no start no tach bounce. sensor or DME?

[greasewagen]

Hi all,
new to this forum, but not to these cars. Have had 1 924 and this is my 3rd 944.
This car is a 1985.5 944
Car would run and stall intermittent. Sometimes restart sometimes not. It sat for a week and now it won't restart.
When you crank engine over there is no movement of tachometer.

Battery is decent, charged it over night , has over 12 volt at rest, spins engine over good.
Had spare DME relay. Tried that no luck.

I made a DME relay bypass/jumper and did not start. Does this test bypass the DME itself, ruling it out as bad?

Measured resistance of speed and reference sensors. Getting 900 ohms between pins 8 and 27 on ref sensor, and 880 between pins 25 and 26 on speed sensor. Those seem within spec.
But between 8 and 23 on ref sensor and 25 to 78 on speed sensor I get nothing measurable. Is this normal or do they both seem bad?


Having trouble deciding if the DME or 1 or both sensors are bad. Any info appreciated,
thanks

[Toast54]

Originally Posted by greasewagen

Hi all,
new to this forum, but not to these cars. Have had 1 924 and this is my 3rd 944.
This car is a 1985.5 944
Car would run and stall intermittent. Sometimes restart sometimes not. It sat for a week and now it won't restart.
When you crank engine over there is no movement of tachometer.

Battery is decent, charged it over night , has over 12 volt at rest, spins engine over good.
Had spare DME relay. Tried that no luck.

I made a DME relay bypass/jumper and did not start. Does this test bypass the DME itself, ruling it out as bad?

Measured resistance of speed and reference sensors. Getting 900 ohms between pins 8 and 27 on ref sensor, and 880 between pins 25 and 26 on speed sensor. Those seem within spec.
But between 8 and 23 on ref sensor and 25 to 78 on speed sensor I get nothing measurable. Is this normal or do they both seem bad?


Having trouble deciding if the DME or 1 or both sensors are bad. Any info appreciated,
thanks

Check wiring in your fuel pump relay socket. I had the DME ground wire pull out and was well hidden behind a larger wire. Nice car

[greasewagen]

Thank you.
I looked under the relay block and did not see any loose wires. It was pretty clean too. I will try and stick my head under there this weekend and get a better look.
I'm leaning towards a faulty DME. I am buying a used DME for $50 from an 86 parts car. It ran when parked, but several years ago. I have an 85.5 and I believe they are the same/compatible.
The car cranks over fine but with no tach bounce. Started a coupe times randomly but stalled 30 seconds or a couple minutes later. It once stalled when I touched the DME but I haven't reproduced that yet. It doesn't start when I hit or move the DME.
Next I plan on measuring resistance of the speed and reference sensors at the DME plug. If I see the same correct resistance I saw at the upper connectors I will figure they are good for now.
I will then buy the used DME and try that out.
I tried the DME jumper and car did not start. fuel pump did run. Does this test bypass the DME? I read that in an old post, but I am not sure it is true.
Any help getting this beauty back in the road is much appreciated.

[Pauld_94S2]

GOOD=Measured resistance of speed and reference sensors. Getting 900 ohms between pins 8 and 27 on ref sensor, and 880 between pins 25 and 26 on speed sensor

NOT GOOD= But between 8 and 23 on ref sensor and 25 to 78 on speed sensor I get nothing measurable.

Was this with the sensors connected to the plugs at the back of the engine?
Sure you are reading the correct wires?
Are the connectors in good condition?

With the sensors connected to the wire harness at the engine you should be able to read the resistance of the sensors from the DME. If you can't then you are close to finding the problem. Carefully check the harness connectors, they are problematic.

[JonA]

https://www.youtube.com/watch?v=zeFOOcLSk60&feature=youtu.be

This is a slightly more sophisticated way to test your sensors. Hope it helps.

Jon

[greasewagen]

Thank you both. Yes that test was unplugging the 2 connectors at back of engine, measuring the wires going to the sensors which are in the bellhousing.
Those plugs are only in fair condition. Rubber boot is becoming brittle.
So I agree next test is of both sensors at the DME plug, similar to the youtube video above.
I'll do that this weekend as i work out of state. If I don't see the correct values, then I will buy the speed and reference sensor harness set from lindsey racing unless someone can recommend another source. If I do see the correct sensor values I'll take a chance on the used DME for $50.

[Toast54]

Originally Posted by Pauld_94S2

GOOD=Measured resistance of speed and reference sensors. Getting 900 ohms between pins 8 and 27 on ref sensor, and 880 between pins 25 and 26 on speed sensor

NOT GOOD= But between 8 and 23 on ref sensor and 25 to 78 on speed sensor I get nothing measurable.

Was this with the sensors connected to the plugs at the back of the engine?
Sure you are reading the correct wires?
Are the connectors in good condition?

With the sensors connected to the wire harness at the engine you should be able to read the resistance of the sensors from the DME. If you can't then you are close to finding the problem. Carefully check the harness connectors, they are problematic.

Got to Clark's Garage...indispensable for working on these..and test using Fuel-16 procedures.