308 ignition problem

hi jon,

you're doing all the right things! About the only stuff left to try is:

1] swap the digiplexes
2] swap the coils (secondaries and the non-solid yellow wires)
3] check the plug between the ecu's and the coils/sensors. Even if it ohms ok, a poor connection could degrade the signals
4] measure the resistance between digiplex connector pin 11 and the wire at the coil
5] check for corrosion in the coil secondary connector
6] check for bad coil/spark plug wires
7] measure the signals with an AC voltmeter when car is running

for the last one, I don't know what the correct numbers would be for the 308. The idea is the sensor wires and the wires between the ecu and coils have pulses on them, so if you use a voltmeter set on AC, you'll get a voltage reading related to the pulse peak voltage and duty cycle.

on the 328, carl rose provided the following:
- tdc sensor should be >0.2V
- tach sensor >2.0V
- primary coil wires from ecu's 2.8-3.5V

the sensors are similar in both cars, so it's more likely those values would be the same. The coil setup is different, since the 328 has ignition modules on the coils that the ecu is talking to, so that value is probably different.

on the bright side, your system is symmetrical, so you should see about the same numbers when comparing the values for each bank. You can make those measurements at the diag connector.

I don't know far a sensor would need to be unseated out of the bell housing before it stopped detecting the flywheel position, but if it's doing that, you'd see a lower voltage on the wire.

 

Agree with everything Phil said above. Jon, when you checked the plug wire & coil leads, did you verify connectivity once they were seated in the cap? (i.e. terminal-to-snap-on connector). There is a Microplex diagnostic check algorithm at the back of the 328 service manual - where the above #s are from - don't know if the 308 has a similar setion for Dinoplex? I also have a pictorial of how to check (for 328) at Microplex, happy to send along if it would help.

Carl

 

Did you run the new ground straps to a location outside the baggage compartment. The new ground straps must go to a ground in the engine compartment (a gound ground there is next to the coils) and not in the baggage area.

 

hi jon,

I think I misread your initial post. Originally bank 1-4 was fine, and the only thing that changed was a new sensor was installed?

besides the sensor not being seated, you could also be picking up noise on the sensor wire if it's routed too closely to a coil or plug wire.

the splices for the new sensors where soldered, right?

I think the poor ground in the trunk tub has to do with the way the rear of the car is assembled. I've never really looked, though. I'd guess that in a lot of cases, just cleaning the ground connection would have worked just as well. The only way it would make a difference is if the ground point is not part of the welded frame, but instead goes through a bolted junction someplace that is degrading electrically.

 

On more then a dozen cars that I know of (I am sure there are hundreds more)the ground in the baggage compartment tends to float (comes and goes) and it is not due to the bolt area where the strap attaches. The ground to the engine compartment ends that problem.
Bank 5-8 runs the tack so one of your 1st symptons is your tack starts doing strange things.

Cleaning the ground will not solve the problem.

 

Did you check both rotors? I know it seems obvious, but on my old Mondial QV, one of the rotors literally disintegrated. The caps cost a fortune, but the rotors are relatively inexpensive. Also, check the caps very carefully and make sure that you don't have a hairline crack in one of them. And try cleaning up the connectors to the digiplex units. Get some connector cleaner, spray them up good, make sure both ends are clean and make sure they are seated properly when you reconnect. Even slight problems can throw the entire system off.

 

Switch over to an Electromotive set up and end all your problems.

Contact Nick at Nicks Forza (a sponsor)

 

Dump the digi's!!!!!!!!!!!!!!!!!!!!!!
and all the crap coils, sensors and the retarded ignition cure that comes with it.

I've installed a dozen and never had a complaint or a service issue since I did the 1st one 5 years ago.

 

since the problem appears to be the digiplex, you got nothing to lose by opening the thing up and taking a look. Check to make sure it isn't just crud on the pins first, but if they are ok, unscrew the lid and check the wires from the pins to the circuit board, and also the wire going from pin 11 to the transistor, which is just attached with a ring terminal. Could be as simple as cleaning that connection.

If you're adventurous, see if you can remove the transistor and get a part number off the top of it, and replace it.

I gotta agree that for $1300, it's better to switch to electromotive or whatever.

attached is a pic of the inside of a digiplex - at least, one version of med-801-a
Image Unavailable, Please Login

 

Jon have you replaced all of the spark plugs?Wen you lost the back bank its possible that you have fouled the plugs as they weren't getting spark.Only a suggestion.Also try replacing the leads if there old.When I rebuilt the engine in my car the car had been off the road for about 10 months and when I put it all back together it would miss at high revs.The distributors were checked as part of the rebuild and everything else was checked except that I was using the same leads.I didn't have a problem with the leads prior to the rebuild but I replaced the coils and it didn't fix it.Then I replaced the leads and couldn't believe what a difference it made.

 

I switched to aftermarket, so have a couple of digiplexes that were working OK when I pulled 'em. Available in Toronto (pretty) cheap to anyone who is keeping their car concours...'82 308. Also 2 coils/ distributors, caps etc.

 

What type of Digiplex do you have for sale?

 

Jon glad to see that you got some better results.If I were you I would swap over to an electronic ignition setup.I have recently done mine and it is hardly noticeable but it made a big difference to the overall running of the car.I have a crane setup with a photo electric sensor mounted in the front distributor

 

Be sure to change your oil ASAP. With all the running with some cylindars down, it's likely quite contaminated by now.

Ken