3.2 cranking but no spark
 

So I've been using the search function excesively all day but am a bit of wits end.

My car ran great yesterday I had an awesome day out at the track, and this morning I did some work on the instrument cluster, fixing the speedometer to use a proper harness (I'm installing 964 gauges). I went to take it for a drive to make sure that speedo was working. The car cranked but will not start.

I pulled the speedo cable off just in case but no change

The car is a 1971 with a fuel injected 3.2 from a 1989 vehicle in it.

I have done the following

1) Jumpered the DME relay with a 3 wire jumper and no change, DME Relay also clicks when I put my hand on it.
2) Replaced the DME with a second one I've had for several years, no change
3) It appears as if there is no spark, I'm not seeing one with a spark tester or when I pull a spark plug and lay it on the block.
4) The flywheel position sensors test with expected resistance at both the 35 pin connector and the engine connector
5) The coil has 12 volts and appropriate resistance
6) The distributor cap and rotor seem to have appropriate resistance.

I'm about ready to have it flat bedded to my local mechanic but any other thoughts would be appreciated.

Check the voltage going out of the DME relay. Make sure it is 12+ volts. I just had a similar problem and it ended up being a bad ground strap at the transmission.

Where would I check that voltage? Is there a good spot for it?

Pop the cover off the DME relay plug and check it on the backside of the connectors

Have you checked the voltage from key to spark plug? When I had the same problem I researched on here and with some local help. While I was very frustrated with the problem I learned a ton about my car and was successful in finding my problem by tracing the voltage.

Have you checked that you are getting proper voltage out of the fuse box?
Have you checked voltage on both sides of the coil? If yes, I would check at the end of the cable out of the coil to the distributor.
You have confirmed that some of the major components are OK (DME, DME relay, Coil, distributor, etc.) If it is a problem with no spark, it will be along that path way that you will find the problem (ground, bad wire, etc.)

Hope that helps.

I just had a similar problem with my '86 Carrera. Car ran great, then stalled and wouldn't restart one day (actually, it happened intermittently a couple of times before dying out completely)

I changed my DME relay out with my spare but no joy. I had no spark, according to a timing light and it ended up being a bad solder joint inside the DME computer for the ignition. I re-soldered it and it's running fine now.

You can do a search for 3.2 no-start and tons of results will come up. A quick diagnosis is to watch your tach as you crank, it will bounce as you're cranking if the ignition transistor is good in the DME computer.

The DME relay has two stages. Does it click once when you turn the ignition to RUN and a second time when you crank?

Does the ICV vibrate with the key in RUN position (DME is powered)?
Do you get fuel pulses (smell of gas after prolonged cranking, NOID light)?

Ingo

Well today I tried a second DME computer with no joy. I don't know what the NOID light is, and maybe I don't have it as I have older gauges in my car (although I'm working on the 964 upgrade now).

As I said in my original post I did a lot of searching and a lot of reading, unfortunately many of the post say things like

1) Check that the DME relay is working
2) Check that you have spark
3) check the fuel pump
4) blah blah blah

without telling you what to do if it fails a step. No complaints the info here is fantastic, but I finally worked out the built in search here kinda sucks, I found that putting in 3.2 no spark got me nothing, but another set of terms found me a post entitled "3.2 no spark" it seems using site specific search on bing might be a better choice just to try and help folks out in the future.

Anyway thank you all for the further suggestions, I finally gave up today as I'm heading out of town tomorrow for a bit and wanted the car to be working when I came back so I had it flat towed to my mechanic (Sigh) I so wanted the satisfaction of nailing this one myself!

2nd DME no joy.

Hmmmm, it is time to check the OHMs on the Ref and Speed sensors.

Next time please check your DME in another 3.2 engined 911, if your car as a fault, it could damage the new/donor DME.

Agreed there are a lot of posts out there and it's kind of hard to find the good ones. However, by the end of the day the DME needs three things to produce spark and I asked you a couple of posts ago to check for each. Not seen any response:

- Power to the DME
- speed signal from the sensor
- reference signal from the sensor

Check for each. Power is checked easily by confirming the ICV is vibrating. Did you do that??
Next, the speed signal can be verified by disconnecting DME, hooking up a digital voltmeter in AC mode to the correct pins inside the 35 pin connector and measuring while cranking. Verify you see a >2V AC signal.

