senihele

Hey all,

I could really use your help. I've been trying to diagnose an intermittent no-start for over two months now. (See this thread https://rennlist.com/forums/924-931-...e-to-test.html). After a week of work, I can get the car running again consistently for a few days, but it always stops working after.

Summary: The car will start with the DME relayed jumpered, but dies immediately after start with various different DME relays installed. It appears the car dies due to a lack of fuel in the rail, as when switching back to the jumpered solution, it takes awhile before I hear fuel come up into the engine and the system fully pressurizes. After a successful jumpered start, the car will start with a DME relay in, but immediately dies. Following attempts won't start at all (apparently because of a lack of fuel). I have tried several DME relays, old and new, without any success.

Background: I won't rehash everything, but basically I suspected the issue to be in the fuel system. The very first thing I replaced however, was the DME relay, figuring this goes bad frequently and I should have a spare anyway. This didn't work and I ended up replacing the fuel filter, the fuel pressure regulator, cleaning the injectors and borrowing another fuel pump. At times various things I did seemed to work and the car would run for several days, only to go completely dead for a week or more afterward. After becoming really frustrated and running out of options, I decided to put the old DME relay back on and the car started up. I went back and forth between the new and old the old started everytime, the new not once.

Today I went to start the car after it had been running for the past few days, and it wouldn't start at all. The new DME relay had the same result. But the three pronged jumper solution did start the car. Again I tried each four or five times, with the jumpered relay starting everytime and neither DME relay working once.


I'm really baffeled by all of this. Does the full DME relay do anything that the jumpered solution does not that might be interfering? Or maybe it starts when jumpered because the fuel system completely pressurizes before I get back to the front seat and start the car.

When I turn it off and switch back to the relay, the car does start, but immediately dies. Following this, it will not start at all, and there does not appear to be fuel in the rail.

Thoughts?

1987 944 NA

951Porschiste

The speed and reference sensor conectors are a real PITA for 951's. I know your car is an NA, but it might be worth looking into your connectors. On startup, the sensor basically tells the DME that the engine is turning over and the the DME needs to send the necessary signal for fuel and spark to fire up the engine. If the connectors get bad, the DME doesn not see the sensor signals and the engine won't start. What makes the problem intermittent are the loose connections. Depending on enviromental conditions, the connections get more of less «connected», and the engine will sometimes work fine for days and then all of a sudden stop working.

Read this post:

https://rennlist.com/forums/944-turbo-and-turbo-s-forum/359468-the-definitive-hot-engine-no-start-thread.html

Hope it helps!

rattlesnakeracing

senihele I read your previous posts and understand the frustration you are facing on the electronic side. If you can, read the FUEL-05, DME Relay Technical Information and Testing at Clark's. http://www.clarks-garage.com/shop-manual/fuel-05.htm

With the jumpered solution you are by-passing the "switches" in the relay. Look at the schematic below - https://rennlist.com/forums/attachme...1&d=1240010621

When the Ignition Switch is turned {86}, current flow pulls the diagonal T down to a normal horizontal T. Current then flows from the Battery {30) across the top of the T to the DME & Injectors {87}. It also energizes the next diagonal T and pulls it down allowing current to flow across the top of that T from the Battery {30) to the Fuel Pump {87b}. Think of each of the two "T's" as switches. The end result is that you have energized and started the injectors and the fuel pump.

What you are saying is that you DO NOT GET POWER from the Battery {30) to the Fuel Pump {87b} with the relay installed, but DO with the jumpered solution because it is a straight wire. It does not have to be energized like the relay.

So, my first suspicion would be the relay. Understand that if you do not have correct voltage applied (12V being the determining voltage) part of that relay may not work. Your battery must be fully charged. That voltage is supplied to the connector underneath the fuse tray by wires. Have you pulled up the fuse /relay tray and checked the wiring to the relay? It sounds as though you are not getting a good connection to the relay itself at the entry point like you do with the jumpered solution.

The grounds are also important DME {85} and DME to Pickup Fuel Pump {85b}. You have to have solid grounds to the point they connect at in the relay or current will not flow correctly.

No fuel pump means no fuel flow.

If you have any questions, PM me.

Good Luck!


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joes

My guess is a bad DME, a cold solder joint.

Bri Bro

Check the voltage at pin 85b. If it isn't close to ground, swap the DME.

tod84944

I had the exact same problem with my daughter's 944, and it was the dme. Luckly I have a couple of spares.

senihele

Hey all,

Thanks so much for the insight. This info is really helpful. This has been my first major attempt at doing repair work and I've learned a tremendous amount about the fuel system, but I'm still trying to get up to speed on the electrical system.

