Re: Trunk Fuel Resistor
 

Merry Christmas Tony,

Please refer to Post 27 —

Attention "theflystyle",

Unfortunately we have become side tracked and must get back on the main line and wait until the resistor in your car is checked for for an internal short to ground. Only at that point can other possibilities be considered.

By the way a Christian name would assist towards personalising this discussion. As you can see we are all friends here. LOL

Please — The analytical procedure must be carried out in a logical sequence otherwise we will completely lose the plot. If and only if the problem has been incorrectly described should an alternative scenario be considered.

At this point a short to ground within the resistor or its connections must first be ruled out.

This current situation again draws attention to the absolute need for a the logical reporting of problems, as I have mentioned in separate post.

Re: Trunk Fuel Resistor
 

Thanks that is a very good reason for the “unlikely” fault.:) The positive leak back from the module would introduce a voltage change, in the series voltage divider, between the pump and 1 ohm resistor. Wouldn’t have to raise the voltage too much to turn it off.
Good one Tony.

Re: Trunk Fuel Resistor
 

Also, I need to ask exactly what gets unplugged and what gets plugged when the engine is able to start. Does the engine start when the control module is unplugged and the resistor is plugged? Or, does the engine start when the resistor is unplugged and the control module is plugged? Or, does the engine start in either case? If the engine starts with either one unplugged is it true that when both are plugged then it does not start?

I am going to assume, for the time being, that the engine starts when the control module is unplugged and the resistor is plugged. I am assuming that there was a mistake at some point when it was said that the engine will start when the control module is plugged and the resistor is unplugged. Given that assumption, I would say that the resistor is probably fine and that the measurement of one ohm is probably correct. In fact, one ohm makes a lot of sense. That resistor forms a voltage divider with the pump when the pump is running. Electric motors tend to have a low resistance so the series resistor would have to be low as well. Therefore, I would conclude that the problem is the control module and not the resistor. Most likely, the final transistor in the circuit is blown (base and collector are shorted) and the control module is supplying 12 volts-ish, through the internal resistor, to the pump when it is plugged in. If that is true, the external resistor is probably getting very hot and the internal resistor in the control module is probably getting very hot.

Fly, can you either measure the voltage across the external resistor or, with a clamp-style current meter, measure the current through it? I would be curious to know how the voltage and/or current changes when the control module is plugged versus unplugged.

1. When transistors “blow” they normally go open circuit and in any event if the power transistor did short base to collector, it would have to simultaneously remain open emitter to collector, and this is in all very unlikely.

2. Furthermore should this unlikely event occur, a positive current would have to be simultaneously supplied by the small driver transistor which can pass only a negative current unless it also “blows” such that it opens to ground and also shorts to positive and in this doubly unlikely event the high current would pass, via a bias resistor capable of very low current, and it would certainly burn out.

3. There is no way the suggested substantial supply could be delivered to the negative pump input so as to form a voltage divider between it and the resistor in the event of the several unlikely possibilities taking place, without a high current becoming available. This would have to be delivered via the TCU which is able to supply only a small control signal, hence the need for a driver transistor. What is more the signal is likely to be delivered at only 5 volts and internal protection would interrupt an excessive load, otherwise the internal TCU switching transistor would be in danger in the event of a fault codition.

4. I agree that if, and only if, the problem has not been correctly described, several other scenarios are possible, including an obscure fault affecting connections or wiring. However a logical sequence of fault location must prevail and unfortunately here the mugwump is now in full flight. LOL

P.S. The word "module" as become introduced, whereas all Subaru technical literature specifically describes the control unit as a "modulator" for good reason. The pump speed is variably controlled in order to adjust fuel pressure in accordance with engine requirements using pulls width modulation. The system does not operate on the basis of simple on/off switching providing only two speeds and therefore only two fixed pressures.

