emaxxowns

so im just honestly confused right now. i plugged the DME back in and alarm control unit as they were sitting unplugged and tried to start it and it started. trying to give any gas would cause it to want to stall but gently hitting the MAF sensor had the fixed. I went to try and start it again today and the only thing that was touched at all was slightly moving the DME and now it wont start again with no tach bounce. would this lean more towards a bad DME or bad speed sensor? has anyone heard of a speed sensor that works intermittently?

emaxxowns

since it was able to start, i would assume that means i did the replacement sensor wiring correctly.

emaxxowns

cranking speed is definitely 200rpm+

peanut

well you kinda answered your own question. If you have no tach bounce then the engine will never start and there has to be something wrong with either the sensor or the associated wiring between the sensor and the ECU

If you throw the DME relay in the glove box and fit a 3x wire bypass whilst you are testing then you can completely and confidently dismiss the DME relay being at fault.

Testing with a DME relay fitted is not a sensible method of troubleshooting because you will never know if the fault issue is due to the component that you are testing or the DME relay.

A 3x wire bypass connects all the connections that the DME relay normally does but it does it permanently rather than relying on 2x crude relays to do the job. A 3x wire bypass takes the DME relay out of the equation

emaxxowns

im still heavily in the learning process here. so to clarify, it could still be a bad DME if i have no tach bounce correct? i know for sure it could be the speed sensor or how it is gapped(not sure if this was ever replaced by someone sel, not by me)

peanut

No not quite .
The ECU uses information sent by the speed sensor to confirm that the engine is turning over at a pre determined speed (approx 200rpm)

If the ECU receives the signal from the speed sensor then it will ground certain components so the they work,... like the ground to the injectors and the fuel pump and igniter etc .It does this by using the DME relays
if all is well the ECU grounds both the DME relays which powers the essential components for the car to start and run The DME relay is the ECUs right hand man if you like

If no pulse or 'signal' from the speed sensor is received then the ECU will not connect essential grounds or 12v supplies ,preventing the engine from being started or running. This is a safety feature built into the ECU by Bosch to prevent a fire in the event of an accident.

The ECU also has to receive pulses or resistance , voltage or other signals from several other sensors in order to work properly like the engine temperature sensor and the O2 sensor TPS , AFM etc .
If any of these are disconnected or faulty then again the ECU will not ground all the essential components in order for the engine to start and run.

Now all that said..... your ECU could also have a fault ,...typically a dry solder joint on one of the pcbs or water ingress etc so your issue could be one or more of the sensors OR the ECU OR both

Dan Martinic

I had an intermittent issue and this was the fault:




Those tiny little lines you see are cracks in the joint; indeed, the joint wasn't loose or anything, but I heated these joints up and added a tiny bit of fresh solder. My problem--surprise stalling now & then--was fixed.

Since you mentioned you moved your DME and the problem returned, you might want to open it up and look for little lines like this on solder joints.

emaxxowns

so i tried starting with a confirmed working DME and it still doesnt start. you said that it has to check other sensors before it will work properly. IF the issue were like an O2 sensor or AFM, would i be getting tach bounce? or is my next immediate thing the speed sensor?

peanut

you seem to have ignored some of my other posts as if you know better than me ?

if you don't answer my suggests and questions how is anyone going to help you ?

First of all DO NOT use the abbreviation DME when you refer to the ECU (electric control unit) it causes a great deal of confusion and misunderstandings when people don't clearly differentiate between the DME (ECU) and the DME relay

Bosch always us the term ECU in all of their manuals and training and repair guides etc so best to follow suite.

lets clear up any confusion or misunderstanding about the speed sensor .

Simple version ....The speed sensor is simply a coil of wire it has no influence on any other sensor whatsoever. The ECU sends out a voltage to the speed sensor which creates a magnetic field which is interupted by the teeth on the flywheel starter ring . The resultant pulses are returned to the ECU telling the ECU the engine speed so that it can determine the correct amount of fuel and ignition advance etc (On the late 944 the speed sensor and TDC sensor iare combined )

If the ECU does not register the engine speed information from the sensor,.... it will not close the DME relay contacts that power the fuel pump injectors ignition spark etc .

The speed sensor is also responsible for indicating engine rpm which is why we look for a slight tach bounce to tell us the ECU is seeing the speed sensor pulses.

No tach bounce,........no start.

emaxxowns

My apologies, I dont mean to sound that way. After re-reading all of your posts it sounds like I need to confirm the other sensors are operating correctly as the next step. I really appreciate the help you have been providing, you sound very knowledgeable.

peanut

hey don't mind me I can be a little grumpy at times.

The most important thing that you can do is do nothing, before thinking about the likely cause of the problem.

Obviously not every component simultaneously failed at the same time so there is no point in just randomly replacing components or making adjustments et before you have applied a little logic to diagnose the problem.

The more things you fiddle with the less things you can count on being as they were when the engine failure occurred which makes diagnosis much more difficult..

When you understand how all the different components in the fuel injection and ignition system work together then it is an easy matter to troubleshoot the problem systematically to narrow down the cause/s

The fact that the engine started and ran means that all the various components involved in starting and running are all functioning . They may not be at optimum but if it hasn't run for a while then thats to be expected.

So we know that the speed sensor must be working even if you cannot see the tiny tach needle bounce so until proven otherwise do not remove or replace it .

When you start the car its really important to make sure that you do not apply any throttle or you will defeat the ECUs cold start mechanism . If the engine only starts with lots of throttle then that itself is diagnostic and tells me something important.

Let me reread the thread and see if I can form a clear picture

emaxxowns

from what im gathering with how it all works together im wondering if my AFM needs to be confirmed working, even when it started i had to hit the AFM to get the car to rev without stalling. otherwise i would wonder if maybe i have a bad ground(s). or if you can think of something else. i have tried to start with pressing down the gas pedal and that doesnt seem to make a difference

Dan Martinic

This.... just cannot be right (?)

emaxxowns

correct, i was wrong when i said that, i read a diagram wrong somehow in my stupidity. it definitely had one lol

Dan Martinic

What do you mean by "hit the AFM"? Describe this 'hitting'