How to test spark coil? (308)

With a mulitmeter/ohm meter.

 

more specifically, about 1 ohm between the primary terminals, and around 3600 ohms between a primary and the secondary.

alternatively, have that useless assistant stick a finger in the secondary while you put 12V across the primaries. When you remove the 12V, if the assistant flies at least 5.624 feet before landing, the coil is good. If it takes an ems team to restart the guy's heart, then the coil is very good. This technique may be frowned upon by workers comp insurance people, but the darwin awards judges will approve.

 

Thankyou for clearing that up.

 

Good to see you figured it out. Was the coil faulty ?

 

if the wires get all mixed up, the schematic will show you how to connect them. Also, stamped into the metal frame on the bottom of the coil are the primary terminal numbers. Should be 1 and 15 on each coil.

Since the schematic has the italian abbrevs for the wire colors, this should help:
A - azzurro - light blue
B - blanco - white
C - arancio - orange
G - gaillo - yellow
H - grigio - grey
L - bleu - blue
M - marrone - brown
N - nero - black
P - nocciola - beige
R - rosso - red
S - rosa - pink
V - verde - green
Z - viola - violet

so yellow/black would be GN

the yellow wires go on the 15 terminals, both the green/black wires go on one coil on its terminal 1, and both the yellow/black wires go on the other coil on its terminal 1. 'course, it's critical that the terminal 1 wires are on the right coil, where right is with respect to which bank the secondary is connected to.

The US spec schematic says the yellow/black wires belong to bank 5-8, but the bank 1-4 wires are solid green, not green/black. Either the euro schem is different, or welcome to the wonderful world of ferrari wire colors.

btw, the extra wire from the #1 terminals are going to the diag connector, as is one of the yellow wires.

 

All 308's are not created equal. My early 308's are totally different. Solid yellow to ballast resistor, solid red to + terminal, jumper wire from resistor to + terminal, and - terminal of coil to points/tach. Primary resistance between - and + terminals 3.1 to 3.4. Secondary resistance between + terminal and center terminal 5500 - 7000. This last one I don't fully understand. Insulation resistance to earth @ 500V, c.c. 50.

Further, it shows in the service manual bench testing the coil via the distributor while its run on a machine, warm and cold, at varying rpm and showing minimum spark length of 7mm at 8000 rpm warm.

Finally, it shows at a temperture of 20*C, the coil withstands a voltage of 1000 V @ 50Hz, three seconds, between primary and metal shell. No discharge allowed. i don't know how to test that either. Probably I will just go buy a couple hot Mallory points coils if I question them.

 

quasimotor has an injected car, and the numbers/wiring I listed should apply to all the injected 308's. Thanks for the info on the dual coil carbed 308...I guess that just leaves the single coil cars.

while coil breakdown under load or high temps does happen, it's not too common from what I've heard on the marelli coils, so just the simple ohmeter check for open coil is probably good enough.

measuring the spark intensity and duration at various temps requires some kind of special measuring device. I've been playing with a KAL spark tester I picked up off ebay, but it's really intended more as a comparison between cylinders than an accurate measurement tool. Still, if someone wants to try it, pm me.

I guess it's still hard to beat a scope if you really want to see what is going on.

 

...and if you want a good scope try www.picotech.com, see automotive section.
Ron

 

I have an old Tektronics scope that I'd love to use to test the voltage on the plug wires with the car running. Where would I get a probe that could handle 30k volts ? Inductive type ? Is there a low voltage output on the diagnostic port for monitoring the 8 plug voltages? (308qv)

 

inductive type pickups/clamps aren't too good for measuring stuff. They work fine when just a sloppy trigger signal is needed (e.g. timing light or sync signal for scope). To get a good waveform, you'll want a capacitive pickup. Same idea, though...just clamp it on the cable.

You can look at all the signals on the primary side using a 10:1 probe, but for the secondary you have to have the capacitive clamp. Poke around on ebay for one.

the mechanic is right. The ohmeter test will detect when either the primary or secondary winding inside the coil is open (burned up) or partially shorted, but it can't find other types of problems that only occur when high voltages are present. However, high voltage issues are uncommon.

on a motor, the laminated metal plates are called the stator. A little delamination/separation at the edges doesn't matter.

The most common reason for losing spark on a bank on the injected 308's is the ecu no longer telling the coil to fire. The three most common reasons that happens is a bad ground on the digiplex, a bad flywheel sensor, or a bad connection in the plug between the ecu and the flywheel sensor.

All those topics have been flogged to death, so if you pop in the right search terms, you'll find lots of reading on what to do. My preference is to do all checking from the ecu connectors before touching anything else. If it's too confusing, pm me for details.

 

"inductive type pickups/clamps aren't too good for measuring stuff. They work fine when just a sloppy trigger signal is needed (e.g. timing light or sync signal for scope). To get a good waveform, you'll want a capacitive pickup. Same idea, though...just clamp it on the cable.

You can look at all the signals on the primary side using a 10:1 probe, but for the secondary you have to have the capacitive clamp. Poke around on ebay for one."


Thanks Wolftalk !!!

 

George,

FWIW, I am in Manhattan, not far from Yonkers, if I can help in anyway let me know (PM is fine - I check it) I have a QV. I share the other's view, coil failures are rare. In all the old cars I have I only exp. one coil failure but I don't know if that counts since it was a Lucas (XKE). Check other threads, could be a faulty ground, a ocilliscope will save you lots of time, but if you are a DIY like me you kinda make up for lack of tools or expertise by narrowing the problem down on the cheap and then do the R/R on parts ( can get pricey with f-parts though) 'till success.