Re: Trunk Fuel Resistor
 

I agree that it is a 1 Ohm resistor.
The early pumps used around 1 Ohm, the later (more efficient) pumps are around the 0.28 Ohm that the Mitsubishi has, the value isn’t really critical, the value controls the pumps flow rate, when the engines demands for fuel is low, like idle, cruise, etc. If the resistance is too big it may cause problems of lean fuel, missing, if too small, the pump would run at high flow, heating the fuel.
When the engine needs full flow, 12V+ it turns the modulator on to supply the pumps ground.

If the pump was to operate at full flow all the time, at times of low engine fuel needs there would only be a small flow through the injector, fuel pressure regulator circuit, back to the tank. The excess would be blown off through the pump’s relief valve to be circulated back through the pump, heating the fuel at the pump. To prevent this happening the ground in the modulator is turned off to leave only the 1 Ohm resistor ground that drops the pump’s voltage to about 6V to reduce the amount of fuel blown out the relief valve reducing the fuel heating.
All the fuel pressure is set by the fuel pressure regulator at 36.3 psi against the manifold pressure, and no other regulation is done.

Re: Trunk Fuel Resistor
 

Trevor,

Are you inferring the STG is within the module/resistor or somewhere else?

I procured a used resistor and module replacement from an SVX that I was looking to test and swap over to my car.

I should also mention no fuel modifications have been made on my vehicle but I have had the grounding upgrade as suggested by svxcess.

Re: Trunk Fuel Resistor
 

Attached is a picture of the bracket containing both the module (left) and the resistor (right).

Re: Trunk Fuel Resistor
 

The resistor is definitely unplugged as I have it on my bench and the car did start in this manner. The module is still in the car but I am going to need to make sure if the module is completely disconnected or not. Based on the wiring diagram (post #7) there are four pins to the module and two of those are for the resistor. With the resistor unplugged I am not sure of the module is still connected to the car. I am going to take a look tomorrow when I get a chance report back.

I do not have any clamp-on/fork style meters unfortunately.

Re: Trunk Fuel Resistor
 

Ok, just to clarify, if you plug the resistor in now, the car won't start. Is that right?

And if you plug in the resistor and unplug the control module, will it start again?

And it better not start with both unplugged or you have serious wiring issues!

There are four permutations:

1. Both are plugged in
2. Resistor unplugged, modulator plugged in
3. Modulator unplugged, resistor plugged in
4. Both unplugged

You have given us #2 by saying it starts in that configuration. We just need to know the other three.

Re: Trunk Fuel Resistor
 

1. Started (This makes me believe it to be an intermittent problem)
2. Started
3. Started
4. No Start

Looking back I tried #1 with both units dangling in the trunk and not attached to the support bracket. I will retry with them reattached just as a sanity check.

I also want to state that I am not an Electrical Engineer and my grasp of electrical schematics and troubleshooting are limited. I apologize for this handicap but will try my best at describing and implementing any suggestions.

While troubleshooting today I removed the module from the trunk and removed its casing. It was easy to disassemble so I took the opportunity to check what I could and take pictures.

The board on the module looks OK to my untrained eye. I checked continuity between all pins/solder points and everything beeped with my multi-meter. According to the schematic pins 1 and 2 connect to the resistor so I checked D1 and R3 on the module as they were the only parts connected to those pins. With my multimeter scale at 2000, R3 read 555 and D1 readh 450.

Re: Trunk Fuel Resistor
 

Yep, you have an intermittent problem. It could be the relay, but unlikely. They usually just fail. Could be the pump, but again, unlikely. Could be either of their connectors. I am voting for connector R19. Monkeying with either the modulator or the resistor could impact that connector.

The problem could also be a weak or intermittent connection to ground. In order for the car to start with just the resistor plugged in, either one end of the resistor has to be grounded internal to it's case and use case to make the ground connection or one of the two wires on the resistor connector's mate has to go to ground. I would check both this assembly's connection to ground and/or the resistor's mating connector's connection to ground. Same for pin 2 of the modulator's connector and ground.

