post I assume you have disconnected the plug at the CDI module where there are 6 connectors. This is not the connector I am talking about. The green wire is quite short and there is a two-pin a connector (on my car at least) partly hidden by the distributor. See photo:


http://forums.pelicanparts.com/uploa...1579612539.jpg

I’m thinking out loud here.

Just thinking through this logically. The reluctor in the distributor creates a magnetic field. Whenever the magnetic field is changed (rotor passes the TDC notch for example), this creates a voltage. The voltage is used to tell the CDI box when to fire.

My thinking is you would connect a test light to the two terminals on the dizzy. Remove the fuel pump relay and then try to crank the car. If the test light pulses, then you have a signal. You may have to flip the ends to get the right polarity. If that is successful, then you should disconnect the 6-pin on the CDI and check there. This would tell you if the signal is making it to the CDI box. If not, then it’s the green wire or somewhere else in the harness.

The reason I think you’d want to use a test light is the pulse would be happening so fast it would be hard to see on a meter.

The other way would be to pull the dizzy and spin the reluctor by hand.

Never tested it this way so I’m just spitballing in hopes that someone will either confirm or correct my thinking or will have an easier/better method. :)

Hope this helps!

Something else I wrote a while back on how I think the ignition system works:



My understanding of spark generation on this car is as follows: the distributor generates a small voltage pulse which you can check by following above. The CDI transforms this to 400 volts which is then relayed to the coil which ups it to 20,000 volts. There could be a problem anywhere on this route. For example there is a connector to the green wire behind the distributor so you can also check there for the pulse if you don't get it at the CDI end of the cable. In my case it was the green wire: Bentley suggests distributor is faulty in absence of pulse but in my case it was the green wire, confirmed by pulling the distributor and bench testing: distributor gave a pulse when rotated by hand.

Thanks guys, I agree with everything you say. I am just trying to verify to correct way to administer the following test for the green cable & distributor because I am new and unsure if I am doing it correctly. This is what I am doing (from original post)

”When checking for distributor pulse following the instructions I have found on threads here do I probe 31D to 7 with my multimeter (VAC) (ie. 1 probe from multimeter on 7 and the other on 31d) or do I attach my multimeter negative to ground and then positive probe 31D and 7 separately? When attaching probes to 31D and 7 I get no pulse which makes sense right, because no power is being delivered to the distributor? Is that correct?“

My reason for doing all this is that I have a non-start and am working through a step-by-step diagnosis. I suspect it’s the coil and have a new one coming and while I await it’s delivery I am testing everything else. The test above for continuity for the green cable which is discussed in many threads here is just a little unclear for a newbie like me and I am trying to work out if I am doing it correctly.

I really don’t want to have to pull the distributor to figure this out and the test above will be the next step to guide me as to whether I need to go that far or not. It would be frustrating to pull the dizzy to test this if I don’t have to.

That’s exactly right but I am testing at the CDI end of the same cable which is easier to access due to the rotation of my distributor. When I test this am I doing it correctly by attaching my DMM negative to ground and producing DMM positive to each terminal (7 & 31d) while cranking?

You are looking for a pulse between 7 and 31d. So you are not doing it right according to Bentley, who says that absence of a pulse means distributor is most likely at fault, completely overlooking that it may be a break in a wire. I don't know why you are getting a pulse between 31d and ground or between 7 and ground UNLESS they are now common. When the green wire failed on my car I had continuity between the two wires in the green cable.So try testing for continuity between 7 and 31d (no need to crank). I am assuming your DMM has a continuity tester.

Right. The reluctor in the dizzy generates voltage from the magnetic field. It’s a sine wave - it increases and decreases. One wire is the signal and the other is the shield (ground). This is why you sometimes have to swap these wires if you replace the original Bosch CDI with an aftermarket (like MSD).

I’m not sure if the original Bosch actually uses this voltage to fire the coil. I suspect it just uses it a signal to know when to fire. Power likely comes from the 12v power feed.

If there is no signal at the CDI plug, then the next step would be to test at the dizzy.

JSV - I followed your instructions here (thank you!!) and **** got weird, check this out!!
So in the pic below you can see at first the sticker of the side of my CDI indicating that 31d & 7 pins are at the bottom, teeth of connector facing left. So I tested continuity here and got nothing indicating a green wire fault right? So I unwrap all the wiring inside the engine bay and discovered this....

