Spark Timing Problems

[cslayman]

Hi,

I'm experiencing some strange problems with a 1988 740 Turbo wagon I'm working on with my nephews. Over the summer we installed the motor from a 1993 940 Turbo. We kept the harness and electronics from the 1988 car. The car ran fine for several months. Now we have a strange problem - under load at low rpm, the engine has no power. There is plenty of boost but absolutely no torque. The engine is not misfiring, but it feels like the spark is being retarded too much. At idle, the spark timing is erratic - most of the strobe triggers are around 12deg BTDC, but there are frequent jumps in the timing. It's not rock solid as I would expect.

What I'm wondering is what devices are responsible for spark timing - which one would explain both a retarted spark under boost at low rpm and erratic timing at idle? Here's my understanding of the parts involved and their role (please correct me where I have it wrong):

* Distributor - Timing adjustment (Hall sensor device)
* Ignition System Control Unit (ICSU) - Above the drivers feet. This unit is responsible for driving the coil to generating the spark.
* LH-Jetronic Control Unit (aka ECU) - Under the glove box. Takes input from the distributor and other sensors and sends the trigger signal to ICSU at proper time.
* Knock Sensor - Generates signal to ICSU (and not LH Control Unit?) if spark needs to be retarded
* Coolant Temperature Sensor - LH Control Unit will take this input and calculate correct spark advance (among other things)
* Charge Air Overpressure Switch - LH Control Unit takes this input and retards spark under high boost (I'm speculating here)

I'm think of disconnecting the knock sensor and Charge Air Overpressure Switch to see what happens.

Thanks,
Charlie

[billofdurham]

Before getting too deep into this problem have you checked the harmonic balancer? It is made up of three parts. The inner part is bolted to the crankshaft. The center part is a rubber ring. The outer part is the one with the timing marks and pulley cuts for the belts. These are all attached when it is new. When the rubber-to-steel bond fails, the outer portion of the pulley can slip away from alignment with the crank, giving the appearance of wandering timing.

A quick check is to paint or chalk a white stripe from the hub to the rim of the balancer, start the engine and watch to see if there is displacement.

It should not affect the timing but it can give the impression that the timing is wandering. If it has failed you need to replace it to get any timing adjustments correct.

Bill.

[cslayman]

Hi Bill,

Thanks for recommendation - I never thought of that. I put marks on the outer edge of the damper and the inner hub. The strobe light shows them tracking together. So it looks like the damper is solid. Now I'm back to tracking down what components are involved in the spark timing.

Do you know what the function of the Charge Air Overpressure Switch is? Does it cause the spark to be retarded under high boost conditions?

Thanks,
Charlie

[jimmy57]

The knock sensor has to be at the right torque on the 4 cylinder iron blocks or it will cause timing retard and weak performance. Torque it to 12 ft-lbs.

Unplugging knock sensor and having the same symptoms will confirm KS problems. The ignition system goes to a very little advance default when the knock sensor signal is missing or erratic.
Floating timing marks on that car would not be unusual.

Proper balancer checking is done by making a stop out of an old spark plug. Braze or weld a bolt into a plug with the ceramic knocked out of it. Turn engine over by hand until the piston stops against the stop you screwed into plug hole. Mark the balancer with a crayon or paint and then turn it reverse until it stops and mark it there. The groove pointer on balancer should be half way between your marks. If not it has slipped. Use of the slipped balancer mark will give you wrong timing base setting and could make engine go into heavy retard from sensing excess knock with no knock sensor issues. If you find it off then mark the halfway point and use that for timing setting.

[cslayman]

Hi Jimmy,

Thanks for advice on the knock sensor. It's good to know that disconnecting the knock sensor causes the timing to go into "limp mode". Conversely, is there a way to fool the engine management system into thinking the knock sensor is good and there is no need to retard the spark? Like hooking up a resistor of a given value to the connector?

I also like your technique for determining if the damper outer ring has slipped.

Thanks,
Charlie

[cgreygoose]

I've posted with some issues on my 89 740 sedan non-turbo with 286K, not running well and I saw your thread. I've got the error code telling me 1.4.3 that my knock sensor is faulty. I've got poor pick up, it's loud and runs kinda rough/boggish at low speeds but once at 60mph up to 85mph it runs great!! So I disconnected the the wire from the knock sensor and the car had zero difference. I should take that as a sign to replace it, correct? I've not been able to rent a torque bar that goes as low as 11-12 foot pounds, which is what I've read is key to proper installation. I'm going to go purchase one today and swap it but what ever happened to your situation? Any insight you can give me? thanks,