Power to ignition coil - can I test with multimeter?

[CharlesJones]

Update:

Well I took the DME out of the car and opened it up, and unfortunately those solder joints (for component T504) look absolutely fine as far as I can tell - nice shiny smooth joints with no cracks in them. Drat! Maybe there is something else wrong with the DME, but I will try some more diagnostics before I send it anywhere to have it looked at.

I still haven't got myself a noid light yet, and still haven't checked for spark coming out of the coil. The only thing I have tested for is power to the coil (which is there) and spark at the spark plugs (not present) by pulling a HT lead and grounding a spare spark plug against the inlet mainfold.

What I need to do next is check for spark from the actual ignition coil (as opposed to from the distributor), and for an injector pulse which I will test when I get a noid light kit. I am guessing I will have neither since I can't smell any unburnt fuel at all, which suggests there is no injector pulse, and if I have no injector pulse then the coil mustn't be given spark either sicne there is no spark at the plugs. I will check this conclusively though and report back.

Thanks.

[ischmitz]

Do you know someone with a 3.2 in your area? At this point I would do a swap test to check your DME. Either plug yours into a known good car or get a known good one and check if your car runs. A short test is safe and won't harm anything.

Good luck and keep us posted
Ingo

[CharlesJones]

Hi Ingo,

That's a great idea, but unfortunately I don't know anyone with a 3.2. Its a real pain - they are pretty rare here in Ireland, partly due to the massive road tax they levy on cars of more than 3 litres, which is 1683 euro a year (which equates to 2228 US dollars)! Very few people are mad enough to pay such ridiculous fees! However, when a car gets to 30 years old it qualifies for vintage tax, which means it only costs 50 euro to tax it for a year. So I only have 2 years to go - yippee!

[CharlesJones]

I have read in another thread that when investigating a no start issue a good test it to measure voltage at the DME. Specifically a test should be done to check for 12 volts on DME pins 1, 18, 35, (the absence of which suggests a bad DME relay) and for a missing ground on DME pins 5, 16, 17 (which would imply ground issues on engine or tranny).

What I am not sure about is how do I check for voltage at the DME? I can't get to the DME pins with the connector connected of course, so should I disconnect that and check for voltage on the pins of the connector rather than the DME? And if so, are these pins numbered in the connector, or is there a particular corner I have to count them from to determine which pin is number 18, for example?

Also, how do I test for ground? Do I check for continuity between the pin and a ground point on the car?

Thanks again.

[ischmitz]

The pins inside the connector of the DME are labeled. When you look at the DME at the black connector pin 1 is on the upper row at the right (where the metal clip is). Count all the way over to pin 18 on the upper row. Then start on the lower right again for pin 19 and all the way over to pin 35. Use a flashlight and magnifying glass and you can see the numbers. That will help you to find the correct pin on the harness connector. That information will help you to find the correct location on the DME harness connector. That is where you want to measure.

When testing for +12V I would use a test light and a MultiMeter at the same time. Connect one side to GND (chassis) and the other to the pin in question. Then measure the voltage across the test light with the ignition in ON position. Same for GND. Use a known good source of +12V and then test the GND points.

Internally all GND points are connected in the DME. So it is not very likely that you will find something with the GND.

I would focus on finding out if you miss fuel and spark. Does the exhaust smell when you crank for some time? If not, make sure the DME is powered (ICV is vibrating) and then on to the sensors. If they check out it could be the DME itself requiring repair. There is one failure mode where it won't detect the flywheel signals any longer. It requires parts replacement.

Remind us: is this problem intermittent or permanent? I remember I read something in the beginning suggesting it happens only sometimes. If it is permanent, I would suspect the DME. If it is intermittent the sensors (speed, reference) are my first guess.

Good luck,
Ingo

[CharlesJones]

Thanks very much for that advice Ingo. That was exactly the information I needed, and the other day I finally got round to testing the pins on the harness connector with a multimeter (I've been snowed under at work recently). I actually couldn't see the numbers you describe on the DME connector, but I counted the pins from the upper right on the DME like you described, and found the corresponding locations on the harness connector where I needed to measure.

The voltage and grounds I was looking for were all good. Stupidly though I didn't think to measure the resistance across the appropriate pins to check the reference and speed sensors - I didn't know you could do that until I read more about trouble shooting afterwards. However, I did check the resistance of these sensors at the connectors in the engine bay, and sure enough the reference sensor showed infinite ohms! What I don't understand is that when I checked resistance of these two sensors before they were both fine - just over 1K ohms each - and it was after I made that measurement that the car started and ran fine. I thought I had read somewhere that these two sensors were called "light bulb" sensors because when they fail they fail for good, but maybe that isn't the case, or maybe it depends how they fail. Anyway, I definately need a new reference sensor, so I've ordered a pair and will replace both sensors this weekend (assuming my new sensors arive on time). I have ordered the cheaper BMW sensors, so I hope I ordered the right ones!

This was an intermittant problem, Ingo, although it only intermitted once, if you see what I mean. The car wouldn't start, so I tried a fiddled with a few things - replaced the DME relay, measured the resistance of those sensors - then the car was suddenly able to start and ran fine, but then at the next attempt it wouldn't start again and it hasn't started since.

Thanks again for your help. I will report back when I have changed those sensors - hopefully it will all be good after that.

[ischmitz]

Charles, some make you believe these sensors encounter light-bulb type failures and want to say once they fail, they fail for good. However, this is not true for either:

Case in point I had an intermittent brake light on the truck for a long time. When I got pulled over I told the cop to watch: while my wife stepped on the brake I banged the rear light housing and the brake light came on. He let me go. Eventually I got a new bulb because getting pulled over became old. So believe it or not even light bulbs can fail intermittently. Once the filament makes contact it sort of spot welds a little and the light works....for a while. Vibration undoes the weak connection.

These sensors are known to fail intermittently - mostly due to heat stressing the housing and eventually breaking in the wire to coil contact. What you see is very typical. Good find and I am sure the car will run fine once you replace them.

Best,
Ingo

[bucktoenges]

sounds like my problem. I couldn't get any spark. I got the isv valve humming and the fuel pump working but no spark. I checked both the reference sensor and speed sensor through the dme harness to the dme. I ohmed them both by checking #25 and#26 pin and also #7 and # 28 pin (I think they are the right pins) to see what it said. It should read somewhere between .6 to 1.6. I found out that the speed sensor was bad. After changing out dme relay and the dme, and going all through my harness, I found out it was a speed sensor.

I put in the new speed sensor and checked for spark. That was it. Started right up. The problem was when the engine was running I heard some grinding going on and tried to shut down the engine by turning the key switch off. It wouldn't. I then unplugged the dme relay to shut down the engine. I tried to start again but no spark. Turns out my mechanic got the reference sensor too close to the reference pin on the flywheel and sheared it off. Without the reference pin the engine would turn off by normal means and it wouldn't start either. Now I have to get a reference pin and drill out the rest of the pin on the flywheel.