New ignition coil is reading at 10,000 ohms

[Franks944]

Hello all!

I’m back again seeking some advice, I have my 1989 Porsche 944 with the 2.7 back together after not running for 15 years. I replaced all belts, pulleys, water pump. Fuel components, injectors, spark plug, cables, ignition coil and rotor and distributor cap. New DME relay as well. Fuel is confirmed to be making it to the front of the car via the rails. I did a compression test and am happy to report that every cylinder is getting 220 - 235PSI. My thing is, the car is at a crank no start. The ignition coil is reading approximately 10,000 ohms when testing the center of the coil when it should be reading lower I know. It is a brand new coil and the old one tested the same so I replaced it with this one. The car has slight tach bounce so I don’t believe it’s the speed/reference sensors even though I’m waiting for new ones. I want to replace these ones. Any idea where a wiring issue would occur causing resistance to my ignition coil creating the crank no start? Thanks!

[walfreyydo]

There is a primary coil resistance and a secondary coil resistance:
https://www.clarks-garage.com/shop-manual/ign-04.htm#:~:text=Ignition%20Coil%20Resistance%20Chec k&text=Check%20the%20ignition%20coil%20primary,be%20 0.4%20to%200.6%20ohms.

Also, have you first confirmed whether you actually lack spark?


The first step to a no start condition is checking for lack of spark or lack of fuel.
https://www.clarks-garage.com/shop-manual/ts-01.htm

You said you replaced injectors - were they the correct low impedance injectors?

[Jfrahm]

I would not worry about the coil resistance. Can you check for spark at the coil wire while cranking?
See the negative pulses at the coil?
You can make and break the coil connection to elicit a spark, and check that spark at the coil wire to see if it'll jump 10mm-15mm and look nice and blue-white.

[Monkey Wrench]

if you put a small test lamp across the coil promary you should see a blink during cranking. that verifies the primary coil has a pulse.

if no pulse troubleshoot back towards the MCE and there is a transistor or amplifier thing probably mounted around the left front fender, itll have wires connecting to the coil you might check grounding and those connections.

if the primary has a pulse, the secondary should too so you can connect a spark plug directly to the coil to check , that's elliminating the cap and rotor.

make sure all your plug wires are fully seated and that none have come apart inside or anythign like that

a lot of the time the coil gets replaced to troubleshoot it and its not necessarily the issue. If your car was parked a while I'd go around cleaning every single ground point you can find . Id use some thing to stop future corrosion,

now there are a bunch of special greases for electrical connections , some are conductive and they can short things out if not applied well but if its a body ground I don't see that going bad.. some use a silicone dielectric greasse on spark plug leads , it is not conductive but it will help the connections not coorode. others help with dissimilar metals. and corrosion caused by differences in each metals electrical potential or "nobility"

since a porsche does have alunimum parts and as well galvanization.(zinc coating) and the voltage of alunimum may be doiffferent than other metals, you may see instances what is called galvanic corrosion.
.
if you look at this link ( below) youll see that alunimum and zinc (galvanization) are down near the bottom of the chart with the more noble metals at the top. Its a well understood and predicatble science but also something easily overlooked.

so where there is a wire to a ground , take it off, clean it up good and replace.. maybe invest in a new fastener if it's rusty.

the one I use is non conductive but contains zinc particles. it is grey so when I see connections that I have previously cleaned up and coated I'll see this grey substance.. that way I remember what Ive already done. and if it looks rusty it still needs doing. I make a habit of not smearing it on dirty connections as that will hide them.

people with boats often have sacrifical anodes made of zinc the idea is they get attacked before other parts and their existence protects other parts, until they become used up.

therefore the anode is "sacrificial"
here is the name of the one I use, but there are a many differnt electrical greases with varying properties on the market.. ( see below)

I'd also check things like grounds for all your lights. if you put a dab on the wire connections to the coil or to ground points I think thats ok. If you put anythign on the actual plug wires a dielectric grease would probably be better there. it helps keep water out. water causes corrosion.

