87-924S Not Starting - DME Test?

[3D914]

Well I've been going back and forth with my 924s trying to solve a starting problem.
It had been failing to start intermittently, so I pulled the DME/Fuel Pump relay and replaced it with the three-wires fix from Clark's site.

It started right up! Cool, so I ordered a replacement relay from the Bird. Put it in and it started right up - but only once. After that - no go. Then I put the 3-wire back in - and now it won't start with that either.

So I decided to do the bench-test on the relays (now that I had most of my tools unpacked) & they tested good.
One thing was strange though. With the 3-wire in, I could clearly hear the fuel pump. Yet even before this problem, I would never hear the fuel pump - even with just the key on.

So that leads me to wonder about the DME. Is there any static or bench tests that can be done?

TIA,

[John_AZ]

Gerard,

This is the simple test to see if the DME is getting a signal from the reference sensors.

When you crank the engine, does the tach needle bounce 1/16 of an inch when you have the key turned to start?

If you see a tach needle bounce, this is a good sign the DME computer is working.

If you do not see the bounce, start by cleaning the reference sensors connections at the top rear of the engine. Look for cracked connectors and broken sheilding. If you can, regap the sensors. Put a piece of tape over the TDC bellhousing hole!

Clean the big negative ground on the bell housing. Put a piece of tape over the TDC bellhousing hole!|
Clean the small ground next to the big negative ground.

Clean the battery cable connections and check for acid corrosion under the sheilding.

Make sure the battery is fully charged. A low battery will send too many amps into the DME computer and burn it out.

Clean the connections at the AFM.

If you want to know all about the DME system go to Clarks-Garage or
http://www.the944.com/

GL
John_AZ
1988 924S 60K + 1987 924S 113K

[thekidd]

That sounds like my car (84) when it went itnot the hot start Id call it, then a few days later wouldnt start. I did the jumper test, and replaced the DME relay with a new and a used one, sometimes would start, other times it would with each of the 3 relays I have. I would have to say it definetly sounds like the speed/ reference sensors. Tach bounce when you turn the key like John asked? If not, very good chance the sensors are bad. Hint: dont buy the Porsche sensors at $100+ each, buy the BMW 5 series sensors- about $100 for the pair. Be very patient with getting the sensors out, tey gut stuck very easily, may take 2 days or a couple hours to get in there, reach them, and then get them out.

[3D914]

Thanks guys. It's going into the garage in a week or so to get new rings, so I'll check all that out before I tear it down.

I'll post results when I get them.

[S ECU Repair]

Give us a call tomorrow, 1-800-328-1425.

Maybe we can provide some help with your Porsche DME.

Perhaps we can fax you some testing procedures that might help you.

[3D914]

OK, replacement engine is in. All connections have been cleaned - all ground connection in the engine bay. When I cranked the engine to build oil pressure, engine started pressure came up to 4-5 bars, ran for a minute or so then died. DME relay was bypassed at the time with the 3-way wire. FP was still running after it died. Now it won't restart.

Replaced the 3-wire with a tested relay. Tach bounces when cranking, oil pressure comes up good while cranking. Both ref/speed sensors were tested good prior to install. Battery voltage is 12+V.

What needs to be checked at the AFM? THe connector is not cracked and appears OK visually.

What's the next step?

TIA,

[JivenJim]

suggest check fuel pressure at Fuel rail while running. If low then:
Maybe fuel Filter or FPR?

[John_AZ]

Check the back of the relay board for burnt wires or bad connections. You said the car fired up the first time. Check the fuses for connections. Wiggle the key in the ignition---it could be a faulty ignition switch.

When you use the DME 3 wire jumper-----do you hear the fuel pump running?

Noid lite test on the injectors to see if the DME is sending a pulse.

Recheck all of your harness connections for bent ends and grounds.

Basic DME tests:
Grab the bic DME connector and wiggle----maybe a bad connection.
Tap lightly on the DME case----maybe a bad solder joint.

Review per Clarks:
DME Computer - Testing and Inpsecting

AFM--check to see the "barn door" swings freely. Clean the harness connection.

Some try a squirt of starting fluid in the AFM door to see if the car will start.

GL
John

[3D914]

Well this gets more interesting.
I took out the new relay and put the three-wire by-pass back in to check some of John's suggestions. It started right up, idled for 5 minutes or so then died. I didn't have the alternator belt tightened yet, so I did that. Restarted it and it idled for 10 or more minutes until I shut it off. It now seems to start on command.

Alternator light is out, but oil pressure light is still blinking (not faint flicker). Oil pressure indicates 4 bars at idle.

I still have to get air out of the coolant system. I'll see if I can get through that tomorrow. The new water pump seems to be making a little noise, but not entirely sure the noise is coming from there.

[3D914]

OK, with the 3-wire bypass in place I jacked up the car to top off the coolant. The car ran without quiting. Once it was warmed up, I started it and stopped it several times - no problem.

I'll have to put the relay back in and try some of the other suggestions regarding the DME and connector. Gotta adjust the clutch first, as it's not disengaging properly.

Almost there . . .

[JivenJim]

by chance did you notice if the fans were on when it shut down.
Could it be a Grounding issue, like the hub on the rear of the Bell housing or main lug next to battery needing cleaning, and thus when the fans kick on the rest of system is short of power, and shuts down? Maybe this is why there is a difference with alternator hooked up?