No spark, No tach bounce, 2 new sensors

Big40ozs · 07:22 AM

Status: Just bought an 88 N/A. Cranks but won't start.
Jumped the DME relay. I can hear the fuel pump.
Installed new Ref and Speed sensors.
Coil chk OK.
I STILL have no spark, and no tach bounce.

Tomorrow will chk continuity between sensors and DME.
Any other ideas? Am I forgetting anything obvious?

mikepellegrini · mikepellegrini's Garage

Have you read through the troubleshooting section at Clarks? I've always found it really helpful: http://www.clarks-garage.com/shop-manual/ts-01.htm and for ignition problems: http://www.clarks-garage.com/shop-manual/ign-04.htm

Are you sure the sensors are gapped properly? How's fuel pressure?

How about the connections between the DME and the sensors? (this plagued us forever on my kid's 86 951. Took months to figure out. Had to replace the harness).

Where have you checked for spark? At a plug wire or the wire from the coil? (you're sure it's not a dirty distributor cap or rotor?).

You have a known good DME you can use?

One time my kid replaced his coil with a new MSG coil and a couple days later, the car crapped out. We screwed with it and screwed with it - what it was was a defective NEW coil. It checked out okay in the tests, but it just couldn't produce a good spark. Replaced it with the old coil and the car ran great.

Lots of luck. Problems like this drive you nuts! Just hang in there....

kdjones2000 · kdjones2000's Garage

Sounds like a DME to me....

I would check Clark's to see if there are other suggestions.

Keith

Big40ozs · 05-03-2008, 04:41 AM

mikepellegrini,

Which harness did you replace?
Are you talking about the harness side sensor connectors or the wiring itself?

mikepellegrini · mikepellegrini's Garage

We replaced about a one foot section of the harness between the speed and refrence sensor plugs and the DME.

The problem was the plugs themselves - or rather the wiring immediately leading into the plugs (on the DME side).

Apparently, the plugs had been handled too much and the wires leading into the plugs had broken (not all the way but it was enough so they didn't have a steady connection). The way we finally diagnosed this was by having someone jiggle the wires while another person cranked the car over. If you jiggled the wires just right, the car would usually start.

We tried soldering the wires back into the plugs and that really didn't work. So we got the harness from another car, and spliced in the one foot section leading to the plugs (both speed and reference sensor plugs) and now it works good. No more starting problems.

I should say he did have another problem that appears to have resolved itself: there was either a problem with the connector at the DME, or maybe the DME itself - if you stomped on the floorboards by where the DME is, the car would cut out. Or sometimes, if you even slammed the door.

The DME connector was securely plugged in, so that wasn't it. But like I said, this resolved itself - hasn't happened again in months.

In my son's case, I'm thinking his whole engine wiring harness is possibly semi-toast and that the way to once and forever fix the problem is to completely replace the harness. If the problems start again, that's maybe what we'll do.

Try jiggling the wires and see what happens.

Another thing to check are your grounds - bad grounds can cause a whole lot of problems like this. We had an 84 NA that had problems with that. After we located and cleaned everything, it finally started good every time.

mattdavis11 · 05-03-2008, 06:56 AM

I had the same problem after I replaced the positive terminal connector. The single lead for the dme (I gather) was not making good enough of a connection. I thought it should have been good, it was bunched in the connector clamp as good as the other wires were. Stripped the wire back some, cleaned it up, good to go.

Big40ozs · 08:11 AM

DME:
DME checked out bad. Had it rebuilt. Still no start, no tach bounce, no spark.

Ref and Speed:
Did a continuity check from DME connector to sensors.
Wires/connectors are ok.
Used oscilloscope to check ref and speed signals at the DME connector.
Signals are strong.
DME connector grounds and 12v+ are fine.

COIL:
Wanted to check coil by itself using Clark's Garage method.
Primary and secondary resistance is within spec. So on to "spark check".
BUT......I cannot get a spark out of it. The ground wire that I am "tapping" to batt. gnd is sparking instead.
So I tried with an aftermarket and same thing.
Its almost as if they are shorting when I tap to ground.
Why does the resistance check out OK, but the thing won't spark?
Do I need to hold it to gnd longer to charge it? Or is something else the issue?

I know this is not the only problem, but I at least want to eliminate this variable.
________________________________________
Update:
9/1/08

The coil is fine. I checked it outside of the car and it generates spark.
The way I did this was by using a 9 volt battery.
It would not work with 12V. This must be an ignition that has a resistor internally. eg. within the DME.

onZedge · 05-17-2008, 09:57 AM

Two coils with the same problem is unlikely. Turn on the ignition switch. You should you have voltage at the small coil wires - both of them.