A few thoughts/questions:

The car will start on starter fluid (and then dies once the starter fluid runs out, again with an empty fuel rail). This seems to suggest the the DME/DME relay problem is related only to the fuel pump functionality, not the ignition switch - yes? I'm not sure if I'm reading the diagram correctly, but it seems power to the fuel pump comes the ignition switching part. Is this right, and if so, does it help me isolate grounds or other wiring to check?

Regarding 12V, all the trouble shooting I've been doing over the past few months has drawn down the battery at times. Jumping the car did start the car several times, but most of the time didn't help. A friend of mine suggested it might be related to a voltage issue not tripping the relay. I've bought a battery charger though, and the battery seems to be in good condition.

I'm hopeful it is an issue with the grounds. That would explain the intermittent nature of the problem, yes? And it would be cheaper to fix than replacing the DME. I really have no experience or insight on this though. Where are the grounds located? Does anyone have any resource on this I could check out? I recently found a lose blue wire in the well behind the passenger side rear wheel coming off the fuel sender. Could this be related?

Any recommendations for resources on how to check the fuse wiring? I've never pulled this apart and am a little intimidated.

Speed/reference sensor: I've read that if the tach bounces when starting up, this should rule out problems with the speed/reference sensors. All instrumentation seems responsive and correct. Is that conclusive, or should I still look into it?

Also, occasionally I have some stumbling at around 3,000, but only on the highway in 5th gear. I've read maybe this points to a problem with the TPS? I've had this problem for years, though. I keep meaning to fix it, but it was so infrequent. It seems a little more frequent now. Is this possibly connected?

I'll check 85b tomorrow morning. I'm still getting the hang of the multimeter, but reading up on how to check for grounds now.

Thanks again!

Charlie

Check pin 86 and see if when you turn the ignition switch you are getting 12 volts at this pin. I have seen cars run with the jumper because it bypasses the ignition switch and not run with the DME relay because of no voltage to pin 86 from the ignition switch.

rattlesnakeracing

[QUOTE=senihele;6492489] O.k, follow along on in theory of what I have below, some of it is cut & paste from faults in this and other forums indicated by [ ] )and may not apply directly to the faults of your car. I will bold in green what I think is relevant. There are a lot of references that I hope will help. You are going to have to do deep research to figure out this problem yourself. I am trying to answer your questions individually to help you understand the theory about how it works as well as eventually arriving at a solution.

The car will start on starter fluid (and then dies once the starter fluid runs out, again with an empty fuel rail). This seems to suggest the the DME/DME relay problem is related only to the fuel pump functionality,{Yes, you are applying power to the fuel pump through this relay} not the ignition switch - yes? I'm not sure if I'm reading the diagram correctly, but it seems power to the fuel pump comes the ignition switching part.

[The Haynes manual does have a schematic that gives a breakdown of the ignition switch. If the engine is turning over and you have 12V on each side of the coil, I wouldn't suspect the ignition switch.The switch is allowing current to flow through the fuse board and the rest of the ignition system. If your problem is not the ignition switch unit at the rotor I would swap out the DME.

DME Control Unit (Also known as ECU). This can be pulled out and you can reheat the solder joints and re-seat the chip. Cold solder joints are a constant problem on the boards. http://www.clarks-garage.com/shop-manual/dme-01.htm

When the ignition key is turned 12V+ arrives at the coil {BLACK WIRE} from the battery. A trigger {PULSE VOLTAGE} is applied from the DME {GREEN WIRE} which makes the coil generate 15,000V through the coil wire to the center of the rotor cap. It is rotated around the plug wires with the rotor button. The voltage travels through the wires to the bottom of the plug where it is grounded creating a spark, thereby igniting the fuel.

Coil operation: Inside the coil current from the battery flows through the thicker primary windings and a magnetic field builds in strength thanks to the help of the iron core. When the switching device opens (or is triggered - DME), the flow of current is broken and its magnetic field collapses over to the thousands of secondary windings. During this collapse, the voltage is stepped up, creating the higher voltage 15KV that is required to jump the spark plug gap and ignite the air/fuel mixture.

So, in your case if you are seeing 12V at either side in the forward (not turning the starter) switch of the ignition, it would be normal because voltage is coming from the DME and the battery. In the start position (starter turning) the voltage changes from the DME as a trigger making the coil rotate in milliseconds the outgoing coil current 15,000V plus out the coil wire to the rotor cap. You may not be getting that trigger from the DME.

In my 88 944 I changed all parts in the ignition to my car. Coil, rotor & cap, rotor button, wires, plugs, DME relay & bypassed the alarm module. I was still seeing 12V at either side of the coil with no start. I put the same parts in my 87 944 and it started with no problem. I used a light and noticed it was dim on the 88 but bright on the 87 during starting. I swapped out the DME's and the 88 cranked and ran.

