Hi guys, I need some help with a 1985 928 with LH-ECU. The injectors are not firing at all and I'm running out of possible solutions.

They changed the engine and re-used the fuel injection and EZF of the old engine. Here are some facts:

1. The car has spark and when you crank the engine the tach needle bounces slightly
2. Power supply to both ECU's is OK
3. When you turn on the ignition, the fuel pumps runs briefly and turns itself off (normal). When you crank the motor the fuel pump is energized and the pump runs.
4. We have pulled the plug of the LH-ECU, jumped the fuel pump relais and fed Ground into PIN 13 of the LH-ECU. They all fire without a problem. We checked each injector (all good) and checked the injector cables for a short. They are all good.

To my knowledge it is very good that we have spark, because the LH-ECU is the slave of the EZF and with no spark there is no injection. The EZF wakes up the LH-ECU via the RPM signal generated by the EZF. The LH-ECU energizes the fuel pump relay and fires the injectors.

In our case the fuel pump relay is energized but no injection pulse is provided, why? We found out that EZF PIN 16 (that is connected to PIN 1 of the LH-ECU) shows 12 Volt + as soon as the ignition is turned on.
I think that there is something wrong here. There should be nothing until the engine is cranked and than a clear RPM signal should be displayed, correct?
Right now when you crank the motor and you check PIN 1 of the disconnected LH plug against ground, you will see the voltage bounce between 9,9 and 10,2 Volts.

What is wrong, is this a fried EZF? BTW we changed the LH-ECU already with a new (used unit out of a working car) no change.

 

Hi Schocki - The signal from EZ-F to pin 1 LH should be a square wave, 0-12v. This is the same signal that feeds the rev counter but also the kickdown relay.

Suggest you unplug the KD relay just to be sure that isn't the problem. If you still don't get 0-12v square wave at pin 1 then it sounds like a faulty EZ-F.

 

Also the ignition/injector grounds could have not been hooked up in the rear V of the engine.

 

Schocki, is it a 5 speed or auto, if 5 speed, make sure there is NO relay in the KD relay position...

 

Jumper EZK relay and try

 

Leave the brains plugged in,
Unplug all 8 injectors,
turn key to ON position, measure each pin of the injector plug to ground. One side should be +12V the other should have nothing.
If you have +12V on one pin of the plug and not the other then clean/attach the grounds SeanR speaks of.
If you have +12V on both pins of the plug, then you have a short in the harness somewhere (ensure all 8 are unplugged!).

 

Hi guys,

thank you all for all your inputs.

1. It is a 5 speed and there is no kickdown relay plugged in.
2. @ Sean I will forward the picture
3. We have spark so jumping the EZF Relay should not be necessary.
4. @ Colin When we jump the fuel pump relay both pins of the fuel injector plugs are energized (plugs disconnected). BUT if we pull the LH-ECU multipin plug and manually feed ground into PIN 13, all injectors fire.

 

What's the actual voltage of the battery in the vehicle? When the last battery in my '85 was on its way out, the battery would sit at 12-12.2 volts. This was usually enough to crank the engine several times (and crank it fast enough), but insufficient to start it, because it couldn't crank the engine and supply sufficient voltage to the EZF/LH. With the key turned off, the battery voltage needs to be around 12.6-12.8. I would check this first and put it on a charger for a while if it's not that high. While it's charging, you can check other things.

I would then check the grounds that SeanR showed in his post. Most of the time, a bad or dirty ground can be corrected briefly by loosening the bolt a few turns and then tighten it.

I would pull the plugs off the EZF and LH. The pins in those large plugs are actually female and very easy to bend when testing with a voltmeter probe. I spent months trying to diagnose an issue with my '85 that turned out to be one or two slightly mangled pins in the LH plug. I used a pick to straighten the pins so they would make good contact and the problem disappeared.

While you've got those plugs off, you can take a minute to place a jumper in the EZF relay socket of swap that relay with one of the others (the horn is a good one).

And since the hood will be open after you crank the starter several times, crack open the fuel rail and make sure the fuel pump is pushing gas to the front.

And one final thing, does the gas gauge work? Does the car have gas in the tank? I know that seems almost ridiculous to ask, but I wouldn't ask if there weren't more than a few instances of no starts that were solved by putting some gas in the tank.

 

Disconnect all 8 plugs, and LH, measure again on both pins to ensure only 1 has +12V.
If both have it then you have a short in the harness.

If you don't then I would be inspecting the LH closely as it should have no voltage from its output.....,

 

What about a pinched or plugged fuel line, plugged fuel filter?

 

OK and thank you for all your inputs. I wrote a Reader's Digest Condensed Version to the mechanic. That will give him enough to do tomorrow. Thank you all for your inputs! I will keep you updated...

Salu2
Schocki

 

I just checked on my car, and 12 volts (actually mine shows a little less ) at pin 1 on the LH plug/pin 16 EZF plug, is normal when the key is turned to the on position.

 

Ok thank you!

 

Can we assume you have the 37 page test plan for 1985 / 6 engine managment system?

Snippet below; not sure it matches your approaches above.

 

And some more stuff. The injectors constantly receive 12 volts from the fuel pump relay. They "fire" when they're grounded. They get grounded by some voodoo wizardry **** that happens in the LH. The corresponding pin for this is 13. That's why when you grounded pin 13, the injectors "fired." This is normal.

So when pin 13 is manually grounded, everything seems to work, and the injectors "fire?" But when it's plugged in, you get nothing. So you've got two possibilities here. Your LH could have an issue which is not letting it ground out pin 13. The LH box has 3 pins that connect to ground (they are pins 5, 11, 25.) The ground point is in the picture SeanR posted above. The other issue is that one or all of the three pins I just mentioned (5,11,or 25) are not attached to a ground point, or are dirty and making little or no contact.

If you've tried both LH boxes with no luck, and you know at least one is good, then I'd rule out a malfunctioning LH. I would test for circuit continuity between pins 5, 11, and 25, on the LH plug, and a suitable ground. Don't be lazy and plug one probe into 5 and the other probe into 11 or 25. Put one probe in the pin, and the other on the body or somewhere like that. Carefully inspect the pins in the plug to make sure they're all straight (5, 11, 13, 25).

On the LH plug, pins 5 and 11 are connected at the ground point by single ring terminal. Pin 25 on the LH plug shares a ring terminal with the brown wire from the O2 sensor.

I also have the added benefit of having a fully intact LH injection harness for an '85 sitting out on the floor of my garage as I write this.

I would bet that when they changed the engine, they made a mistake and forgot to attach the ring terminal for pin 25 and the brown wire from the O2 sensor to a proper ground point.