86 Carrera Problems - Page 2

2.70Racer · 09-04-2009, 04:40 PM

The DME relay is really two relays in one box. one part controls power to the ECU, the other relay controls the fuel pump.
Many parts on a 20 year old Carrera are at or well past their design life.
Replace these parts unless you are ok waiting for tow trucks.

kidrock · 09-04-2009, 06:03 PM

The DME relay serves as the "fuel pump relay" in the '84-'89 Carrera 3.2 with Motronic. The DME relay, as explained to me, also serves as the relay to the various sensors associated with the Motronic DME (speed, reference and cylinder head temperature sensors).

Your fuel pump will operate only while the motor is running, ***UNLESS*** you bypass the DME relay. To do this, you need to disconnect the relay from it's housing. Then, with the aid of a jumper wire, you will "jump" the relay housing by connecting the wire from junction #30 to #87b of the relay housing. By doing this, you are bypassing the DME system, and you should hear the fuel pump whirr to life. Careful with the wires down there under the front seat!!!

Before you test the fuel pump by jumping the DME as explained above, you asked which hose to disconnect to test your fuel system. Do you know what the Idle Control Valve (ICV) is? It's the silver cylinder located near your throttle body, and has a hose going vertically into it, and another coming out below. You can safely and easily disconnect either of these hoses. When disconnected, spray a small, short burst of "quick start" (ether) or carb cleaner into the hose, and quickly reconnect (no need to reinstall the hose clamp yet). Quickly, attempt to start your car. If you have a helper, have him (or her) start the car as soon as you put reconnect the hose.

If your car starts immediately then subsequently dies, then it is probably a dying, intermittent fuel pump. They usually won't work well when hot...but your car will start after the pump has had some time to cool down.

Let us know how she goes, keep us posted. Good luck.

widgeon13 · 09-04-2009, 07:31 PM

Ok, thanks for the recommendations and detailed explanations. Hopefully I will get to it tomorrow for some further troubleshooting. ICV will be my first option and then jump the DME plug to see what that tells me. If neither of those options give any good info I will get to the fuel pump and tap it with a rubber hammer or handle of a wooden screwdriver. Appreciate the time and info. Will keep you all psoted.

jimbauman · 08:32 PM

Like Kidrock says .... Did you try bypassing the DME to check the fuel pump? Here's what I do... Jump a wire from fuse 4 to 6 and you should hear the fuel pump whirrrrrrrrrrrrrrrrrrrrrr..... If not, bad pump.

If you CAN hear the pump, leave the jumper attached and start the car. If it starts, bad DME (more than likely). This is a good SHORT TERM solution to limp home when the DME goes bad and you don't have a spare...

JB

ischmitz · ischmitz's Garage

The DME relay has two stages - it really is two relays in one box: The first stage switches power to the DME itself (used for the digital part of the DME and the AFM). It is controlled by the ignition switch and the alarm unit (if installed). The second stage is the fuel pump relay. It controls power to the fuel pump and the O2 sensor heating element. It is controlled by the DME itself. It is activated when cranking (key in START position) or when the speed sensor detects engine speeds above 200 RPM.

Ingo

widgeon13 · 06:51 AM

Quote:

Originally Posted by jimbauman

Like Kidrock says .... Did you try bypassing the DME to check the fuel pump? Here's what I do... Jump a wire from fuse 4 to 6 and you should hear the fuel pump whirrrrrrrrrrrrrrrrrrrrrr..... If not, bad pump.

If you CAN hear the pump, leave the jumper attached and start the car. If it starts, bad DME (more than likely). This is a good SHORT TERM solution to limp home when the DME goes bad and you don't have a spare...

JB

Ok, I've jumped the fuel pump at the fuse panel and get a whir, jump it at 30 & 87b at the DME and don't get anything. Keep in mind this is a new DME but not the Bosch one. I know where the ICV is but it doesn't seem necessary to fog there if the fuel pump is working. May try later today to have someone start the car as I jump the fuel pump at the fuse panel.

Where are the flywheel speed and reference sensors located, maybe those are the next things to check as well a any spark.

ischmitz · ischmitz's Garage

It appears some use DME and DME relay for the same thing. They are two different items:

DME is short for Digital Motor Electronics and describes a silver box about 8 by 5 inches bolted down under the drivers seat. It has a 35-pin connector going to it. Is is a micro-computer with analog inputs and outputs and generates spark and fuel signals based on its sensor inputs.