Then, checking for reference signal requires an o-scope. With a voltmeter it's a little more difficult but you will see a small signal, too. An O-scope is better though.

If you don't have that alternatively, check if the DME produces a fuel signal either directly (NOID light plugged into the harness instead of an injector) or indirectly by confirming long cranking leads to strong fuel smell out of the exhaust.

A NOID light is an LED. Use Google as your friend and eBay to by it cheap for under 20$.

Based on your findings (still outstanding) you decide where to look for a fault: If you get a fuel signal but no spark its either the DME or the coil/distributor downstream. If you don't get fuel and spark it might be the reference sensor or the DME. In your case you swapped the DME and if you are reasonably confident the other DME is healthy I would look at either the coil or the reference sensor.

Testing the coil is described: Measure primary resistance and secondary resistance and verify it has +12 V on the primary.

Good luck,
Ingo

Any Update
 

My '87 suddenly will not start after sitting for a few weeks. Ran great up until then and then suddenly "no start." I changed the DME, all the fuses, put a fresh 5 gallons in and now have to dive in deeper.

Thanks

Any Update
 

My '87 suddenly will not start after sitting for a few weeks. Ran great up until then and then suddenly "no start." I changed the DME, all the fuses, put a fresh 5 gallons in and now have to dive in deeper.

Thanks

You must have done something while you were working on the inst cluster. retrace your steps.

follow up as I just realized I never closed this, it was one of the sensors on the flywheel the one which causes this, I had inadvertently checked the other one so I didn't find it. Car ran great (well until it just did the same thing again last sunday, but I don't know yet if it has spark even as I've not had time :))

Today I've found I have spark and I can hear the fuel pump running loosening a fitting and we get fuel coming out so what's next? What can cause injectors to not fire?

Simple answer:

-Bad DME
-Bad harness
-Bad injectors

How did you determine that the injectors didn't fire? Best use the NOID light or test light.

How did you determine you have spark at the right time?

Just want to make sure you're not jumping to conclusions.

Which type of noid light will work on Amazon there are ones like this http://www.amazon.com/OTC-3054E-Noid-Lite-Test/dp/B0071JJYLO/ref=sr_1_1?ie=UTF8&qid=1444606186&sr=8-1&keywords=Noid+Light will that work?

For spark I used an in line spark tester and it flashes so I know I'm getting spark at the right time? No way to know, but it's sparking, and it ran great until it just quit while I was driving it so I think that's all good

Tried jumping the dime relay after replacement didn't change anything and no dice, haven't checked fuel pressure yet not sure how to do that, advice welcome. Bad dme? I have a spare but want to isolate anything that might have roached it before I try, fuses are intact

In my case, similar symptoms, it was the reference sensor (IIRC) checked at the suggestion if ischmitz. What was odd was it would occasionally check out fine then later it would show bizarre, out of spec, readings. At first I thought I was just checking it wrong. Replaced it and it's worked perfectly ever since. Just waiting for the other one to fail...or something else to go wrong. Moronic no start issues are a complete PITA!!!

My understanding was that the reference sensor going bad would mean no spark. That happened last summer and it was replaced then.

Correct: If one of the sensors doesn't work there will be no fuel and no spark. If you have proper spark the sensor are fine. During cranking the frequency of sparks is low enough that you can see each event with a tester. Better yet get a timing light.

Let's see what the NOID light will tell you. The one on Amazon is fine. Here is an even better deal on evil bay: 9pc Electronic Fuel Injection Signal Noid Lite Tester Light Set GM Ford Bosch | eBay

They all work just fine

Ingo

Cool thanks, I just OHM'd out the sensors, and they're good. I ordered the light from Amazon it'll be here in a few days, and we'll see what we have then, beginning to suspect computer, as everything else seems good. It's that or fuel pressure, as I've not validated that, I would think that if I let the pump build though (Since it runs with the key in run, and not just when the DME tells it to, have to fix that one day) it would at least sputter or cough.

Also I noticed the tach doesn't move when it's cranked, is that important.

Electronically all three (tach, injectors, coil ) will work once the sensors see flywheel motion. The tach isn't always good enough to show the ~200 RPM during cranking though. So I'm not sure I'd put too much into that observation. Let's see what the NOID light will tell us.

