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740 Ballast Resistor Bypass or Compatability?

Step one is get power to the injectors and then work on the control side. I find it difficult to believe both sides of the circuit are bad. Ground side should be over 1000 ohms. I don't trust your readings. If you unplug the ecu, that same test would go to OL if you did things properly.

Do the tests I developed in that previous post while looking at a Haynes manual on the John Crapper this morning before work. I was late to the auto shop.

A wiring diagram is more valuable than 1000 posts. Study up. I have nice clean ones available online in interactive format at the shop. Old Volvo diagrams are tougher to read and tougher to find, but you gotta learn to use one and do it well. Parts swapping is no bueno. Even with duct tape, that part may have still been good.
 
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dl242gt said:
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.
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Good to know, I'll test that soon.
ZVOLV said:
Step one is get power to the injectors and then work on the control side. I find it difficult to believe both sides of the circuit are bad. Ground side should be over 1000 ohms. I don't trust your readings. If you unplug the ecu, that same test would go to OL if you did things properly.

Do the tests I developed in that previous post while looking at a Haynes manual on the John Crapper this morning before work. I was late to the auto shop.

A wiring diagram is more valuable than 1000 posts. Study up. I have nice clean ones available online in interactive format at the shop. Old Volvo diagrams are tougher to read and tougher to find, but you gotta learn to use one and do it well. Parts swapping is no bueno. Even with duct tape, that part may have still been good.
Click to expand...

The wires were not making any contact, and I wasn't able to get them attached in any way. I knew the resistor needed to be replaced anyway for that reason, so I replaced it.
I'll do these tests in a little bit and post back.
 
Did some testing... everything was good, except I am NOT getting power at the blue/yellow wire on the radio suppression relay, therefore I am not getting power at the ballast resistor. This testing was performed with the key in the "on" position.
I tested the relay, and it is working. It seems there is a short somewhere in that wire.
I will check the other points noted in the wiring diagram where the blue/yellow wire goes.
 
Ok nice. I'm sure you are getting better at electrical diag already. I learned this stuff in my back yard while drinking beer and now I do it professionally.

Have you checked for power to the MAF blue/yellow wire as i mentioned?

The LH relay, aka fuel pump relay, powers up the MAF and injector relay. (Radio blah blah blah relay). Reference wiring diagram. I used Haynes manual to learn the circuits on my old 740s.
 
Okay, I don't know what I was thinking when I typed that post... I got that all mixed up. Pardon my brain fart.
I AM getting power at the MAF. The green/red wire coming OUT of the LH relay is what was not getting power. I am getting power at the relay.
After some more tinkering, I realized the issue is a bad ground wire on that relay. I used my meter to test the ground wire to the right fender ground (proper ground point for this relay) and it was jumping all over. Came to 585 ohms at the battery negative.
After connecting an alligator clip between the left fender ground and the wire, I got power at the green/red wire on the relay as well as at the ballast resistor. I am about to test the injector power right now, then repair the ground wire. Or if I could re-route it to this fender, that might be easier... but I don't want to take a shortcut that would cause problems.

I am getting better every day with electrical diagnosis. Still not my favorite thing, but I enjoy it more as I understand more.
 
Using the Mitchell wiring diagrams is easier. Using the Mitchell wiring diagrams for 1989 740 shows four different fuel system types. Make sure to use the diagram for LH-Jetronic (LH-2.2),Turbo. Non turbo would be LH-2.4 in 1989.
 
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Nice! Sounds like you are getting closer.

On a relay ground circuit i wouldn't want to see more than 10 ohms resistance.

We are still confused about your claim about "0 ohms to ground on injector ground wires claim". If that's true, you could instantly fry your new resistor, ecu, and/or start a fire possibly. Maybe that's why your resistor wiring failed....

I would recheck your work and thoroughly inspect the wiring bundle going up thru the intake manifold. You may have to cut the insulation back to see bare wires. Repeat your test, and like I said, if you really do have a measured short to ground unplug the ECU and see if that short goes away.

And I wouldn't leave the injectors plugged in either.

Use your test light with a FUSED (3 amp) jumper wire and connect the light between injector positive and negative, key on, is that bulb on!?? I would really hope not.
 
ZVOLV said:
Nice! Sounds like you are getting closer.

On a relay ground circuit i wouldn't want to see more than 10 ohms resistance.

We are still confused about your claim about "0 ohms to ground on injector ground wires claim". If that's true, you could instantly fry your new resistor, ecu, and/or start a fire possibly. Maybe that's why your resistor wiring failed....

I would recheck your work and thoroughly inspect the wiring bundle going up thru the intake manifold. You may have to cut the insulation back to see bare wires. Repeat your test, and like I said, if you really do have a measured short to ground unplug the ECU and see if that short goes away.

And I wouldn't leave the injectors plugged in either.

Use your test light with a FUSED (3 amp) jumper wire and connect the light between injector positive and negative, key on, is that bulb on!?? I would really hope not.
Click to expand...

