944S Cuts Out on Straight Road - Won't Start - Wait 5 and She Starts

DaveBoettcher · 08-18-2008, 08:10 AM

I have an intermittent fault where I can be driving along in my 1988 944S (never raced or rallied, driven by me since 1991) and she suddenly cuts out as if the ignition has been cut. No spluttering or misfiring, just a sudden total loss of power. The radio stays on. This just happened to me on a straight road as I was driving in to town at 40mph (speed cops hot on that stretch).

I try the starter. The engine cranks as normal, but does not fire. Usually I try this a couple of times without success, and then wait a few minutes - 2 to 5 minutes maybe - and she then will fire, and runs as if nothing had happened.

I am guessing that it is an electrical problem, maybe an insulation problem where some wiring has been rubbing and the insulation has worn through. But I have no idea where to start looking. Any suggestions? Thanks!

Regards - David

Jeff Laurence · 08-18-2008, 08:17 AM

EXact thing happened to me (several times)--try to borrow an ignition control module--heat sink mounted to an aluminum plate in the drivers headlight well.
Apparently, same module is used by Volvo (wrecking yard < Porsche dealership!!!)
Cheers,
Jeff

Jeff Laurence · 08-18-2008, 09:05 AM

Bosch # 0-227-100-124, Volvo #3501921 ( 1987 Volvo 240)
Jeff

DaveBoettcher · 08-18-2008, 10:54 PM

Thanks Jeff, I have seen the ignition control module thing you describe, but what does it do?
David

Outback Porsche · 08-19-2008, 12:32 AM

I had a similar incident. The small vacuum hose Y junction at the throttle body came apart.

Jeff Laurence · 08-19-2008, 06:40 AM

Ignition module recieves a signal from the main brain to fire the coil- they are only on the "S"
( maybe others , but not on the 8 valves).
Jeff

DaveBoettcher · 08-19-2008, 08:33 AM

Thanks Jeff, I have done a bit of investigation and these ignition control modules are not only expensive, they seem to be difficult to get hold of due to the age of the car. Is there any way of testing mine to see if it is faulty?
David

hpservertech · 08-19-2008, 08:53 AM

Have you tried the DME relay? it sounds intermitant and that would be my first thing i would look for.

When it doesn't start, do you hear the fuel pump running? It's either not getting spark or fuel, my guess would be fuel.

Jeff Laurence · 08-19-2008, 11:37 AM

I agree-- check the DME relay first, but if the fuel pump is running.......
BTW our host sells the ignitor for $89. I paid $CDN160 at a Porsche dealer (did fix the problem, though)
Jeff

DaveBoettcher · 10-23-2008, 03:06 AM

Quote:

Originally Posted by hpservertech

Have you tried the DME relay? It sounds intermittent and that would be my first thing I would look for.

When it doesn't start, do you hear the fuel pump running? It's either not getting spark or fuel, my guess would be fuel.

Good question: I have never heard the fuel pump running since I owned the car. And as I mainly drive it alone it always does this fault when I am on my own, so I can't get underneath to listen to the pump while it is cranking. I thought they were pretty silent anyway?

Quote:

Originally Posted by Jeff Laurence

I agree-- check the DME relay first, but if the fuel pump is running.......
Jeff

Well it stopped doing it for quite a while, so of course I ignored it and hoped it had gone away - until it came back hard last week and did it a couple of times in quick succession.

So I had a word with Bert Gear of Berlyn Services (a very good source of advice and parts in the UK) and explained the problem, and the suggestions I had picked up here. Bert also fingered the DME relay as the likely culprit, saying that they are "notorious" to the extent that some people keep a spare in the glove box! This was news to me, but they are also pretty cheap, so it was no-brainer to try swapping that out first.

So far, after two days with the new DME relay, the problem has not recurred. Although it has done this before, so I am not certain that it is cured.

Looking at the old DME relay, all the solder joints are firm and there is not much sign of a problem. Just one wire to the upper coil looks a bit odd - purple in colour as if it has got hot.

This is the wire that connects the frame of the relay to the moving plate that does the switching to connect 87b to 30: my wiring diagram uses different numbers so I am not sure what these do. Other than that, the DME relay is a pretty simple device so I am not sure how it could produce the intermittent fault I was experiencing: But I will keep my fingers crossed and hope that it was!

paper1122 · 10-23-2008, 03:27 AM

+1 on keeping a spare in your glove box. I've already used my spare... for another stranded 944 owner!

rickharrison · 10-23-2008, 05:45 AM

Jeff...Tottenham!

A buddy of mine lives there...Al Bumstead...know him?

oh, and DME relay...yes that is likely the culprit.

Jeff Laurence · 10-23-2008, 06:23 AM

Sorry, Rick -don't know him, and I've moved to Pottageville recently.
Jeff

MrWrong · 10-23-2008, 06:27 PM

When my dme relay half died this summer it wouldn't run the fuel pump when it was cold, like first thing in the morning, unless you tapped the dme. Funny thing was though it would run the dme and the tach needle would bounce like it's supposed to on starting.