Noid light shows injectors not firing, next up trying to check grounds


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Injectors don't need GND. They get +12V from DME relay and GND pulses from the DME. If the coil works your GND to the DME is fine.

You start to narrow issues to the DME. Send me a PM if you need it checked/repaired.

Ingo

I'm going to try my dme in another car first, I have a spare but don't want to blow that up, what can kill a dme?

DME killers would be shorts in the harness (injectors, coil) or a miss-wired tachometer

Hmm, seems unlikely those would have cropped up in a car that's been running for 20 years but that's where I'll look. I'm going to try my ECU in a friends car this weekend, and go from there. Is it more likely it just failed or that something killed it? I'm also thinking I should trace the harness for the injectors back and make sure it didn't just come unplugged, the engine was out earlier this year, it's seen 2 tracks days and maybe 200 miles of street driving since then but we could've screwed something up when it went back together.

Besides the injector harness coming loose, an over-voltaging alternator (bad regulator)
can damage the DME control unit's injector output resulting in no injector signal.

Has anyone mentioned to check for tightness the ground connections on the intake manifold above #1 cylinder? A number of them come together there. Saw this happen before. Wasted a lot of time doing a lot of checks. Problem solved by simply tightening the ground connection. Duh.

When mine exhibited those symptoms, it turned out to be the 12v power to the DME fuse. It had worn thru the metal on the top. Replaced the old fuse and haven't looked back.

Where can I measure for 12v to the DME At the relay or pull the power on DME? Would this be the case if I have spark and it cranks?

For grounding I attached some jumper cables to that bolt on the runner and ran it tot he front of the car where the batteries - terminal are attached to the chassis, no difference so I doubt it's a poor ground.

I'd check alternator but how do I do that when the engine won't run? It won't be measurable while cranking!

One shorted injector will take out the other 5 because all 6 are wired in parallel. I suggest you unplug the injectors on the LHS cyl1-3 and see if it attempts to start. Basically you are eliminating the 3 LHS injectors to see if she starts. Then do the opposite RHS the same way. If you find that disconnecting the LHS injectors now allows the engine to start then one of those 3 are shorted, continue plugging them in one at a time till you find the shorted injector.

I mention the above because you already said you can't detect injector pulses with a noid light. You can also try the noid light test again but unplug all injectors except just 1, and test at that 1 injector to see if you now have pulses.

I don't see this as a common problem but I have seen it a few times.

Right, you can't until the engine runs and you still might not determine over-voltaging
if the alternator is intermittent. But if the DME ECU's injector output turns out to be
your injector problem, an intermittent alternator can cause that. As mentioned, a shorted
injector/injectors or wiring can cause the same problem, but it's rare.

Thanks I'll check these things next. Seems like all I've got left are rare failures, as I've already found the common ones are good.

Hopefully it's a simple wiring problem and not your control unit.
Make sure the up-shift wire from the control unit (pin 11) is not being used
nor connected incorrectly as this will also damage the injector output.

Has anyone checked the flywheel sensors. I had one of mine fail so I put in both new to head off any future issues as they were a fita to replace....Put them in and it fired right up! These are either good or bad no in betwen...If there is no signal to the DME from these sensors the DME will do NOTHING, frustratinf when it happens suddenly.

Yup above in thread plus if either of those fail you won't get spark. I haven't checked CHT sensor just yet, so I'll also do that but next up is looking for a shorted injector. I doubt that the upshift wire is the issue as the car's run in this configuration for 23 years now...

CHT shouldn't effect spark, it will effect fuel mixture. Car will run REALLY rough with a bad CHT or one that looks connected but isn't (as was my case when my car started running like crap)

You need to be very clear on what works and what does not. If you have spark then both crank sensors are working. So if you have spark but no injection pulses we know it's something in the injector drive circuit or the injectors. I test for coil and injector pulses with a simple LED test light, you can get one at Walmart for about $5.00 but it MUST be the LED type. Just unplug an injector and put the light across the 2 pins in the injector harness and have someone crank the starter, the LED must flicker if you see no LED activity you have no inj pulses. You can use the same test light across the coil's '-' and '+' posts and it will also flicker if you have coil pulses. The LED test light can even be used to test both crank sensors, unplug each sensor and put the light across the pins that go to the sensor, then crank motor and again the test light must flicker.

Where are you located?

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