I will definitely check the injector grounds again tomorrow. I'm hoping I was wrong, and it's very possible. My mind is all over the place with this car... and on top of that I just picked up a 2000 V70R that needs transmission work. But first thing's first, my 740 needs to run!

I tried starting once I had everything buttoned up (and yes, I have power at the injectors now). Still won't start up.
I realized that I wasn't hearing the fuel pump engage after I let off the key. For a short time, I remember the pump engaged after I cranked it, but that was it. Now it's back to how it was and I don't hear the pump running. Tried my jumper to run the pump and it still wouldn't start.
I didn't check for power to the injectors after that, but I know there is an issue with the connector under the left/driver's side hood hinge that prevents the starter from engaging depending on the angle of the hood (hinge pushes on the wires). I need to unplug it and make sure all the wires there are making good contact.
 
Fuel pump relay jumper wire needs one wire going in and two wires going out. Should be battery is 30 and 2 87's feeding fuel pumps and radio interference relay.
 
lummert said:
Fuel pump relay jumper wire needs one wire going in and two wires going out. Should be battery is 30 and 2 87's feeding fuel pumps and radio interference relay.
Click to expand...

Good to know... that's probably exactly why it wasn't running. With the single wire jumper I was using, the injectors wouldn't be getting power.
Still weird that the relay wasn't engaging. Do these fuel relays go in and out before totally failing?
 
Take the cover off the fuel pump relay, and manually close the tab to get the pump running. On my build thread mine pooped out while driving and I closed it to keep it running with a pair of sunglasses :)

And yes, they can be interminnent and make you wanna light the car on fire...
 
"No starts" are easy to diagnose. I diagnosed two no starts yesterday in 20 min each. This thread is taking too long!

It runs on ether?! Why isn't your fuel system working?

Got injector pulse yet?

Got current to fuel pump?
 
No injector pulse. Been working on it all day, and I know I'm getting close. I properly repaired the ground for the radio suppression relay.
The fuel relay clicks when I turn the key to on, but I don't hear the pump. One thing I noticed is that there is no connection showing on my meter between the relay connector 86/1 and ECU pin 21 on the connector. It passes through Connector "A" on the right a-post, but I can't find that connector anywhere. Does anybody have any guidance to offer on that?
It appears this wire is a ground... pretty important. I'm willing to be this is a problem, if not THE problem.

Then there's this under the dash... I have no idea what this is. Lots of red wires. It MAY be factory, but the PO messed with a lot of wiring and screwed some stuff up, which is in part what has taken me so long to do this. I really hate when people mess with cars who clearly have no idea what they're doing.

https://photos.app.goo.gl/7EvHoFSKvkwJrNOl1

And the injector grounds... 420-425 ohms when measured to battery ground. Should be good.
 
With LH 2.2 the fuel pump relay doesn't prime the pumps when turning the key on. It primes the pumps while cranking the engine over and for a couple seconds after letting up on the starter.
 
Okay, I thought so. In my testing before, there was only voltage at the pump while cranking.

Anybody know where the connector is for this relay ground wire to the ECU? I can't find it for the life of me.
 
bmdubya1198 said:
Okay, I thought so. In my testing before, there was only voltage at the pump while cranking.

Anybody know where the connector is for this relay ground wire to the ECU? I can't find it for the life of me.
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Thinking there's a boost pressure over-pressure switch under the dash of an 88 740 Turbo. 88 765 has this switch mounted next to the coolant reservoir. Boost pressure over-pressure switch is between the LH 2.2 ECU and the fuel pump relay. Track down the hose that connects to this switch.
 
Tested both grounds on the fuel pump relay connector. 86/1 shows 79-80 ohms against ground (ECU casing). 86/2 shows 0 ohms, perfect. Both, however, show an open circuit when tested against their respective ECU connector terminals. Should these terminals be grounding to anything beside the ECU terminals?

lummert said:
Thinking there's a boost pressure over-pressure switch under the dash of an 88 740 Turbo. 88 765 has this switch mounted next to the coolant reservoir. Boost pressure over-pressure switch is between the LH 2.2 ECU and the fuel pump relay. Track down the hose that connects to this switch.
Click to expand...

Yes, I saw the switch under the driver's side dash, next to the EZK. I'll take a look tomorrow.
 
Just read through this thread, and the last post caught my attention... http://forums.turbobricks.com/showthread.php?t=32343
The other day while testing, I heard the fuel pump run for a moment after cranking, indicating that it was working properly. I don't remember what I did, if anything, to change that, but it doesn't do that now. As I said before, the fuel pump isn't running at all except if I jump that terminal. The relay isn't the issue, already tested a brand new one.
I really think these grounds are the problem. It's now just a question of where they are so I can fix them.
 
I don't follow your work, but diagnosing a relay circuit should be easy. You could also just bypass it.
 
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