The problem was the DME. I took the DME apart and used a soldering iron to reheat the soldering joints and I also reset the plug in chip.I put the DME back in the car and it started with no problem. In my opinion the $500.00 plus cost of a used DME that I saved was well worth the effort. It took about 6 weeks to go through that process and was extremely frustrating because the 88 is a daily driver.

Reference: http://www.the944.com/ ...at the bottom of the page is "Workshop Manuals and PET". Select that link and all of the manuals are listed. You need Vol5-electrical.pdf, this covers the S model. Direct link: http://www.cannell.co.uk/944_worksho...electrical.pdf]

Is this right, and if so, does it help me isolate grounds or other wiring to check?

Regarding 12V, all the trouble shooting I've been doing over the past few months has drawn down the battery at times. Jumping the car did start the car several times, but most of the time didn't help. A friend of mine suggested it might be related to a voltage issue not tripping the relay. I've bought a battery charger though, and the battery seems to be in good condition. {No matter what, you must have 12V. Always use a multimeter to check and see EVERY time before you start. Electronics are precise with very little variables plus or minus a constant point.}

I'm hopeful it is an issue with the grounds. That would explain the intermittent nature of the problem, yes? And it would be cheaper to fix than replacing the DME. [{A reference for DME repair - Call him! memoriter.....call this guy, he REPAIRS DME's for all the Porsche shops around here, and is well know nationally......and he is reasonable. His name is Loren (310) 545-3716 Tell him Phil at Noirtech said to call}]

I really have no experience or insight on this though. Where are the grounds located? Does anyone have any resource on this I could check out? I recently found a lose blue wire in the well behind the passenger side rear wheel coming off the fuel sender. Could this be related?

Any recommendations for resources on how to check the fuse wiring? I've never pulled this apart and am a little intimidated. {Disconnect the battery. There are 2 screws (referencing my 88 944) in the fuse box. Standing on the drivers side, looking down inside the fuse panel, cover off, 1 screw should be at your lower left of the box, the other at your upper right. Release the screws and lift the connecting panel up. You will see all of the wires & connections. Look at the relay for the DME and see if the connecting wire is pushed up far enough. Check all of the other wires out for corrosion (white/green powder - rust - etc...). Some wires that may be corroded will be thicker than the normal wire, like a bump inside the wire. You will not be able to see the corrosion because it is covered up by the plastic wire covering. Do not pull too tightly since you may disconnect or dislodge wiring. When finished, put the screws back in & reconnect the battery. }

A wonderful reference to electronics & theory: http://www.tpub.com/content/neets/14177/index.htm

Speed/reference sensor: I've read that if the tach bounces when starting up, this should rule out problems with the speed/reference sensors. All instrumentation seems responsive and correct. Is that conclusive, or should I still look into it? {I would check out the DME before I looked into the speed/reference sensors. Even if you were to replace the sensors, they depend upon a good DME. Great info on speed/reference sensors - http://forums.pelicanparts.com/showthread.php?t=457162}

Also, occasionally I have some stumbling at around 3,000, but only on the highway in 5th gear. I've read maybe this points to a problem with the TPS? I've had this problem for years, though. I keep meaning to fix it, but it was so infrequent. It seems a little more frequent now. Is this possibly connected?

I'll check 85b tomorrow morning. I'm still getting the hang of the multimeter, but reading up on how to check for grounds now.

Hope this isn't too jumbled up and helps. Good Luck!


.

Charlie

When the ignition switch is on do you get 12V at pin 86???????????

Monte951

I have a similar problem with my 951 right now only i have a working dme relay that was modified. I'm really close to fixing my problem and i would like you to try something. My DME Relay has the lower switch permantly closed. I'd tell you what legs but its easier saying the lower switch. This allows my car to start. If you take the cover off of your dme relay and put a small folded peice of paper in the bottom switch closing it, it should make the car work like normal.
The bad side is this will keep your DME unit energized and kill your battery in a few days. Lot of fun for me. Brian Boderick is on to something and i'm working on the full solution. Try modifying your DME and see if it works and maybe we can both can get these fine automobiles working. If it doesn't help you just pull out that paper that closes the switch and you should be back to were you were! (Plus it's nice to see if the switches open and close when you try to start the car.)

Monte951

Check my current Thread and see if my problem sounds like yours i'll find the link:

https://rennlist.com/forums/944-turb...on-i-hope.html

Monte951

Hey i fixed my jumpered DME relay problems yesterday by bypassing the Alarm Modual. (5 minute fix for a month long problem) Turns out you have to have it and it causes all sorts of hell when it malfunctions. Bypass it using two wires as described in clarks-garage and try your car then. Worked like a charm for me! Hope this helps

Jusii

You're correct, the immobilizer cuts fuel pump via DME-relay.

Monte951

Any update? I'd put money on the alarm thing, sound exactly like what mine was doing...
If you need help making the jumper as the directions are simple yet a lil general i'll send ya one, or walk you through it.