 

Once you've grounded the digiplexes, and clean the connectors, if you still have the problem, Switch the two coils & see if the problem changes banks.

By switch I mean:

Lable & remove every wire from both coils, then reconnect every wire from coil #1 to coil #2 & vice versa. If necessary to get the wires to reach, switch the mechanical position of the two coils.

If the problem follows the coil, then you've found it, if not, then you need to keep looking. You can do the same thing with the distributor caps & digiplexes.

 

For those that have access to an Oscillocope, I found a test probe for checking ignition at the plug wire on Ebay- $50-

http://cgi.ebay.com/ebaymotors/TEX25A-Automotive-Ignition-High-Voltage-Probe_W0QQcmdZViewItemQQcategoryZ43989QQihZ002QQitemZ4602380740QQrdZ1QQsspagenameZWD1V
Image Unavailable, Please Login

 

you can use an ohmeter to test the TDC sensors. As usual, if the sensor has a problem where an internal wire is busted, the sensor may look ok until things expand from heat or you hit some bumps. However, that's pretty rare. Usually the internal coil burns up and the sensor measures infinite ohms, but occasionally the sensor will short and measure almost zero ohms.

check your owners manual for a picture of the diagnostic connector and how the sensors connect to it. It may even be the correct picture of the diag connector, unlike in the 328 manuals I've seen. It's in the maintenance section, and is a nice pic showing the coils, digiplexes, sensors, and diag connector wiring.

However, even if the sensors are good when measuring from the diag connector (where good is around 700 ohms +/- 100 ohms), it doesn't mean they are connected to the digiplexes if there's a problem in other connectors. That's why I'd recommend you unbolt the plate with the digiplexes on it (left side of trunk under the carpet), unplug the connectors to each digiplex, and measure the sensor resistance directly between the digiplex connector pins. The diagram in the manual will show you which pins, I think it's pins 1 & 5 in each connector.

You can also check the digiplex ground and connector condition, which you really should do anyway, and search the site for info about running a separate ground wire back to the digiplexes.

The flywheel sensors aren't switches. They are just coils, and the system is using a magnetic field/hall effect to detect when a little block on the flywheel passes the sensor tip. You measure the sensors when the flywheel is not turning, and you'll get a constant value regardless of flywheel position. You'd need a scope to see the blips on the wire that the flywheel block causes when it's spinning past the sensor.

Having a problem with the yellow wires on the coils is a little odd sounding. I think the only way it could be messed up is if both ends of the yellow jumper wire that goes between the two coils are connected to the same coil.
The yellow wire is +12V, and it daisy chains from the battery/fuse box -> coil A -> coil B -> diag connector. Which coil it connects to first doesn't matter.

The digiplexes are controlling the ground side of each coil. On each coil, one wire is coming from a digiplex, the other (same color) is going to the diag connector.
You must have these wires correct (well, mostly correct...the wires going to the diag connector don't really matter, but the digiplex that is watching the sensor for bank 1-4 had better be connected to the coil that is sending spark to bank 1-4).

The other thing to check is the connectors between the coils and digiplexes, and the connector between the sensors and the digiplexes. Both those connectors are tucked up above the driver's side rear wheel well liner, and you can usually see them from above, but it's best to take out the liner so you can really check those connectors well. Common problems are corrosion on the pins and pins pushing back out of the connector shells.

so much for theory...do this:

1] unbolt the digiplex plate
2] remove each connector from a digiplex.
3] check for around 700 ohms between pins 1 & 5 on each connector
4] check for +12V between pin 8 and 9 on each connector with ignition key in run position. If the black ground wires were under one of the ecu plate bolts, you'll need to connect them to ground first.
5] check for around 700 ohms between pin 2&3 on each connector...that's the tach sensor, but it really only matters for the digiplex that is outputting the tach signal on pin 10 of the connector. You can't easily test pin 10 with the meter, but if your tach works, it works.
6] pin 11 on each digiplex is the bank coil ground wire. You can measure about zero ohms between pin 11 and the other end of the wire at the coil. You can either figure out which color wire, or just disconnect all the non-yellow wires at the coils and it'll be one of the four.

If you want to use the diag connector, you can use the diagram in the manual to see where the pins in that connector attach to. For example, you should measure about zero ohms between diag connector pin 2 and one of the digiplex connectors pin 11.

oh, and the manual diagram doesn't tell you which bank is which. In the 84 USA manual, bank 1-4 is the top coil and the left digiplex. If in doubt, look at the schematic to verify the color of the wires between pin 11 and the coil.

note also that you can swap the digiplexes by swapping the connector plugs. If the problem moves to the other bank, you've got an unhappy digiplex. Fortunately, this isn't too common.

'spose might as well mention the 328 is different. It has one TDC sensor instead of two, and one ecu instead of two. Still has a tach sensor. So if you see comments in threads about 328 ignition problems on one bank and a person suggests checking the TDC sensor, that makes no sense. If one bank fires, the TDC sensor is ok.

 

Hey, you could always check the coil like I checked my dad's electric fence unit for his ranch when I was a kid. I just plugged it in and grabbed each of the two electrodes with each hand... it worked. It REALLY worked.