No start - traced about everything - coil explanation please

jhilb · 10-25-2010, 04:32 PM

One morning the car starts. I shut it off after 5 minutes. A few minutes later I attempt to start it again. It runs rough and finally dies after about a minute.

It wouldn't start again. That was about 2 weeks ago. I've tried starting multiple times since.

Fuel is being delivered. DME relay tested. It energizes, etc. Ignition switch is good. I've tried two.

I am not getting spark. No 12v at the coil on the black wire (should there be)?

Previous owner had 12v from ignition switch run position hot-wired to black wire on the coil. If I hot wire the battery to the black wire similar to what PO did, car starts, runs strong. Which is basically bypassing the DME and all its sensors to power up the coil regardless and get it running.

If I remove 12v from battery it dies immediately. Sounds like the DME is not triggering that black wire. I thought the DME triggered the green wire though?

I've traced all the DME connector wires (sensors) to their respective end points. All checked out good.

Speed and Reference sensors show correct resistance, but I have no tach 'bounce' during cranking.

I've had a starting issue before. Previously I pulled the DME conenctor off and proceeded to 'pin out' all the connections. They all came up clean, like they did just now. When I put the DME cable back on the back of the DME, it started right up.

So my issue is either the coil, the speed and reference sensors, or the DME itself.

I think the coil is good but the previously hot-wired ignition switch to the black wire (not green wire) has me concerned about that. I've done all the reading I can but still do not understand the green vs black wire on the coil. I am first trying to eliminate that as a possibility.

Next I suppose I should eliminate the sensors.

I suspect it's the DME or connections but will save that for last.

The car has started a bunch of times previously. Mostly would start in the mornign (I know it sounds weird). Now doesn't like to start at all.

bazar01 · bazar01's Garage

You have started some good troubleshooting there. Knowing that you don't have 12V at the black wire in the coil is a dead give away. The 12V to the black wire is supposed to come from a switched 12V power from the ignition switch. All the DME does is provide the switching ground path for the ignition coil to generate high voltage spark, just like the contact point on older ignition systems.

Your coil is good. You just don't have the 12V from the ignition switch that powers up the coil from either a bad ignition switch or a break in the wire which can be tested with a digital VOM.

Good luck.

wild man · 10-26-2010, 07:17 AM

Quote:

Originally Posted by jhilb

Next I suppose I should eliminate the sensors.

I suspect it's the DME or connections but will save that for last.

Consider them already eliminated! And the DME also, as it does NOT trigger the black wire (+). Only the (-) wire on the coil is switched while the ignition is on.

It has got to be a connection, if not a wire itself. That is assuming that the ignition switch is not to blame.

S ECU Repair · 10-26-2010, 07:52 AM

Hi , most likely you can have a problem with the reference sensors or the DME , i have never have to replace a Coil in a 944 before ,they just don't go bad .

Check RPM sensor Pins # 8 and 27 at the DME connector = 600 to 1600 Ohm
Check TDC sensor Pins # 25 and 26 at the Dme connector = 600 to 1600 Ohm

Average readings for this sensors and 1000 some Ohm ,if they check fine ,then the DME can be defective and we can help .

Jose

wild man · 10-26-2010, 08:14 AM

He already said that if he jumpered the (+) coil terminal directly to the battery, it runs fine. In my book anyways, that would eliminate the speed and reference sensors.

S ECU Repair · 10-26-2010, 08:18 AM

Ok so then attach the positive side to terminal 15 .

bazar01 · bazar01's Garage

Quote:

Originally Posted by S ECU Repair

Ok so then attach the positive side to terminal 15 .

Terminal 15 in the electrical manual is the coil + terminal. He needs to find out why the +12V is missing in that terminal.

jhilb · 10-26-2010, 12:36 PM

I've tried two different ignition switches. One was brand new.

Both the RPM sensor and the TDC sensor check out to 1020 Ohm.

What should I do with Terminal 15?

I should check the 12v switched ignition to black (+) for continuity right?

Thanks guys!!! I did my best to follow the Clarks Garage troubleshooting but kept coming up blank.

Jeff

jhilb · 02:34 PM

Update..

I get 980Ohm on 8-27 terminals, and 985Ohm on the 25-26 terminals. Just tried starting it again and I do get tach bounce. I am pretty sure we just ruled out the sensors.

But still no start after re-seating the DME connector.

I am going to trace the ignition switch wire to the black coil wire (after disconnect batt). I know the switch itself is good.

bazar01 · bazar01's Garage

The wire from the ignition switch is black at terminal 15 and goes straight to the ignition coil terminal 15 which is also a black wire. The green wire on the coil goes to the DME at pin 1.

The problem is that the 12V from the ignition switch is not getting to the coil. Test to make sure there is 12V at the ignition switch in position 2. If there is 12V there, there is a break in the wire or a loose connection between the switch and the coil.

The 12V wire is marked red in this diagram.

jhilb · 10-26-2010, 03:21 PM

Great minds think alike. There is 12v getting into the switch.

So it looks like (well it is) the problem is the wire from the switch to the coil. Which explains why the PO had a wire running from Term 15 to the coil.

I tried putting 12v to Term 15 just for kicks, no luck. I could once again run a wire from the switch to the coil but I'd rather find where in the harness the issue is. Although without tearing through the whole harness that is going to be near impossible.

Maybe I might get lucky and it's just a bad plug connector. I tried the 9 pin plug on the firewall but I don't think this runs through that.

RC924 · 10-26-2010, 07:03 PM

There is simply a break or intermittent connection SOMEWHERE between the ignition switch and coil +.

Another possibility is that there is, or may have been, either a kill switch or immobiliser installed by a PO. PITA I know getting under the dash, but first look for a concealed switch or added on relay. May be necessary to trace the individual wire but test conductivity between every single connection first. Check / clean / lube with silicon grease all connectors.