1991 940t injection problem

I'm new on this list. Have a problem with a 1991 940T wagon - intermittent no start. When it won't start there is no injection pulse. If I disconnect any one injector, the pulse comes back and the engine starts.

It APPEARS this system fires all 4 injectors at the same time, from one driver in the ECU. We have tried bypassing the wiring harness from the injectors to the ecu, as well as jumping all the grounds at the ecu to be sure all of the grounds are good.

Strongly suspect a bad driver in the ECU - got a used ECU from wreckers - exactly the same problem. Do we have 2 bad ecus or is there another (common) problem I am unaware of???

Reply to
clare
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a 91' 940t will be ci (continuous injection). if pulling one injector solves the problem, it is probably not a logic error. ground fault seems more likely, but it can be subtle. 20 yr old injectors could be bleeding enough power individually to shut the system down. clean them up or replace them all. there are a number of ways to test the injectors to see if they are w/i spec. & one injector may be much farther out than the others. hit me again if you need the protocol.

Reply to
edasduke

ANY ONE of the injectors, if pulled, will restore the pulse, and when reconnected it runs well. The resistance of all the injectors is the same. I wondered too if the injectors could be overloading the driver. My brother is actually the mechanic working on this and he's called me in for a "second opinion".

I have not personally looked at the car. How would you recommend checking the injectors?

Reply to
clare

Hi Clare, 1991 940Turbo uses LH-jetronic 2.4. When starting there are five injectors available (four normal and one start injector). As power for injectors is routed through the Interference Suppression Relay to all five, the earth drive pulses come from pin 18 (ECU) for the four injectors, the start injector gets it's earth drive pulse from pin 32 (ECU). Problems often arise from the Fuel Relay going intermittent. I would check that the start injector is OK, then the Fuel Relay, then the Interference Suppression Relay. As a last resort make sure the throttle switch is correctly set (uses both contacts). Hope that is helpful, all the best, Peter.

Reply to
Peter K L Milnes

Since disconnecting any one injector makes the pulse come back I think we can rule out the start injector and the throttle switch. Since there is power at all injectors (but no ground pulse) that rules out the interference suppression relay. Pin 18 does not pulse low until the load is reduced by disconnecting one injector.

As soon as (any)one injector is disconnected the pulse train is re-established and the engine starts. It continues to run with all injectors reconnected and will GENERALLY restart. A day or two later it does not start - [ull one injector lead and away it goes.

It is a 563 ECU. I've been told they are notorious, and a 951 from a 93 240 will work???

Reply to
clare

we are getting closer. now test the individual injectors -- apply current & see what happens. this isn't a resistence test. this is a function test.

just for a giggle, disc>>

Reply to
edasduke

I've been a mechanic since 1969, and my brother since 1975.

The car starts as soon as it gets fuel. It gets fuel as soon as the injectors get grounded. The injectors don't pulse when the engine doesn't start, but as soon as one injector is unplugged the injectors pulse and the vehicle starts. It isn't the ECU, becuase after trying another used ecu and having the same problem, a "known good" ecu (951 - no cold start from a '93 240) was plugged in - same problem when the engine got warm. No restart. I'm wondering if possibly something wonky with the injectors? 1.7 ohms each, within 0.05 ohms - spec is 1.5 to 1.9? No way the resistance of the current limitting resistor could have DROPPED causing a high current draw??

Reply to
clare

That is a weird one. Check if you have solid 12V when cranking at the common point of current limiting resistor cluster. This lead carries current for all four injectors. If there is something rotten along that path to 12V source that it could act as an additional current limiting resistor for all four injectors. That may be too much resistance to fire all four at once. Take one out and you have barely enough for other three so it starts.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

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void _-void-_ in the obvious place

Reply to
Boris Mohar

No -at least my brother claims the 12 volt power has been verified - it is the GROUNDING that does not happen. My thought was perhaps the current limiting was not working and the injectors were drawing TOO MUCH current, kicking the driver into overcurrent protection = no ground pulse. Removing one of the 1.7 ohm injectors from the parallel load lowers the current draw (raises the resistance) so the driver goes out of "overload" and fires.

Bit the injectors are only a small part of the resistance of the circuit - 1.7 ohms with the ballast resistors being 6 ot 7?

I'm going to have to shoot over to his shop and test the injector current (and the RI relay)

Reply to
clare

Reply to
edasduke

So, whatever happened with this? I'm curious because I have a 91 940T sedan sitting in my yard. Runs fine, but it would be good to know just in case....????

I think you have a bad ground connection somewhere in the injection system.

/glenn

Reply to
/g

Have not talked to my brother lately but I think he was looking at shorted injectors. He told me they were 1.25 ohms each and spec is

2.5.

Yet to confirm

Reply to
clare

OK, here's my GUESS on what happened.

The car had a fuel pump issue previously - as well as a plugged fuel filter. Both were replaced just before he started working on THIS problem.

My SUSPISCION is the engine was running a bit lean due to poor fuel delivery, so the computer compensated by increasing the duty cycle of the injectors - and the guy kept driving it.

High duty cycle means higher average injector current, which means more heat. The injectors overheated and the insulation in the coils broke down, causing the injector coils to short when warm - lowering their resistance.

So on a warm start, in particular, but possibly even on a cold start, the injector resistance was below spec, and the currrent over spec - causing the injector driver in the computer to shut down on overload protect. Disconnecting one injector reduced the current so the driver came back "on-line" and fired the injectors. At speed,(higher frequency) the inductance of the injectors was higher than the DC resistance, so the current did not get into the overload range, and the driver kept pulsing the injectors.

This is my suspicion from what I HAVE been told. I have not been told for sure what he ended up doing to fix it, but since I suggested this might be the problem, and he has not gotten back to tell me I was wrong (you know how younger brothers are????), the chances are pretty good I hit the nail on the head.

Reply to
clare

i would have gone w/ ground fault. the odds of 4 parts failing simultaneously are just too high -- unless, there has been some type of catastrophic event. it does look like you need 4 new injectors. hmmm... wasn't that my suggestion in the 1st answer

Reply to
edasduke

They may have failed sequentially - not all at once - and when the last one failed the problem became critical.

Still speculation - but I'm calling my brother tomorrow to see what he found.

Reply to
clare

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