Big40ozs · 05-17-2008, 12:18 PM

I have 12v at both of the primary leads of the coil.
I'm just not sure why it checks out ok electrically (primary and secondary resistances in spec), but it won't generate spark when I ground the neg terminal.

KY944S · 05-17-2008, 02:38 PM

If you have a factory alarm you may want to try and buypassing it. This is also found in clarks.

onZedge · 03:04 PM

Quote:

Originally Posted by Big40ozs

...Just bought an 88 N/A...Am I forgetting anything obvious?...

If you are using the engine to ground the spark plug for the spark test - and you can't get a spark with two different coils, you have to ask yourself "Hmm, I wonder if the engine block is actually grounded"

jmd_forest · 05-21-2008, 03:28 PM

Check to make sure the timing belt is not broken. Broken timing belt = stationary cam, stationary cam = stationary rotor = no spark at spark plugs.

Also, as KY944S states, try bypassing the alarm. It will eliminate that variable.

If possible, transplant your DME into a known running 944 to verify your DME is good.

jmd_forest

hpservertech · 07-18-2008, 07:50 PM

Did you ever solve this? I'm having the exact same problem!

Spidey944 · 07-19-2008, 01:50 PM

I did this on my car, well, went through it anyway about a year ago. Turns out the bolt holding the distrubuter rotor on fell out and the engine would catch once in a while but never start. Took me 3 days to find the issue, and i'm an ASE cert. mechanic for 10 years! Just a thought. I felt so stupid, because i pulled the cap off and saw the one bolt, but forgot that there are two bolts holding everything together.

onZedge · 07-19-2008, 02:52 PM

After re-reading this thread, I don't see where the injector wires were checked. With the ignition on, there should be battery voltage (or close to it) at both.

hpservertech · 07-19-2008, 03:12 PM

Ok. You got me thinking. Made sure that the rotor was at least turning, so at least the belts aren't broke.

I did take the distributor off as well as the rotor. Should I be able to get continuity between the middle of the rotor to the end of the rotor? Also, shouldn't I be able to get continuity on the distributor on the outside pins to the inside? This isn't making any sense to me.

onZedge · 04:01 PM

You should have close to zero ohms between the inside of the dist cap and its corresponding pin on the outside. I have to check about the rotor. I will edit this post when I measure a spare (gimme a minute)

EDIT: WRONG! I read 1-1.5K ohms across the dist cap terminal pins and 1K ohms across the center tower. I read 1.5K ohms across the rotor. Both caps were used, one Bosch, one Beru. Rotor was used Bosch.

hpservertech · 07-19-2008, 05:03 PM

Well I have followed Clarks page for the ignition coil. http://www.clarks-garage.com/shop-manual/ign-04.htm

Everything checkes out ok, except from getting spark directly from the oil. So if I read it correctly the coil is bad. Going to see if i can borrow someones coil & dme tonight and at least get it started.

Outback Porsche · 07-20-2008, 05:00 AM

I'm assuming that you have no tach bounce, as the thread title states.

This would mean that you have no switching ground signal (or pulsing ground) leaving pin 1 of the DME. This is signal that 'drives' the coil and is split at the 9 pin connector on the firewall of the engine bay (pin 1 or the heavy green wire) and goes to the tacho.

One of two things is happening;
a) At the coil you have 12V on both primary terminals and no evidence of a pulsed ground from the DME. The fault exists from the DME back, and could include the speed sensor.

or b) You have 12V permanently across the primary of the coil. which would indicate that the output of the DME is being shorted to ground somewhere. Alarm system, DME, wiring, etc..

Cheers,

Outback Porsche · 07-20-2008, 05:41 AM

If you want to check your coil, could could try this;

This is the disclaimer part. There is some risk here of being electrocuted by the back EMF (Electro Magnetic Force) of the coil. Care need to be taken (But it does make a funny story to tell your mates if you do electrocute yourself and any spectators will get a good laugh). Insulated pliers will help.

Rotate the engine so that you have electrical continuity through the rotor to a spark plug and remove the spark plug. Disconnect the wire on the coil from the DME (the green one). Whilst grounding the spark plug to the engine, tap the DME terminal on the coil with a flying lead that you have connected to ground. You should see a strong spark on the plug.

Cheers,