Re: Trunk Fuel Resistor
 

Subaru in all of their technical literature refer to the control module as a modulator for a very good reason. The pump is in fact pulse width modulated as I stated way back in post No 8 detailing how the pump control system works.

It is now being asserted that I was and am wrong. Five minutes with Google has quickly turned up confirmation that at 89.75 yrs I am far from senile. The system presented does not include a resistor but the identical principle involved is clearly stated and is exactly similar.

Go here and Scroll down to the heading “Fuel Pump Driver Module” and read about pulse width modulation:-

https://www.autoserviceprofessional....sis-and-repair

I repeat what I have previously posted in an effort to get the thread back on track.

Thread 8 :-
The pump receives its negative supply via two alternatives, i.e. the resistor R34 or the fuel pump modulator switching transistors which are controlled via the ECU.

The resistor will provide a reduced voltage and therefore limited pump speed in the event of the pump modulator or its control failing. The resistor will also prevent full zero voltage in respect of pulse width modulation speed control of the pump. (An innovation also used by Subaru in respect of the control of transmission pressure.)

Under normal circumstances pump speed and therefore pressure, will be controlled via the modulator and ultimately by the ECU by means of pulse width modulation.

Thread 19:-
(1) “When I got the engine running normal, I would unplug the fuel control module from under the deck lid, yet the car would continue to run normally.”
Therefore the resistor is completing the pump ground circuit.

(2) “Yet when I unplug it (The resistor), the car starts. When its plugged in it does not.”

Therefore when the only the modulator is in circuit and the pump is running at a controlled speed and pressure the car starts, but when the resistor is connected in parallel with the modulator it does not start, possibly due to high pump speed and abnormal fuel pressure.

The final power transistor in the modulator is NPN and is switching on when the driver PNP is switched on via the ECU at c14. The ECU ground switching circuit must be OK as the pump relay is operating OK.

Conclusion:-

A short to ground within the resistor housing would result in the pump always running at full speed and this would account for the strange situation as per (1), the failure to start as per (2) and also other erratic engine behaviour. Furthermore the pump must draw a lot more current than one would expect.

Alternatively, both resistors measured are faulty and have reduced in value, which would be extremely unusual and unlikely but not impossible. Unfortunately the correct specifications can not be located. Check the resistor for an internal short to ground.


Post 10.0 included:- I would expect the resistor value to be in two figures but it can it not be estimated without full details covering the pump motor and its requirements.


"The photo now posted appears to show."

The resistor is marked A1,5 000R20 which means that its correct resistance is 20. ohms. The six digits allow for figures in K ohms.

QED.

Re: Trunk Fuel Resistor
 

Maybe the ground point for that circuitry, that is bolted to the top left corner of the deck.

Re: Trunk Fuel Resistor
 

Post 41 sets out the answer to the problem, contrary to posts 32, 34 & now 42.

Re: Trunk Fuel Resistor
 

Trevor,

I hope there isn't a misunderstanding. I never intended to say you were wrong or that I was looking to say that you were. I was simply replying to the questions that were asked of me.

There is another tidbit of information that I wanted to share through my troubleshooting today. As my car is a garage queen and sits during unfavorable weather, it is connected to a battery maintainer. This trickle charger has an LED indicator stating if its charging (Red) or full (Green). While the car has been inoperable I have noticed the charger flashing red more then it does typically. I didn't put it together until tonight when I had the car sitting with the module (and resistor for what it's worth at this point) disconnected from the car. Now the battery charger has been sitting at fully charged for hours with no drop back to charging. Does this align with the module being the issue? I presume it could also be the wiring to the module but I have not yet followed that path. The wiring harness combines with the security system hardware then quickly goes deep behind the trunk liner and then seats.

Re: Trunk Fuel Resistor
 

On a side note, I've been having trouble with this same part. If its plugged in, car wont start. If I unplug it, car will start. I pulled another one off a junk yard svx and still in the same spot.