944 intermittent starting problem

[ASTAR911]

Trying to work out what is wrong with my early 85 944 auto with an intermittent starting issue.

It cranks but doesn’t start and then every no and then starts perfectly.

Starts perfectly runs for about 20-30 seconds then dies, then it cranks and doesn’t start.

Car has had little use over recent years.

Battery is good, dme relay replaced, speed and reference sensors replaced (FAE brand).

Could this be an old aftermarket alarm playing up (immobilising)? Old fuel?

When it cranks and no start, there is no tach bounce and bit of a fuel smell (not strong).

[Jfrahm]

It's best to test with a DME relay jumper.
With no tach bounce you should have no spark and no injector pulses. But it's still worth trying a shot of carb cleaner or starting fluid and not make assumptions.

Check and clean the 9 pin connector. Test for AC voltage from the speed and reference sensor at the DME, you can do this with a DMM instead of an oscilloscope.

https://www.clarks-garage.com/shop-manual/ign-02.htm

While you are at it, see if you are getting power to the DME. If there is an old alarm system who knows what it might be doing.

[ASTAR911]

Status update;

DME relay has been replaced recently

Speed and reference sensors have been replaced (FAE brand) recently and look relatively new, plugged in correctly to DG/BG wires. The bracket looks like it has never been removed, so I am assuming depth has been set correctly. The depth of the magnetic part of the sensor on the FAE's is the same as the original/old Bosch ones.

Removed old alarm system (no change, still no start)

New/fresh fuel (no change, stil no start)

DME relay bypass, fuel pump runs (no change, still no start)

Will now focus on checking for spark….,,

[Jfrahm]

With no tach twitch there shouldn't be a spark but you also smell fuel and there shouldn't be fuel either.

You could look for the fuel pump turn-on signal at the DME relay socket. This is easier to isolate than the wiring at the DME connector:
This would be a ground appearing on terminal 85b when cranking and with the DME relay jumper in place:
https://www.clarks-garage.com/shop-manual/fuel-05.htm

If that doesn't show up when cranking then either the DME is not powered up, not seeing the S/R signals or there is a wiring or circuit issue. A test lamp is good for this as it pulls some current where a DMM will show you voltage that might disappear if there is any load.

If that ground is not present during cranking you should be looking for DME power/ground and S/R sensor signals at the DME. However for completeness I would also look for the negative pulses at the coil to initiate spark, maybe with a test lamp if you are not familiar with this sort of signal on a DMM.

For an intermittent problem it should be something like green crusties in the wiring harness, loose sockets in the fuse/relay panel, dying DME computer, etc. That's the sort of thing that can really fool a DMM also (or fool the guy looking at the DMM).

[Jfrahm]

Er, I had the pin wrong for the fuel pump relay coil ground in the DME relay socket.
Socket 85b gets grounded when the DME wants to turn the fuel pump on. This is when the DME sees the signal from the S/R sensors so fuel pressure is only provided when the engine is starting or running.
(A DME relay jumper powers the fuel pump all the time though).

[ASTAR911]

Update;

Checked battery reading 12v and approx. 10.5v when cranking.

Cleaned ground connections including one behind engine/bellhousing.

Checked for any loose wires behind fuse box and couldnt find anything.

Speed sensor reading 882 OHMS from sensor plug
Reference sensor reading 885 OHMS from sensor plug
DME plus pins 8-27 reading 882 OHMS
DME plug pins 25-26 reading 885 OHMS

Not sure if this means the speed and reference sensors and the wires going to DME are all OK??? They are FAE brand, I have read that although they read ok they could still be duds!

Checked the voltage on the coil wires and both reading 12V with ignition at on position.
Checked spark to spark plug using in line tester, no spark.
Check for spark on from main lead coming from coil, no spark.

Not sure what else to check at this stage.

[curtisr]

Hi,

You mentioned removal of the 'old alarm'. Have you tried 'jumpering' it?

https://www.clarks-garage.com/shop-manual/elect-23.htm

[ASTAR911]

Originally Posted by curtisr

Hi,

You mentioned removal of the 'old alarm'. Have you tried 'jumpering' it?

https://www.clarks-garage.com/shop-manual/elect-23.htm

It was an aftermarket alarm, pretty much removed the whole thing, so nothing left to jumper.

Anyone now an easy way to check if I removed everything correctly?

[ASTAR911]

I noticed when I turn the ignition to on only the oil light is on and the goes off when I start crankkng.
Is this normal, should there be more ligh when the ignition is at the on position.

[Monkey Wrench]

the light normaly shows no oil pressure which is normal when the engine isn't started, it should go out once there is oil pressure. same with any older car, there is normally a pressure switch fed off an oil galley usually on the block somewhere.

if you like features you can connect an actual guage, the idiot light is there to warn you of a no oil presure condition which will likely become critical very quickly. it may come on briefly if you are low on oil and take a corner causing the oil to slosh and then the pump can run dry, if you see any of that pull over, pay attention immediately. never drive it if the light is on.

best to leave the lamp intact even if you prefer or want ot add a guage. if it is a guage it can either have a sender and an electric guage, or a small copper line to the mechanical guage. I prefer the copper line. less stuff to fail.

[Monkey Wrench]

Checked the voltage on the coil wires and both reading 12V with ignition at on position.
Checked spark to spark plug using in line tester, no spark.
Check for spark on from main lead coming from coil, no spark.

yes when key is on you shoudl have 12 v at one of the coil wires.

try hooking a small lamp across between that and the coil wire near it, this is the primary ignition coil.

hop in and crank the engine, the lamp should flash on and off when cranking, if it starts it will just glow dim.

