CDI / coil

[upwardr]

Having troubles getting my 911 to start. I don't know if it's CDI or coil or both.
I turn on ignition and can heat the CDI unit buzzing (good news) but get no spark any ware. I have power to red wire feeding the CDI but no power to +side of coil. Should there be some power out to coil?
Was running well when it went into storage. Fuel pump is working and have about 5 PSI at the carbs.
Seems electrical. Any thoughts?

[T77911S]

no power to coil.

did you verify spark?
put a plug on a wire and crank it to check for spark.

I would look down in the carb to see if fuel is spraying when you operate the throttle linkage.

is the fuel any good?

[upwardr]

No spark. Should there be power at the coil after the CDI box? Fuel is old but. I'm not getting any kick.

[T77911S]

no power to coil.

did you verify no spark or just basing it on "no kick".

check points, make sure they open and close.
check to make sure the wire form the dist to the CD is good.

[dfhtrhjn]

You can pull cable from the center of the distributor, hold it a mm or two from any ground. When you crank you should see spark arcing to the metal it's near.

If you don't see a spark, then work your way backward to the coil. I keep a spare coil for diagnostics, swap one in and check the spark. If you know the coil is good, then go back to the CDI box.

If you're pretty sure your wiring is healthy as well, then order a CDI/MSD/CDI+ etc. Having a spare CDI unit (MSD for example), is the cost of two hours shop time if you have to take your car somewhere.

[upwardr]

Plugged plug wire no spark. Put in spark plug tester(light style) no light so looks like coil?

[ischmitz]

let's get some things clear. A CDI-based ignition doesn't have the classic +12V power to the coil. As a matter of fact one side of the primary is connected to GND and the other is connected to the CDI box.

A good working CDI box will not pose any danger when there is no activity (cranking) but an aged box and an open coil can present up to 450V at the other primary connector of the coil. So please do take the warning stickers seriously.

A humming Bosch CDI box is partial good news. There still can be issues inside the box that prevents it from generating a spark. Your first test should be to disconnect the CDI box and then measure primary and secondary coil resistance. Primary is around 1 Ohm and secondary around 1000 Ohm. If one of them is open the coil is bad. If not I'd look at the trigger. From the "carbs" comment I assume we are talking about the '71 and a 3-pin Bosch CDI.

Ingo

[upwardr]

I go thanks for your help.
How does one perform the test you suggested at the coil? I do understand basics but this I have no idea. I have a decent multi meter so I will give er a go.

[ischmitz]

The coil has two screw terminals. Disconnect the 3-pin connector at the CDI box. Then set your multimeter to OHM. If it is auto range no more settings required. Else use the lowest range (e.g. 0 -200 Ohm). Then touch both test leads together. You should see something like 0.0 - 0.6 Ohm. Record that value. Next measure the primary of the coil by touching each screw terminal with one test lead. The take that value and subtract the previous number. That’s the primary resistance.

Next set the range of the multimeter to the next range (e.g. 0 - 2kOhm). Disconnect the center high voltage connector at the coil and measure from any of the screw terminals to the center connector. That is your secondary resistance. It should be around 1000 - 1500 Ohm or 1.0kOhm - 1.5kOhm depending on how your multimeter displays results.

[upwardr]

Primary=3 used the 0-200
Secondary=7.46 used the 20k setting


Doesn't look right I'm thinking.

[upwardr]

Anyone know if these values indicate a bad coil or CDI box?

[mysocal911]

Originally Posted by upwardr

Primary=3 used the 0-200
Secondary=7.46 used the 20k setting


Doesn't look right I'm thinking.

If it's 3 ohms you measured on the primary, then the coil is an inductive ignition coil
(Bosch blue) used with points on early 911s. This will function with a CDI ignition unit
without problems, but may be too long when mounted.

[upwardr]

Thanks Dave. So do these indicate a good coil?
I have used this blue coil for many years but for some reason can't get any spark.

[ischmitz]

Have you checked that your points are working? This is done by measuring resistance from the correct pin in the 3-pin CDI connector to GND while moving the crank.

Pull the 3-pin connector from the box and measure inside the connector. There are 3 contacts inside the connector. The center one is +12V (with ignition on). We know this works because your box whines.

One of the outer ones runs straight to the coil primary. It will measure 3 - 4 Ohms to GND if your coil is connected. This is because the other side of the coil primary is connected to permanent GND. Check that. One lead to the outer contact in the plug and the other to a metal part of the engine in lowest setting of your multimeter.

Then the other outer contact is connected through the points in the distributor to GND. When you rotate the crank the points will close and you should measure either OPEN or around 1 Ohm or less depending on crank position. Check that. If both check out your CDI box is bad and needs repair.

[theiceman]

no one asked the question ???? what colour is the coil ?

[upwardr]

It's a bosch blue coil.

[doswald]

I’m working from memory, but on my ‘71T there is a small wire that plugs into the distributor and powers the points. If it is unplugged, broken or corroded, there will be no spark. Also, if the points are corroded-no spark.

dho

[upwardr]

Thanks Ingo, still trying to get to the bottom of this. So outer prong in the 3 prong connector to coil I get 3 ohms. Other outer prong to ground when cranking gets no reading on meter. Doesn't change. I get noida. Does this indicate faulty points or CDI box.

Thanks everyone for the help, I have several other coils from my stash but think coil seems ok?

[ischmitz]

Larry,

the 3 Ohm are correct and this is the coil primary.

On the other outer prong you need to see either open or <1 Ohm against GND depending on where the engine is in its cycle.

I don’t know how fast your meter updates. During cranking the point open and close multiple times per second. Maybe the meter is a tad slow. I prefer to turn the engine slowly with a big wrench on the pulley bolt instead of cranking.

As a next step I’d take the distributor cap and rotor off and inspect the points. One side of the points is connected to GND. Measure resistance from the other side of the points to the outer prong in the CDI connector. If it’s close to zero your points are bad or not closing. If it’s open your harness is bad.

The points are like a simple switch between GND and the CDI box trigger input.

[upwardr]

Well good news, opened points give a crank and the spark plug tester lite up.
Plug everything back together and spark but no start. Kinda sounded good but no run.
Check fuel pressure 4//5 psi.

Time for beer, very frustrating.