1984 944 no start, no tach bounce...

[Hell2go]

Help! Im in Lockport, NY. I have researched the hell out of this and am at my wits end.

Ive replaced: dme relay, dme, speed and reference sensors, ignition switch, did alarm bypass, and a new battery of required specs...

When i turn the key, i get the fuel pump running, dash lights, and the starter turns over, but no tach bounce. I checked the reference sensor pins at the DME socket and they read around 890 ohms and it jumps when i tap the sensor end with a screwdriver, so i know there is a signal there. I also have 12v at the main power lines at the DME socket. I have 12v at the coil. I did the clarks garage factory alarm bypass, and the alarm goes off when i turn the engine over?
I don't know what to try next. Any porsche experts near me with an oscillascope that wants to try and help me out?

Any reasonable ideas? Before i resort to firing it up with a can of gas and a match?

[9FF]

Did your tach work before?

New sensors correct type and gapped right, plugged into correct terminal, sensor bracket not loose.

Have you checked for spark, pulsed injectors (noid), checked for fuel from injectors.

That’s all I got for now, gl

[Hell2go]

I plugged in sensors one at a time while ohming out at the DME plug to make sure they are plugged into right spot, there is no bracket.

The car was not running when i bought it, PO tried to change head gasket and messed up timing and bent the valves... I removed, rebuilt and reinstalled motor, so I've never had it running previously.

[Hell2go]

[QUOTE=9FF;10838325]Did your tach work before?
New sensors correct type and gapped right, plugged into correct terminal, sensor bracket not loose.

Have you checked for spark, pulsed injectors (noid), checked for fuel from injectors.
I bought it not running, the PO changed head gasket and got timing wrong, bent the valves. I removed, rebuilt and reinstalled motor.

I dont have signal from DME to coil that pulls side of coil to ground causing spark. I'm thinking something is not allowing computer to send signal, but I don't know what. I need a flowchart for the DME for atartup so I can track it down, if thats it.

I messaged with edredas from youtube who is as close as i have found to be an expert with these cars, and I've already done all the things he would have done so far... this one is a real conundrum.

[Hell2go]

Originally Posted by 944 Ecology

Probably a speed or reference sensor. You will need an oscilloscope to reliably test...

Here's a cheap one...

https://www.amazon.com/Pico-PicoScope-2204A/dp/B00GZMRZ3M

Like i said in the first post, i replaced both sensors , they ohm test in range and show fluctuation when testing, there are 3 wires, is one of them a power line? Should it have dc voltage on one of them?

[Hell2go]

Like i said, i replaced both sensors and they ohm out in range and show fluctuation when testing, there are 3 wires on each, is one of them a dc voltage supply line?

[Jfrahm]

You should not get fuel pump action without a signal from the S/R sensors. They are passive sensors:
Speed and Reference Sensors - Checking, Replacement, and Adjustment

If you are getting fuel pressure when cranking and have not installed a DME jumper I'd look for spark and, and use a spark checker or otherwise make the spark jump 10mm to be sure it's strong enough.

Then if you've got a strong spark try a puff of starting fluid just to be sure it's not lack of fuel.

You can test the S/R sensors with a DMM even if it does not have a Hz setting, you can look for an AC voltage signal when cranking. But w/o a crank signal you'd have no fuel pump action and no spark.

The tach could be dead, I had one fail. Or it could be the DME (not the DME relay).

Always good to test with a DME relay jumper just in case there is some sort of issue with even a new DME relay.

-Joel.

[mikseymono]

Yes...test with DME jumper first....make sure jumper wires go into the other wires with a solid connection. I had a similar problem, bought a couple of DME relays and it turned out one of the wires behind the domino relay housing was loose and not connecting to the actual DME relay. when you plugged it in...sometimes it is the simple things..... Bon chance! M