The DME relay is a little black box the size of a match box and is a special VW/Porsche relay. It is recommend to have a spare since these are known to fail. They are around 25$ and our host sells them

Ingo

widgeon13 · 09-05-2009, 07:18 AM

Haven't done anything to the DME but have changed out the DME relay and also that is where I jumped the 30 & 87b connections to see if the fuel pump was operating. Thanks for the clarification on the two different items and sorry to use the incorrect name when speaking of the DME relay. I've done nothing to the DME (silver flat box).

ischmitz · ischmitz's Garage

When you jumpered it ,was the ignition switch in the ON position?

widgeon13 · 09-05-2009, 08:23 AM

No, ignition at key was off.

jimbauman · 09-05-2009, 08:23 AM

Quote:

Originally Posted by ischmitz

It appears some use DME and DME relay for the same thing. They are two different items:

I think when most people refer to the DME they mean the DME relay. The ECU (engine control unit) is how some refer to the main computer...

JB

widgeon13 · 09-05-2009, 01:08 PM

I went back and checked the contact between 30 & 87b and got the whir of the fuel pump so it appears the fuel pump is working. May check grounds next.

Quote:

When you jumpered it ,was the ignition switch in the ON position?

kidrock · 09-05-2009, 03:42 PM

I'd fog the vacuum line to the ICV anyways, just to make sure it's not a fuel starvation issue. It only takes 30 seconds.

If a burst of carb cleaner doesn't turn the motor over, you can now focus on spark (or lack thereof).

But since you're checking your grounds, I need to ask if you are aware of the big batch of ground wires that are attached to intake runner #1. This is a common problem, as is the ground strap under the chassis near the tranny. All these should be clean and tight.

I believe that speed and reference sensors are checked with an oscilloscope...as if I'd keep one of those around the garage?

I think many people just bite the bullet and replace them without testing? Maybe an expert will chime in here.

anyways, make sure you're not fuel-starved...then you can move on to the next issue, which is spark.

widgeon13 · 09-05-2009, 05:39 PM

We moved a few weeks ago so have just today found the Bentley manual and that is helpful, also lists all the ground contact points. I will fog the vacuum line tomorrow and see what happens.

Can you get to the sensors w/o doing a drop or partial drop?

This same (no start) thing happened to me a few years ago and all that was necessary was replacing the DME relay.

Thanks to all for the encouragement and guidance. At least this is not my daily driver.

kidrock · 09-05-2009, 06:23 PM

The speed and reference sensors are accessible without dropping the motor. You will need to jack up the rear driver's side, and probably remove the wheel for easier access. Both sensors are side-by-side in a small metal housing, located near the flywheel housing where the motor meets the tranny.

Make sure you fog first...then check your grounds as described earlier. If you're getting fuel and your grounds are clean and tight, you will need to check your plugs, wires or coil for spark. There is no need to start throwing money at sensors yet.

You're probably closer than you think. don't give up yet!!!

widgeon13 · 09-05-2009, 06:35 PM

Thanks again, we'll keep working on it.

tomphot · 09-05-2009, 07:06 PM

Quote:

Originally Posted by kidrock

The speed and reference sensors are accessible without dropping the motor. You will need to jack up the rear driver's side, and probably remove the wheel for easier access. Both sensors are side-by-side in a small metal housing, located near the flywheel housing where the motor meets the tranny.

These sensors were my problem - after much searching, once they were replaced I've had no troubles.

widgeon13 · 09-06-2009, 04:40 AM

Checked the battery voltage and had 11.74 volts at the terminals. Is this sufficient to start the car? It turns the starter fine but I'm wondering if it is not sufficient to run fuel pump at the same time. Bentley manual says 11.7 volts is fully discharged. Could this along with a faulty ground be my problem. Bentley says 12.4 is where you need to be for adequate charge, battery was in the car when I bought it 3 (+) years ago. All accessories work when in the key on position.

I'm clearly no electrical guru so I'm grasping at straws at this point.

jimbauman · 09-06-2009, 06:31 AM

Just curious - you were going to try starting the car while the fuel pump was operating (with the jumper attached across fuse 4-6). No luck with that?

JB

widgeon13 · 09-06-2009, 06:37 AM

Quote:

Originally Posted by jimbauman

Just curious - you were going to try starting the car while the fuel pump was operating (with the jumper attached across fuse 4-6). No luck with that?

JB

Haven't tried that yet but will do that shortly. Car is at the airport hangar so everything I do requires a trip over and back and I had some home projects yesterday.