...my Green Wire runs from the dizzy all the way up (no connector en route) to the top part of pins at the CDI connector. I check continuity here and have it! So my "Green Wire" seems to be ok (resistance across pins measures 575 Ohms).

I can't figure out why my system would work if wired backwards to the "original" sticker on the side of the Bosch CDI unit so I check Bentley which shows the pins per my setup, 31d & 7 at top of connector when connector teeth face left. Stranger still is the fact that any replica stickers also show this opposite configuration.

I am not sure what any of this means but it seems that every replacement sticker produced is incorrect or my system and Bentleys manual are backwards.

Regardless off all this it seems that I do have continuity through my Green Wire so it should;t need replacing. I am hoping I ave done this all correctly because reading up on a green wire replacement and it looks like a total PIA - Pelican's wire is much shorter with a connector on the end of it so there seems to be no way to splice it into the socket at the CDI - strange.

http://forums.pelicanparts.com/uploa...1579623920.jpg

What CDI do you have? It sounds like someone swapped the green and violet wires for an aftermarket unit that triggers on the opposite slope from the original Bosch.

I wish it were that simple - nope, I have the original Bosch CDI. I was just told on my instagram post that this sticker is for the CDI connections rather than the plug itself and is therefore a mirror of the plug. It's a poor way to illustrate this in my opinion as it should either be mounted the other way up. (Either that or this is my excuse for being an idiot!)

I'm not familiar with your cable set up; maybe it is an aftermarket replacement? I'm not sure what continuity you are checking. There should be infinite ohms between 7 and 31d when no cranking is taking place; at least that is my understanding. The green wire on my car failed when I had continuity between the two wires; there is still continuity end to end. The problem is when the shielding makes contact with the inner wire, so you could well have continuity on your green wire end to end, but you definitely don't want continuity between 31d and 7.
The fabled green wire is actually a cable with a center wire with metal shielding on the outside encased in green insulation. I am assuming that there is also insulation around the inner wire to prevent shorting with the shielding. I think that it is this inner insulation (not visible) that breaks down. The green wire you are looking at is not the green cable as at that point it has been separated into two wires. What does the cable at your distributor end look like?

I followed the Green Wire I show above from the distributer up the wiring harness to the CDI and it's 100% the same cable which splits and goes into the correct pins on the CD connector. At the distributer end it looks like this, I just now unsure how to check it properly:

http://forums.pelicanparts.com/uploa...1579630028.jpg

Still not clear if there is continuity between 7 and 31d. If there is a short between the two then that is perhaps why you get the pulse to ground on both of them; they are both transmitting the pulse from the distributor. Put your probes between 31d and 7 and measure the resistance or have you done this and it was 575 ohms? If it is possible, wriggle the green cable while you do this. Should be a very high value of ohms or it's a short. Do not crank during this test.

Thanks for helping with this mate! Below is a pic of the continuity test. This is just for the purposes of a photo (had to hold the camera with one hand) usually my hands would be much further up the probes but reading is the same: 575 - and a wiggle of the Green Wire made absolutely no difference at all, not a single Ohm, stayed steady at 575.

http://forums.pelicanparts.com/uploa...1579634897.jpg

Your meter looks to be showing 575 K Ohms, so that's quite a high value (575,000 ohms). So cable would appear to be good. How exactly do you check for pulse between 7 and ground? What do you see on your meter?

You are measuring the coil value in the distributor.

Oh this is good news! Thank you - I assumed the .575 k - mean fivehundredseventyfive, another learning curve! Thank you.

So for the voltage test I setup previously as the pic shows below (DMM set to VDC, DMM negative to ground by coil, DMM positive to terminal 7 in pin, then crank) but right now I don't have a helper to crank the engine for me. I'll have to wait for the Mrs to get home and grab her as she walks in.

http://forums.pelicanparts.com/uploa...1579638162.jpg

Hang on! I didn't see the decimal point so think you are right with 575 Ohms. Perhaps Dennis can confirm that this is a good value for the coil at the distributor end where the cables connect. Still suggests cable is good. I'm looking at the wiring diagram on page 280-9 of Bentley.
What value did you get on the meter for the pulse?