I'd check your battery grounds and any grounds related to the MCE board and any other ground poiint you spot as you work around and look at things like actually follow the big ground wire, pull it away fro the body and clean it so it is definitely a good contact.

the high side of the coil has fine strands and many of them , it is in essence a step- up transformer and so to make higher voltage it takes many turns. Id be less worried about the actual resistance reading but it shouldn't be open or shorted, wheather it was 1000 ohms or 2000 ohms, it may be ok


the number of windings hence the voltage is already decided back at the factory by the original porsche engineers but many are sold on coil with a "higher output."

well you dont actually get something for nothing , energy is neither created nor destoryed. you can consider wheather the engineer of the replacement coil had a better idea about the number needed than thre porshe engineer did. hence the ratio . I dont have a dog in that fight ;-)

the higher the voltage the further the spark can jump and some have troubles with things like carbon tracking or the plug wires conducting through the inulation to ground. increasing voltage may actually exhasperate such issues.. How do you get more wire in the same package , you make the wire thinner.. so that may be a consideration too, as well as the price of copper.

if you were to start varying the wire size and length it would equate to different ohms between the coil wire and body ground.

if you had a wire break, the coil would be open and non functional. if the bundle of wire had an internal short part way through the bundle than you may have a lower resistance as it is basically a shorter length of wire, these things are possible but not so often the cause.

if you had a blinking light, IE there is a pulsation at the primary and as well, no output ( no spark) then Id suspect bad connections an the terminals and then the coil itself.

when your key is on it makes the coil hot wire see 12 V, the ground constantly interrupted by the MCE electronics so dont think you can just put a meter across and see 12 Volts. the ground is intermittent when cranking which is why youll see a test lamp pulse, when running it will pulse but the filament ( heat up and cool down ) will not react as fast as the pulses , so when running the lamp will glow but a bit on the dim side.

Id verify this and nto go playing with the MCE ..do that if you have NO pulse otherwise you may dig some other hole, be careful not to change parts and in so doing create two reasons at once why the car wont start. You almost certainly have only one issue.

replacing parts at random is not the best way to troubleshoot , really by whatever method you use you should be narrowing the problem down until you isolate the cause, then you can go changing parts.. if you want ot change an engine sensor good , but Id try to do that wiht a runnning engine so if its lets say too close or too far, you havent created a source of confusion.

if you change your sensoer and things as a precautionary measure thats ok but try to do that after you solve the issue , if possible to avoid such confusion granted some stuf is hard ot change and you may not wanto keep replacing things but keep it in mind that anything you do could create a second issue. if you touch an electronics board it is possible that you have put on a new sweater from the dryer and you are fiull of static..or maybe you just pet the cat,, then you go touching electronics and you get a tiny electrostatic shock, that stuff blows boards. you cna ground yourself to make sure YOU arent; charged up.or wear a ground strap on your wrist is normal for people doing this. improvise if you like.



If I do replace a part as a part of diagnosis under a no start condition and I see no difference, back on it goes, that wasn't it. I dont want to change the conditions, at that point, im looking for the cause.Im ot replacing parts at frandom for reasons of reliability..

you have a tach pulse so you probably do have a pulse at the coil , but check.

follow the ground from your battery thats a good one to check and any engine grounds too.

you may not know it because all you did was clean a ground and no difference but this will sometimes elliminate an issue BEFORE it causes future issues. then no troubleshooting is needed and you avoided the problem by doing it preemptively. sure perhaps unknowingly...



PENETROX-A

https://en.wikipedia.org/wiki/Galvanic_series

[Monkey Wrench]

I found this older thread and thoughtit worth mention.
hes basically explaing the corelation between the coil impedance charge time and the basic takeaway was that if you go to some new fangled aftermarket coil expecting that it will work better , well the engineers din't do such a bad job of designing it.

it seems to allude to the suggestion that some of the aftermarket coils may actually damage the transistor in the MCE and that the Eprom may be upgraded to prevent this. and that if both are done in conjunction you may improve the spark at higher RPM's.

my laymens analysis is that it may not be the best move to just blindly change the designof the coil without further understanding of how different ratios , inductance, impedance etc will affect the charge time and the load on the transistor, the coil is hot by default with the key on eand as I understand it the MCE basically intermittently grounds it , there is a transistor between the MCE and the coil which is also involved. i wondered if he was referrring to that with the comments about the transistor not lasting.

I dont know everythign about the system, or as much as some others here, but I did feel this older post was worthy of a mention because he went into a lot of detail.

https://forums.pelicanparts.com/porsche-924-944-968-technical-forum/733588-tech-944-951-ignition-system.html