Laco of common sense?

Vic Rattlehead · 11:45 AM

For a while now, since around the end of march my 994 S had a bucking/kangarooing problem. Violently lurching upon acceleratioin, or stumbling upon cruising.

So, back to basics I thought. Checked all the HT leads, dizzy cap, rotor arm. Everything was fine. (my old spitfire had frayed wires on the aftermarkert electronic ignition module whith the same results) I noticed the prob worsened if the car went over a rut, specificaly the left of the car. So I checked the wiring around the ignitor unit. All fine.

Next was the fuel filter. Me and my Dad fought with this bugger for days, trying to get the old seized nuts loose. Spraying with penetrating oil every night.

It bucked for a day then the problem went away. And I felt victorious.

Until last Monday when it returned with avengeance. Worse than ever.

I had a quick brainstorm at home when I realised, the problem went away when I took out a friend of mine who is 6'3 and he adjusted his seat back. Then the problem returned when I took out the promoter of my band. Who also is about 6' and moved his seat back.

So I tore back the carpet to find the wooden panel in front of the DME held in place with one screw. On removing the panel, I found the bracket that the DME is secured to, wasn't secured to the car body and was just free to bounc around. And worst of all, it had been placed uspide down!

The guys adjusting their seats must have knocked the whole thing.

So a few self tapping screws and washers later, the bucking has gone and I'm a happy man.

Obviously though, whoever had the DME out in the first place didn't have the common sense to put it back in the right way up.

Sorry for the long post, but some peoples stupidity just astounds me sometimes.

SoCal Driver · SoCal Driver's Garage

Grounding the DME is critical. Good find!

The early 944 DME's are secured to the bottom of the steering column house so if they are loose you will get hit in the knees.

You might want to open the DME up and see if there are loose solder connections from it's bouncing. Check the wire to pin connections on the plug too.

Vic Rattlehead · 06-10-2006, 11:42 AM

Thanks Socal.

I originaly thought that it could just be the harness plug was loose and needed pushing home. I didn't expect to see the whole unit free to flop around.

944 Ecology · 06-12-2006, 01:48 PM

The frame of the DME is not grounded, so whether or not it is attached to the car's chassis is immaterial.

Most likely, you had a loose connection causing your problem.

I have run several cars at the track with electrically isolated DME and KLR boxes with no problem.

SoCal Driver · SoCal Driver's Garage

If the DME is bolted to the chassis as it should be then the case of the DME is grounded. Now wither this has anything to do with the PCB boards grounding I would have to open one up to check. In all honesty I haven't been inside one in quite a few years. However in looking at the schematics that are on FRWilks there are ground points for the components.

As the 944S has a problem with burning up some of it's PCB traces if the engine is not grounded to the chassis the ground points I see may not ground to the case.

Eldorado · 06-12-2006, 07:40 PM

just had mine opened up (granted it's for a 944) but i didn't see any obvious grounding points from the pcb to the case..

Vic Rattlehead · 06-13-2006, 03:42 AM

Well, securing the DME how it was intened to be seems to have cured it.

Thanks guys.

gtroth · 06-14-2006, 04:21 PM

Do you guys think the ungrounded case might make the DME more susceptible to electrical (radiated) interfence from, say, the ignition?

Eldorado · 06-14-2006, 04:44 PM

i'd say so..
grounding the DME case is essential for a safe and happy DME.

SoCal Driver · SoCal Driver's Garage

The closest path back to the DME for the spark is the wires for the injectors. Read a post a while back where the owner replaced one of the IC's that drives the injectors and fixed his problem.