I must of gone through 6 cans of starting fluid before I figured it out.

I'd get in and crank my life away, usually late to work, and nothing.

Get out give it a spurt of ether. It'd fire and sputter for a sec or so untill the dme relay vibrated enough to catch. Then I'd swear like hell at morning rush hour traffic trying to make up lost drive time.

Then in the late afternoon with the temp inside the cab at 200 degrees F the relay would be warm enough to work right. I'd hop in and fire up on the first crank.

I finally figured it out when the relay fully gave up the ghost and I didn't have the tach needle bounce last month.

Now to being a cheap student I cann't afford to have a spare dme relay in the glove box. I just keep a Y shaped piece of wire there with male spade terminals to temporary bypass the relay when needed. Like for the week to took local parts house to find me a relay.

fastfredracing · 10-23-2008, 06:36 PM

+ 1 for checking the dme relay. You need a buddy to listen down the filler neck hole, or have his head right near the lower portion of the gas tank to hear the fuel pump. The fuel pump only operates when the engine is cranking or running, so it is helpful to have someone nearby to listen while you crank. Also, carry around a spare spark plug, and the next time the car quits, jump out, pull one plug wire, put the spare plug in the removed wire, and verify if you have spark while the car is not running. Once you have verified which system is not working, it makes the diagnosis a lot easier. It may also be crank/ref sensor related , although I believe the S model is different from an 8v, and only has one of the two senors.

DaveBoettcher · 10-24-2008, 01:19 AM

Quote:

Originally Posted by MrWrong

When my dme relay half died this summer it wouldn't run the fuel pump when it was cold, like first thing in the morning, unless you tapped the dme. Funny thing was though it would run the dme and the tach needle would bounce like it's supposed to on starting.

Not sure what you mean by the tach needle would bounce like it's supposed to on starting? Can you describe what you mean, and why it's supposed to bounce?

MrWrong · 10-24-2008, 05:45 AM

On the early 944 and probably the rest from what I've read.

The needle on the Tach will bounce slightly on start up before the engine catches during normal operation, someplace between 0 and the first line on the tach on mine.

Basically its the dme reporting that its receiving a signal from the reference sensor.

If you want a better explaination of my problem pull the fuel pump fuse and crank the engine. It should run off the residual fuel in the line for a second and die. Then try starting it again while you watch the tach.

DaveBoettcher · 10-24-2008, 09:18 AM

Quote:

Originally Posted by MrWrong

On the early 944 and probably the rest from what I've read, The needle on the Tach will bounce slightly on start up before the engine catches during normal operation, someplace between 0 and the first line on the tach on mine.

Ah, OK I understand. It doesn't do this on my car, the tach just goes with the engine speed. No funny bumps. Unless it's so slight that I have not noticed it - I looked specially today and didn't see anything.

Lapponia · 10-25-2008, 02:37 AM

DaveB! What a fine picture of your DME relay. It looks the same as mine when I pulled the can off. What strucks me when I see your relay is that you point out, is the wires from relay frame to relay yoke, they seem to be welded differently to the other left relay, more heath I think. Also, they seem to be too short for the yoke to move all the way down against the coil and the contacts they are supposed to close. I think it should be a distinct bow of the flexible wire between relay frame and relay yoke thereby making yoke more easily attracted to the coil when operated that part of DME-relay.
BTW, relay looks good and I would suggest you to polish or rub the contacts with fine sandpaper and clean them with alcohol. And WERY important, operate the relay coils with a battery charger (more than 12V) many times and look at the relay moving parts...do the close as supposed to? Check the ohmic resistance over contacts with a volt/ohm/ampere meter when closed. It shouldn´t be more than 0,4 ohms (resistance from metering wires). If you have this you are fine. But I cant recommend to lubricate the yokes with oil as relay part become hot and oil will probably evaporizise to the contacts......not good. Perhaps a small amount of hot resistant grease will do.
For the time beeing I keep my DME-relay in the glove box and run the car with a common available double relay 2x30 Amp wired exactly as the original one. Works just fine during 2000 km this summer. Only luke warm after a 100 km run as an example. Now I´m going to modify the DME again to a single coil relay with double throw contacts 2x30 Amps with same security of function as original.
Sorry for all the words but I´m having the flu and I´m bored to death and my fingers were itching to write this

_____________________________________
Lapponia 1984 944 na

DaveBoettcher · 10-25-2008, 07:52 AM

Hi Lapponia,

Excellent info, thanks very much. As far as I know the DME relay was the original - I have had the car for 17 years since it was 3 years old and I have never changed it (I have only recently replaced a rad hose for the first time, but that's another story) so the old relay is over 20 years old and I am not going to waste time tinkering with it.

The new DME relay seems to have done the trick as the problem has not recurred since I installed it.

However, I am interested in your comments about using a double relay 2x30 Amp: do you think the original DME relay was under-specced, and this is why they cause so much trouble? Do keep us up to date with your plan to install a single coil relay with double throw contacts - can you do this in the original relay housing so that it just plugs in?