DME relay question

[wjdunham]

Did some searching but would take me a lot longer to piece it all together than to just ask for some help... MY 85 Carrera died while out on a drive last night. I found a safe place to pull over and replace the DME relay with my spare, but did not get the car to re-start. Cranks fine, and today I verified that I have spark and injection pulses with my Noid light and timing gun. So that rules out the Motronic DME and the Speed/Reference sensors as the culprits (thankfully!).

So my guess is either a bad spare relay (URO part which I read are suspect) or the fuel pump died. I haven't checked for fuel pressure as I don't have a pressure gauge.

My question is, will I still get spark and fuel pulses with a bad relay? If that URO replacement was bad then it was a very expensive ($160 for my tow home) mistake...

Thanks for any help.

Bill

[Joe Bob]

Reference sensor....?

Can you hear or feel a click from the relay when turning the ignition to "ON"? If not, either the relay is bad or their no power to it.

If the relay is powering on, give the intake a blast of ether. If it tries to start, it's a fuel issue.

If the engine is still a lump, do the dance and check the sensors on the flywheel.

[GH85Carrera]

The DME relay is of course a two part relay. One side is for the fuel pump, the other side is the signal from the reference sensors.

The easy way to test your fuel pump is squirt some starting fluid like ether or carb cleaner into the intake when cranking. If it sputters and coughs and tries to start, you have no fuel pressure, and a bad fuel pump or no signal from the DME.

Get the solid state relay available and that will fix the relay forever.

The solid state DME relay is a upgrade any 3.2 should have even if it is running fine now. As far as tracking down the problem, that will takes some diagnosis. There are a few hundred no start threads. Read through them and you will see what to test, and how to diagnosis each possibility.

[cabmandone]

You can jumper the fuel pump to make sure it's working. You'll hear it humming when you put the jumper in. No hum, no fuel pump.

As Glen mentioned, it's a two part relay so you could have spark and power to the injectors but no fuel to the injectors if the fuel pump side isn't working.

[Joe Bob]

I once had a No Start problem. It ended up being the crappy torpedo fuses. I used to live at the beach and was always fighting corrosion. I would spin them every few months. Wore a bald spot on top the fuse that fed the DME power. THAT was a PITA.

I've never had a reference sensor go but plenty of bad DME relays. Last time I purchased the solid state relay. Still keep the analog relay as a spare but doubtful I'll ever need it.

BTW, friends don't let friends buy URO products. they would have to improve to just suck.

[wdfifteen]

You can hot wire the fuel pump to see if it is working. With the holes in the relay socket facing you and the wires going down, jumper the top and bottom holes on the left row of the socket terminals (terminals 87b and 30). You should hear the fuel pump buzzing.

[wjdunham]

Originally Posted by cabmando

You can jumper the fuel pump to make sure it's working. You'll hear it humming when you put the jumper in. No hum, no fuel pump.

As Glen mentioned, it's a two part relay so you could have spark and power to the injectors but no fuel to the injectors if the fuel pump side isn't working.

I'm trying to rule out the reference sensor and speed sensor as the culprits, because I really don't want to have to try and replace those with the engine in the car :-( I'll try the fuel pump debug by jumpering/bypassing the relay, but I'm not sure my original question is answered: will the DME generate spark and fuel pulses if either one of those sensors is bad?

I know it shuts off the fuel pump if the reference sensor drops below 200 RPM, or if the reference sensor just fails. Does it also cause the spark/fuel to get shut down, or will it generate those pulses even if the sensor is dead?

Seems like it would need the sensors to get the timing of the fuel/spark pulses correct so my thought was if I'm seeing pulses then that means the sensors are OK and it's a fuel problem or a bad relay?

As a preventative measure, I've already purchases a Focus9 solid state relay...

Thanks,
Bill

[GH85Carrera]

Replacing the sensors with the engine in place is not fun, but totally doable. And replace the head temp sensor "while you are in the there" unless it has been replaced fairly recently.

[wjdunham]

Originally Posted by GH85Carrera

Replacing the sensors with the engine in place is not fun, but totally doable. And replace the head temp sensor "while you are in the there" unless it has been replaced fairly recently.

Yes, that would be my plan, but getting that bugger in and out with the engine in place looks to be a real pain. I didn't want to have to tackle it all right now unless it's absolutely necessary.

[wjdunham]

Pretty sure it's the fuel pump:
Turned the key to the run position, ICV is vibrating
Inserted a wire on pin 85b of the relay, with the key in the run position see 12V, key in the Start position see 1.5V so looks like the DME is trying to pull it low and turn the fuel pump on
Shot some starting fluid into the engine, catches and fires just fine
Pulled the fuel pump fuse and cleaned the contacts, fuse itself was fine, reinserted, no start

Is there any other possibilities before I replace the pump?

Thanks for all the help, I know there are hundreds of threads on this subject, but many of them go well off track and it takes hours to get to the right one...

Bill

[GH85Carrera]

It does sound like your fuel pump. All the signs are sure pointing at it.

If it is the original pump, it is likely time anyway. Buy a new filter and change the fuel filter as well just to have a clean system.