Roy

Thanks Dennis - and does this look correct/good to you? We are getting 575 Ohms through 7 & 31D. We can't find a reference to what this should be.

I managed to grab the Mrs to help - this is what we got. Looks like a 1volt pulse which, if I have read threads and Bentley correctly, is good.... right?

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I can’t remember the value for the reluctor coil, but it is likely fine as they rarely fail.

Definitely something happening but notice you have meter set to AC in video but to DC? in earlier snapshot. Do you get the same result if meter set to DC voltage?
Still odd that you don't get a pulse between 7 and 31d but did you have meter set on DC volts?

I know that you don't want to remove the distributor but to remove the wire at the distributor end it's pretty much essential as there's so little room. There is no need to lose the timing if you mark where it is clamped and set it to TDC on number 1 before removal it should be possible to replace it in the same spot. Once you can get at both ends of the cable you can check the cable out. On my faulty cable I was getting almost no resistance between any pair of contacts with the cable off the car.
Maybe you should wait for your new coil to arrive before going any further.

Yes earlier snapshot was incorrect (you have good eyes!!). I double checked Bentley and verified that reading should be done VAC. I actually did test it on DC first - just to be sure, and didn't get a reading that made any kind of sense. Bentley 280-9 instructs:

• With engine cranking test for voltage (VAC) between terminals 7 and 31d.
• If meter shows a voltage pulse, CDI control unit or coil are suspect.

...and I agree, I think so far it feels like I am ok on the Green Wire. Perhaps we are wrong but once the new coil gets here (which is taking it's sweet time) we'll know 100% and can go from there. Thanks for all your help so far Roy! You've been fantastic!

FULL END-TO-END GREEN WIRE SUPPLIER FOUND!!!

No update as yet on the non-start but this exploration has shown me that finding a solution for the Green Wire is hard to find as a simple plug'n'play option. The one provided by our host and most their suppliers is too short and need to be spliced in to the original cable and removing/bypassing the older, cooked section.

I hunted everywhere for a solution and ended up reaching out to the guys at Kroon Wire who make pretty much every kind of wire/engine harness you can imagine for all our cars. The can provide the CDI harness as a full replacement which includes the original Bosch Distributer Wire:

• https://www.kroonwireharnesses.com/index.php?route=product/product&product_id=711&search=6+pole

It is expensive but very well made and a great replaceent preserving the original wiring specs of our cars. However, after emailing them they are very aware of this issue and while it is not advertised on their site that explained the following"

• "We could also deliver a green wire at the right length with terminals that you can "click" in to the 6 pole connector. Please note that the distributor connector is not available as a single part (unfortunately) so will need to mount a 2 pole Bosch connector at the other end of the cable and that will connect to the famous short Bosch green distributor wire. (male to female). The price for this will be EUR85,- ex shipping ( the original Bosch wire is included)"

So thought I should share this here because it looks like a great option for anyone who needs to replace their Green Wire (perhaps me in a few days... we shall see)

I cannot tell you how happy I am to report back that the new coil arrived today, it went in and she started faster than ever before!!!

I cannot thank you guys all enough for your help, experience, wisdom and patience with me! For most of you this is simple stuff but for a newbie like me it's a milestone. Taking a car from a non-starter to a nice clean start without taking her to a shop is a big deal for me and I couldn't have done it without this forum!

THANK YOU EVERYONE!!!! And now enjoy the sweet sound of success!! ;)

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Congratulations on fixing the fault. As a matter of interest I measured the resistance between 7 and 31d on my '78SC and it was 560 Ohms so pretty close to your value and confirmed that your green wire was OK. I bought my new green wire from my local Porsche dealer. Bit pricey but did the job. Originally bought the URO product but it failed after a month. Turned out it was a poor fit electrically and they did refund me.

Unfortunately true for our first production run. Once we heard of the issue, we modified the inventory at the time to tighten the connection, though it's certainly possible there are still some wires with with the original loose connectors out there in the wild.

We updated the connectors for the next run, and they have a nice secure electrical connection and no complaints so far. If anyone has one of the early wires, please PM us with your purchase details and we'll be happy to send you the new wire at no charge. Or we can provide instructions on how to tighten the connector with a small nail or icepick, takes only seconds.