SwedeSpeed - Volvo Performance Forum banner
1 - 8 of 8 Posts

·
Registered
Joined
·
52 Posts
Discussion Starter · #1 ·
Is it worth it to open up the ECU and clean the connections. I have a couple of minor performance issues and MPG is in the low 20's but in general things are running pretty good. Reason I ask is that cleaning connections on the fuel injection relay and fuel pump relay made quite a difference in MPG. Hate to disturb the unit if it may not help since it seems to be a fairly happy 42 year old.




Having fun!




Craig
 

·
Registered
Joined
·
62 Posts
I have no input of value for the ECU question but I definitely appreciate hearing that there is a benefit to cleaning the FI and FP relays. I will be doing that tonight.
 

·
Registered
Joined
·
2,625 Posts
If it is running OK, I would not go opening up the ECU box. The location of the main relay and the fuel pump relay, at least on my 142, make the connections susceptible to corrosion. The controller is in a more controlled environment and the termination plug is fairly well shielded. Unless there is evidence of moisture or a lot of dirt accumulation around the termination plug, I would not go fiddling with it. Also, the termination plug on the ECU is a multi terminal plug unlike the individual spade connections on the relays. Cleaning the male and female contacts on the plug would be a real pain.

Low voltage can cause problems for the D jet system, although highway mileage dropping to the low 20s seems to be a bit extreme. If the connections on the main FI relay have deteriorated or the relay itself has deteriorated, you could be giving the system low voltage. Check the voltage between the 87 terminal on the main FI relay and ground with the car running. If it is not the same (within about 0.3 v) as the alternator voltage, you likely have some bad connections or the main FI relay may be getting old. The main relay can be replaced with a good quality SPST or SPDT automotive style 12v relay for about $5 or less from an electronics vendor rather than paying the $$ for the OEM Bosch. The primary difference is that the Bosch has a polarizing diode in the coil circuit. The polarizing diode prevent the ECU from being energized and fried if somebody connects the battery up backwards. If you want the polarizing diode feature, you can add a $0.30 1n4001 diode in series with the coil on the automotive relay.

Corrosion of the bolted terminals on the main 12 v distribution block which the Djet gets its power from can also cause low voltage for the D jet (and low voltage for all the stuff inside the car). I was getting a 0.9v drop across these bolted terminals before I cleaned the terminals and bolted them back together with Ox-gard conducting grease. A little Ox-gard applied to the various spade terminals and connections in the car can do wonders for improving things (like brighter lights!). However, it is a conducting grease so small amounts applied to the connections is the operative word. You don't want to create new problems. Ox-gard is available from Home Depot and other places.
 

·
Registered
Joined
·
250 Posts
I'm trying to figure out how cleaning electrical contacts on the relays can translate to an improvement in fuel economy. I'm not denying it, but really can't imagine how it's possible.

Cameron
PDX
 

·
Registered
Joined
·
2,625 Posts
I can't attest to the specifics of how low voltage will alter the operation of the D jet; but, it certainly has the potential. The obvious one is injector opening time which is a highly non linear function of the injector supply voltage. As the voltage drops, the injectors take longer to open. The injector opening time has to be added to the injector time calculated to deliver the required fuel to run the engine. The applied injector pulse width is the sum of the two. Modern digital ECUs monitor the supply voltage and scale the opening time adder based on the measured voltage. This works over a fairly limited range because the opening times are very non linear. Opening times go way up when the supply voltage gets down into the 10 v range. I know this directly because I had to characterize the Bosch injectors when I switched from D jet to Megasquirt.

I suspect that the D jet being a first generation analog controller probably does not have the injector voltage dependency built into it. As a result, if the voltage is dropping significantly, the effective PW for delivering fuel is shortened and should tend to cause the engine to run on the lean side.

Most modern ECUs have a board operating voltage of 5 volts and the sensors operate from that 5 volt supply. Modern ECUs have an on board power supply that drops the 12 v supply to 5 volts and tightly regulates that 5 volt since it affects the signals that come back from the sensors. Modern ECUs can be remarkably tolerant of reduced supply voltage and can probably operate successfully down to supply voltages in the order of 8 - 9 volts (that is just the ECU, fuel pumps and other stuff maybe not so happy). From what I recall of seeing the internal schematic for the D jet (for a Porsche 914 application - slightly different than the Volvo), the manifold temperature circuits and the MAP circuit were operating at 12 v. I don't recall seeing any internal voltage regulation in the schematic. Since the D jet controller is interpreting sensor output voltages as temperatures or pressures that are scaled from 12v, if the supply voltage to the sensors is tanking, it has the great potential to cause the controller to calculate erroneous fuel PWs. I don't know which way that error is going to go.

As our cars age, you can get some pretty spectacularly bad connections that give some really low voltages. I fabricated an ersatz R sport instrument cluster for my 142 using analog gauges driven by stepper motors and digital electronics. I fabricated it as pretty much a plug and play replacement for the stock dash (albeit different termination plugs) connecting to the existing wiring harness (except for the oil pressure gauge). The dash voltmeter low voltage warning light was always coming on (less than 12 volts) even though my alternator was chugging out 13.8 volts. Between the connections at the 12 v distribution block on the fender, the connections on the ignition switch, lots of wiring and other bad connections, I was losing 2 volts if I had the interior heater fan running!
 

·
Registered
Joined
·
250 Posts
Seems like performance would noticeably suffer if the overall system voltage were low enough to futz with the ecu's ability to activate the injectors. They need only 3 volts to operate.

Weird.

Thanks,

Cameron
PDX
 

·
Registered
Joined
·
2,625 Posts
On the D jet, there is an 8 ohm resistor in series with each injector. The resistor is located inside the ECU case. The Bosch injector is about 2.4 ohms resistance. The series resistor is how they get the injector voltage to approximately 3 volts. (2.4 / (2.4 + 8)) x 12 v = 2.77 volts (3.1 volts if the system is running at 13.4 volts). Drop the supply voltage to 10 volts and the injector voltage drops to 2.3 volts.

Have a look at this data provided by Injector Dynamics for one of their injectors.

http://injectordynamics.com/injectors/id1000/

Notice how the offset time (which is what most people incorrectly call opening time) increases dramatically for operating voltages below 12 volts. The ID injectors are high impedance injectors; but, the same sort of % impact on offset time occurs with the D jet injectors at reduced voltage.

Edit - small correction. The D jet resistors are 6 ohms, not 8 ohms. You can make the corrections to the equations if you wish. Also, the paired D jet injectors are switched by a common power transistor, so you would have to also consider the reduction in applied injector voltage associated with Vce of the power transistor if you are concerned about accuracy.
 

·
Registered
Joined
·
250 Posts
That/those dirty connection(s) on the relay(s) must have equated to a substantial voltage drop. If it really happens as described, I'm surprised that this isn't a more common discussion. Glad it's running better.

Thanks,

Cameron
PDX
 
1 - 8 of 8 Posts
Top