The pump replacement is another no fun project. Hopefully your tank is not full. If you are like I am, you will make a mess. Nothing much more fun than gasoline running down your arms to your armpit. I would recommend talking to Len Cummings and getting the proper metric sized, gasohol safe hose. Finding the metric size hose is a challenge at all the parts supplier placed around here. Much less a fuel injected rated alcohol safe hose. I would replace the hose from the tank to the pump, and have that step done.

The fuel hose goes from the pump banjo fitting, over the steering rack, and down the tunnel to the engine compartment. My hose started leaking and I had to replace the lines in the tunnel. It too is a dirty pain in the butt project.

[tx63conv]

Originally Posted by wjdunham

I'm trying to rule out the reference sensor and speed sensor as the culprits, because I really don't want to have to try and replace those with the engine in the car :-( I'll try the fuel pump debug by jumpering/bypassing the relay, but I'm not sure my original question is answered: will the DME generate spark and fuel pulses if either one of those sensors is bad?
Bill

I had a similar issue and turned out to be the ref sensor. My relay tested fine and I was able to jump the fuel pump. However I could not get spark or fuel pulses. Replaced the ref sensor (and speed) and started right up.

[cabmandone]

Originally Posted by wjdunham

I'm trying to rule out the reference sensor and speed sensor as the culprits, because I really don't want to have to try and replace those with the engine in the car :-( I'll try the fuel pump debug by jumpering/bypassing the relay, but I'm not sure my original question is answered: will the DME generate spark and fuel pulses if either one of those sensors is bad?

I know it shuts off the fuel pump if the reference sensor drops below 200 RPM, or if the reference sensor just fails. Does it also cause the spark/fuel to get shut down, or will it generate those pulses even if the sensor is dead?

Seems like it would need the sensors to get the timing of the fuel/spark pulses correct so my thought was if I'm seeing pulses then that means the sensors are OK and it's a fuel problem or a bad relay?

As a preventative measure, I've already purchases a Focus9 solid state relay...

Thanks,
Bill

Pretty sure you won't get spark if the sensors are bad. If you're seeing injector pulse your sensors should be good because injector pulse is signaled by the ECU which gets power from the DME relay if I recall correctly. If no signal from the DME relay to the ECU, no spark, no pulse.

[Joe Bob]

Have you shot ether into the intake yet?

[wjdunham]

Originally Posted by Joe Bob

Have you shot ether into the intake yet?

Yes, see post #11

I also pulled the relay and jumpered pins 30 and 87b - my jumper wire got hot fast after I turned the key to the on position. I'm pretty certain I had the correct two pins on the socket jumpered, I checked it many times before applying power. I would think if the fuel pump is shorting that the fuse would blow, but it definitely is another indication that something is not right with the pump.

Next step is to pull the cover off and check the voltage right at the fuel pump and see what's going on. Once I get that far I'll probably just replace it for good measure.

[Joe Bob]

Missed that. Yes, likely culprit is the pump.

[wazzz]

Try to hot wire 30, 87b, and 87, not only 30 and 87b as you first did. You should then be able to crank and start the engine. If it does not start, then the fuel pump is indeed a possible culprit. When doing so, you should hear the pump buzz before you crank the engine. If no buzz, then the pump is likely failing.

But there is one last check you can try to make sure all parts of the DME relay are ok. Your previous tests showed that the main internal relay is correct (the ECU is powered on and the ICV is vibrating). The voltages measured on pin 85b when ignition key was on Run (12V), then Start (1.5V) seem both correct. Like you said, the second 1.5V value shows that the ECU is pulling a ground on 85b when key is in Start position. That means the second internal relay coil of the DME relay is energized correctly. But its contacts could still be bad, in which case the fuel pump would not get power.
So if you can manage to insert a wire in pin 87b of the relay like you did for 85b, check to see if a +12V is present on this pin when the relay is in place and when key is in Start. That way you will know that the DME relay is good and the fuel pump is probably bad.
If you can get a hand on the Bentley manual, see a simplified diagram on page 240-37. Very useful.

[BoxsterGT]

The late style "Flow-Through" Bosch Fuel Pumps have come down in price a lot, since so many of this style are now used worldwide.

FYI-
Bosch 69458 is available for just over $100 shipped. Been using them with great results, and they come directly from Bosch.

Hope this helps.

Len

[wjdunham]

Thanks Len! I just purchased this one Bosch® 69467 - Front In-Line Electric Fuel Pump which was a bit more but still very reasonable. Have you had any experience with that one?

@wazzz - I did one additional measurement today which I should have just done right away - measured the voltage on the downwind side of the fuel pump fuse while cranking the engine. I measured approx. 10.5V which would indicate that the fuel pump is trying to pull a fair bit of current, likely damaged but not completely shorted as the fuse isn't blowing. The fact that my wires heated up significantly when I jumped the two outer pins on the left side of the connector would also confirm a lot of current flowing.

[wazzz]

10.5V while cranking isn't that bad, considering the current drawn by the starter. Battery voltage always drops a little while cranking. Wherever you measure it, including upstream the fuel pump, you'll see the dropped voltage.

Seeing 10.5V at the pump fuse does not mean that the pump is drawing any current. It could be just dead and cut open (not shorted). If you don't hear any buzz, it is probably dead.