If you do not see those pulsations at the coil , you do not have a pulse to the primary coil and the problem is between the coil and ECU or maybe that the ECU isnt; synching up and alllowint the ignition system to function.


if the primary coil has no pulse dont sustpect the ignition coil or the distributor cap or wires, yes it is possible they could ALSO have issues.. If you have proven you do not have a primary coil pulse so then you can focus upon why there is no pulse.

likely since you had issues withthe alarm it isnt; allowing the ECU to enguage , basically. youmight check that the ECU is grounded and if it gets it's 12 V or perhaps the DME relay is preventing it from becoming energised.

maybe someone else can explain more clearly how the system prevents starting, It sounds like you are on the right track.

the DME relay is a dounble relay, part of it ins involved in starting the fuel pump , another part is involved in allowing 12 V power to the DME to function I suspect that the theft prevention system is involved with related wiring.

the car may have a way of sensing a stalled engine and that may turn off the fuel pump so it may not normally run for more than a short prime without the car being started.

the car will not run without the main fuel pump, you cna check for fuel flow and you can check for fuel pressure too.

If you do not have a pulse at the primary the car wont start because it will then have no spark.

I believe that your car, like mine is early 85 and so it will have a metal tank and no fuel pump in the gas tank..

unrelated but :
have a look at that hose coming out of the tank, if it looks old , change that hunk of hose because mine let go of its whole tank of gas ,when the hose rotted. the hose is cheap. it can burn down your house , so I'd check that out even though it is unrelated. some cars had rust issues on top of the tank causing the stink of rawgas in the car.

to remove th tankmeans removing the transaxle first so if you need a new clutch or axle shaft bearings or similar its probably good to do those jobs together.

our cars used a rubber centered clutch so it is suspect that there could be an audible clunk when letting thre clutch out because of the rubber becoming degraded, if you have the starter off you can look for bits of rubber.

mine meeds love beofre it hits the road even for a test run but i suspect that at some point Ill clean and recoat the tank with por 15 ( there is a gas tank kit) and a clutch, and probably the driveshaft bearings in the torque tube , maybe CV joints as well.
someone mentioned that the CV joints from a VW van may work.

[ASTAR911]

Haven't had a lot of time to work on car...

Quickly checked volatge on fuse board for dme relay;

Pin 30 reading 12.88v
Pin 86 reading 0v with ignition off and 12.55v with ignition on
Pin 87, 87B & 85B reading 0v with ignition off and 0v with ignition on

Not 100% what this means but I think it means the really is getting power from battery and ignition, but not sure if the DME is getting power?

[kdjones2000]

It's probably the ignition switch, it's notorious for the early cars to have issues.

[Monkey Wrench]

wont hurt to check and actually clean the battery ground, to frame and to engine.

everythign electrical depends upon a good ground back to battery but for some reason it so often becomes assumed to be ok. you care more likely to find a weak intermittlent or high resistance connection than total loss of connection , otherwise nothing in the car that is electrical could function at all either. no connection would be identiacal ( electrically) to pulling the negative cable off .

a weak ground can cause intermittens and various troubleshooting may still see a functional ground as it is actually may be still grounded , somewhat.
if anything is loading the DME relay, pump has bad bearings but turnes for examle hat may cause more load on the contacts of the DME. pitted contacts can often present as intermittent issues.

I had similar issues on a volvo and I did have a pulse with a test lamp across the coil , the issue was that the actual nuts on the coil and the studs on the coil , cleaning fixed that.

also there is a little amplifier on the ground its proabably around the inner fender of the drivers side, the coil wies will lead you to it, check the connections there.. they may be dirty. thats part of the circuit between the DME and the coil. its bolted to metal with heat transfer putty behind it. my plate 80's Volvos use the same part. ive never seen one fail but the connections could be cleaned to assure contact. I believe it has three wires so someone else can perhaps better expliain how it functions electronically . I realize that amplifying a ground is probably weak on the technical side.

I believe the key provides the hot and the ground is provided by the dme through this item.

since it is intermittent , a lamp across the coil may go out when you have it under the no start condition. if its out as youare diagnosing, is your test lamp bulb loosing it's hot or is the lamp loosing the the ground?

if it blinks and youare cranking and it is still not starting then the issue like past the coil wires, of course from the high side of the coil its passing the distributor cap and on to the plugs. If its loosing the pulse intermittently then it may be a problem back through the transistor through the DME and through the relay contacts. back throuhg the chassis ground and back through the battery cable to the battery post. of course the DME wont send the grounded signal if it looses its hot beacuee the computer is down basically.

if you suspect the key is not providing consistent power Id tcheak that at the coil not at the key itself, hook up a test light and see if the power ever faulters, if it does then look towards the key switch. no pont messing wiht the actual key switchif its working electrically, and you can check that .. if the 12V + power wire is not consistent then yes, look at the key switch. maybe its flakey. or maybe the power supplying the key switch is flakey. ( intermittent or connected but with high resistance) )

I would not give up and order a DME unless you first chack all the contact points mentioned for a bad connection.


as mentioned a multimeter may mask the issues

if it sees 12 V and almost no potential it reads 12V

whereasa bulb or an old style analog meter with a needle that sweeps that may be less easily "fooled" so that could help.

the term "erroneous voltage" may fit the description of that scenario , it can throw people off track if they rely on their digital meter without questioning the situation. also shoud be obvious but the meter can't be on the AC scale if you are measuring DC. It happens ..

if you measure voltage near 12 V on a DMM and it wont light a light bulb because it has not enough current, that would be the same situation just worded differently.