No Start (No Tach Bounce) Help

[TrailRider]

******SOLVED*******

While driving the other day my '87 944 Turbo shut off. Was able to pull to the side of the road and haven't been able to restart it. There was no warning and was running fine right prior.

The car cranks over fine but no tach bounce while cranking.

What I've done so far:
1. Swapped the DME Relay with spare
2. Swapped the DME with spare
3. Measured Resistance and Voltage to both reference and speed sensors at the DME plug and at the wiring harness connection. Both are in spec.
4. Bypassed Factory alarm module (have some doubts about this... so could still be a culprit)

What else can cause no tach bounce?

I really don't think my no start has anything to do with fuel delivery given that I have no tach bounce so ruling all of that out (injectors, fuel pump, etc). something is causing no spark...

Any additional suggestions would be appreciated!

[Chalt]

You said you did the resistance test. How did you test the voltage output? With a scope or a multimeter? What were the Peak-to-Peak voltage readings?

To test mine, I bought a cheap oscilloscope and was able to verify that the speed sensor had a peak-to-peak voltage of > 2.5V. I checked the reference sensor and it had a peak-to-peak voltage of > 2.0V. The testing specs and procedures I got from Clark's Garage Speed and Reference Sensors - Checking, Replacement, and Adjustment

The scopes are all over ebay and Amazon and the one like I got was in the $30-40 range. I didn't need a complicated one to test these sensors. The brands vary but they all are essentially the same thing. Mine looks like this:



Here is a pic I took of the scope's screen while the engine was cranking:



By the way, before this I had never touched an oscilloscope. It was easy to use. I just popped in a 9v battery, hooked up the test leads, and cranked the motor. I didn't have to have any kind of advanced knowledge to use this scope. I just plugged it in and got the reading. I am sure this cheap scope is useless for an advanced electronics guy, but it was all I needed to check the speed/reference sensors on this car.

[Chalt]

This is a really cool bench test by Tom86951. I appreciated it because it shows how the speed and reference sensors work in a very visual way. It showed also how the gap plays an important role in the signal they send. Another interesting note is they are just Hall type sensors. They generate the signal from magnets.

[Igeaux413]

A friend had a similar issue about a year ago and mentioned he found the ground wire that either bolts to the rear of engine or the ground connection between the engine and body was loose. After tightening it back down, problem solved.

[Chalt]

Tom Mc'Guiness made a specialized tester for speed and ref sensors called The Carmonica. I don't know if he is making them anymore. Pretty awesome little tool build. He goes by username "Tom M'Guinn" on Rennlist, and by "Tom M'Guinness" on youtube.

Here is a video of his in operation:

[TrailRider]

Thanks Chalt!

Yes, correct, did the speed / reference sensor test with a multimeter so just saw the Voltage numbers... but as you noted could not test the Peak-to-Peak numbers.

I really don't want to throw parts at this so picking up the Scope might be the best option going forward.

Did you solve your issue by replacing the sensors?

I haven't inspected the distributor / cap / rotor fully so will have a look at that more closely today.

[Chalt]

Originally Posted by TrailRider

Thanks Chalt!

Yes, correct, did the speed / reference sensor test with a multimeter so just saw the Voltage numbers... but as you noted could not test the Peak-to-Peak numbers.

I really don't want to throw parts at this so picking up the Scope might be the best option going forward.

Did you solve your issue by replacing the sensors?

Mine didn't quit running while driving. Mine just became increasingly difficult to start when cold. Would start immediately if warm. My problem turned out to be a bad DME Temp sensor. So mine was starved for fuel due to the bad sensor.

I'm glad I didn't throw lots of parts at it instead of spending more time troubleshooting. I have seen way too many threads of people who just replaced everything they could think of and still didn't solve their problem.

My speed and ref sensors tested good – I didn't replace them.


If your Ref and Speed sensors test good, that will save you a pain in the neck replacement. I have heard they are at times difficult to remove.

I think the cheap scope is a good investment for your diagnostic tool kit. As far as I know the scope should work on other make and models for ref-sensor testing too.

[TrailRider]

After reading further online about other experiences with no starts, someone had noted that when turning the key to the on position (but not cranking the car) the boost gauge (for a turbo car) should go to 1.0 Bar. Mine would stay at 0, indicating the problem. They noted this was tied to the factory alarm.

I thought I had bypassed the factory alarm module successfully. Per Clarks-garage -
Alarm System - Bypassing

Either a previous owner tweaked the wires for my alarm (not likely since it's been running fine) but the color of the wires for the late model 944 chart that Clarks-garage references for wires 7 & 8 is wrong.

My 7 & 8 wires are Yellow and Brown/Black. Clark's indicates Yellow/Red and Blue wires -- these are actually wires 5 & 6.

Once I realized this, was able to bypass the alarm, got the 1 bar of boost when key set to ON and now the car starts up no problem.

So, issue was the factory alarm module failure.

Will try to post up pictures later to fully document this.

[Chalt]

I don't know the colors of the wires but I know which terminals to jumper: