740 Ballast Resistor Bypass or Compatability?

I finally got my 740 Turbo to start on starting fluid, so my no-start issue is for sure related to lack of fuel. After removing more duct tape (the PO really like it...) I discovered that the wires on the ballast resistor aren't connected. After a rudimentary "repair," I hear the fuel pump engaging after cranking, but the wires still aren't able to make full contact.
Now I'm not 100% sure that's the issue, but I'm 98% on it due to the fact that I wasn't able to get a good connection between the wires and the resistor.

I noticed that LH 2.4 cars do not have this resistor, and unfortunately all of the 740s at my local Pull a Part are N/A. I'd like to be able to get it there, but if not I'll just order one on eBay. But before I do that, what other cars have this resistor? They have a few 940s and possibly some 240s, so if those cars have this resistor, that would be great.
Until then, is there any way I can bypass the resistor?

All 740/940 turbos used the ballast resistors. Even the LH2.4 cars. So if you find any of those cars you can use the resistors.

No, you cannot bypass if you keep the low Z injectors. You could bypass the ballast if you change to a high Z injector. I do know people like to use injectors from the 5 cylinder turbos but I don't remember which one though.

Verify what injectors are presently installed. The Green ones need the ballast or you can fry the ECU. If someone installed different ones from 850 or something they may have bypassed the ballast.

Bypass the resistor=fry your ECU injector driver with greentop injectors.

^^^^He beat me to it.

Use a noid light and test light to diagnose the injector circuit.

Good to know! I'll have to pick up a test light or noid light set, because that's one of the few tools I don't have.

I'm going to look into an injector upgrade as well. I have much more experience with the 5 cylinder white blocks than red blocks. I've wanted one of these cars for a long time, and I'm happy to start learning more now that I have one.

Test light the injector positive side wires to ground. Key on. Illuminate? If no, resistor wiring or injector relay wiring suspect.

Test light between the injector power and ground wires. Crank. Blink? Or use a noid light here instead.

The test light is better than a meter because it tests current as well as voltage. A meter can show 12v when there isn't enough current to operate the circuit. Pro tip to use a basic tool.

ZVOLV said:

Test light the injector positive side wires to ground. Key on. Illuminate? If no, resistor wiring or injector relay wiring suspect.

Test light between the injector power and ground wires. Crank. Blink? Or use a noid light here instead.

The test light is better than a meter because it tests current as well as voltage. A meter can show 12v when there isn't enough current to operate the circuit. Pro tip to use a basic tool.

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Awesome, I'll try this out tomorrow. Thanks!

And if you discover you have injector pulse, the next thing to do would be use the test light at the fuel pump positive wire while cranking.

Since it runs on ether, that's great. I work on trucks all the time with dead fuel pumps. First I pull codes ,( no codes) then blast ether down the throttle and see if it starts, yes!?, then spend 5 minutes with a test light at the fuel pump relay, then drop the tank and repeat the test light procedure at the fuel pump wiring, if power=dead fuel pump. Cake. If the injectors are accessible, we plug in a noid light too, on the quick, before dropping the tank just to make sure the injectors are alive and it's not something like a crank sensor.

Picked up a resistor at the junkyard today, still won't start. I'll be testing with a light later today, I have to go look at another car this afternoon..... I might have a Volvo addiction.

You should verify the unknown injector type before trying to connect things.

Put down the parts gun and pick up some tools, a wiring diagram, and your thinking cap.

Was the low-z injector connected to the ECU without the ballast resistor? If so, I'd consider the ECU already fried.

But there had to be a bypass harness connected or all wires just taped together for that to happen. Lets wait and see what the injector findings are.

Allow me to explain further... I replaced the ballast resistor because the old wires were not even connected anymore. The PO had duct taped the wires to their respective terminals, but that didn't work very well. That's the main reason I got a new resistor, not just because I was throwing parts at it. I'd rather diagnose things properly before replacing things. Now...

I probed the injectors with the test light yesterday and they are not getting power with the key in the "on" position. I looked over the wiring diagram and did some more testing. All of the injector grounds are good. Since I could not find a 1988 diagram, I am using a 1989 diagram, and it appears a green wire is missing from connector "D" on the right wheel housing. Either that or the car didn't ever have that terminal, because it really doesn't look like it. Everything else on the green wires from the injectors to the ballast resistor are showing continuity. I repaired the connection on connector "G" on the left wheel housing because the terminal had some of that rubber seal blocking the connection. Tested again, still nothing, and still won't start.
Is the radio interference relay on the left wheel housing something that would cause this? All of the wiring I have tested seems to be good.

I recommend a wiring diagram and chase the injector power circuit.

Yes, a bad injector relay would cause no power to the injectors. But these mid 80s 740s are notorious for bad wiring.

I'm gonna go look at my Haynes manual......ok...so on page 317 the diagram shows the LH relay sends a 12v relay control signal to the injector relay. (Pin 2. Blue /yellow). Test light illumate here with the key on? Check the MAF blue/yellow wire too.

Then pin 3 of the injector relay is the power output of the relay to the ballast resistor. Key on, test light illuminate?

Does the green/red single wire going into the center of the ballast resistor have power?



Report back.

One other thing that could prevent the pumps and injectors firing is the connection at the boost pressure over pressure switch. This switch makes a ground connection between LH 2.2 ECU and the fuel system relay. 1988 and 1989 740 turbo wiring diagrams should be identical. Check here:

http://www.volvowiringdiagrams.com

bmdubya1198 said:

All of the injector grounds are good.

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Since injectors should always be powered (+) through the Radio Suppression Relay and only Pulse Grounded through the ECU when the engine is being started or running what does it mean when you say that Injector grounds are good?

It means he likely thinks the grounds at the fuel rail are for the fuel injectors and they look good. I recently explained this to PromiseRing.

ZVOLV said:

I recommend a wiring diagram and chase the injector power circuit.

Yes, a bad injector relay would cause no power to the injectors. But these mid 80s 740s are notorious for bad wiring.

I'm gonna go look at my Haynes manual......ok...so on page 317 the diagram shows the LH relay sends a 12v relay control signal to the injector relay. (Pin 2. Blue /yellow). Test light illumate here with the key on? Check the MAF blue/yellow wire too.

Then pin 3 of the injector relay is the power output of the relay to the ballast resistor. Key on, test light illuminate?

Does the green/red single wire going into the center of the ballast resistor have power?



Report back.

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Thank you, I will test this tomorrow morning and post an update.

lummert said:

One other thing that could prevent the pumps and injectors firing is the connection at the boost pressure over pressure switch. This switch makes a ground connection between LH 2.2 ECU and the fuel system relay. 1988 and 1989 740 turbo wiring diagrams should be identical. Check here:

http://www.volvowiringdiagrams.com

Click to expand...

I've been using the 1989 diagram from that website. That one wire being absent from my car is strange, but being that there are absolutely no signs of it existing, I'll ignore it for right now.
That would make sense, I'll check that out in the morning.

PCH said:

Since injectors should always be powered (+) through the Radio Suppression Relay and only Pulse Grounded through the ECU when the engine is being started or running what does it mean when you say that Injector grounds are good?

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I tested the ground side of the injector connector to the battery ground cable. My meter zeroed out, indicating a good ground.

The injector drivers in the ecu provide the ground to the injectors. You should not measure a ground from injector negative to ground of the car. You should measure from the injector to the ecu connector to test the negative circuit for the injectors. If the injector drivers are shorted that is a bad ecu.