• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

740 Ballast Resistor Bypass or Compatability?

I know it SHOULD be easy, but this has been quite a pain just to get to this point. The previous owner thought he was a mechanic and played with some wiring, so this has been a bit of a task just to get to where I am.

I am now able to get a good connection to the ECU from all of the fuel pump relay terminals. Those are not an issue.
The injector grounds are making a good connection to the ECU.
I have power at the fuel pump relay.
I have power at the ballast resistor as well as the injectors.

But I do NOT have pulse at the injectors or power at the fuel pump. That's where I am now.
What am I missing?
 
Is the fuel pump relay getting the ground signal from the LH2.2 ECU? The fuel pumps and the radio suppression relay won't work without it.
 
Yes, the two grounds are showing continuity with the corresdponding ECU pins.

Ok, great.

Does the ECU actually ground the relay while cranking?

Or apply a redundant ground for the relay control and does the relay close?

Pop it open and pinch it closed works too. I do it all the time to trigger fuel pumps and starters at the shop.
 
I’ll try manual closing it today. 86/1 shows 665ish ohms to ground with the key on. 86/2 shows open circuit. I take it the ECU isn’t grounding them correctly.
When I turn the key, the further back relay closes, but the front one does not. Not with the key on nor while cranking.
 
I don't understand, or trust, your readings at this point. No offense. It's getting too complicated. Diagnosing a relay circuit just requires a test light for the power wires, and yeah measure to ground on the ground circuits, but if you do it wrong such as a poor choice of ground, the readings will be bunk.

Try the pinch method.
 
I don't understand, or trust, your readings at this point. No offense. It's getting too complicated. Diagnosing a relay circuit just requires a test light for the power wires, and yeah measure to ground on the ground circuits, but if you do it wrong such as a poor choice of ground, the readings will be bunk.

Try the pinch method.

Complicated... tell me about it. I tested the ground point (left fender ground) to multiple ground straps on the engine prior to using it, it's a good ground.
I'm not an expert with automotive electronics, but I'm learning. I much prefer engine stuff, that's what I enjoy. Bear with me.
I will try the pinch method in a little bit. It's a rainy disgusting day, so that puts a bit of a damper on things.
 
Just take a pocket screwdriver and pop the cover off the relay and then turn the key on and observe if the contact closes. Then use your fingers and close it manually and see if the circuit operates.
 
I reflowed the solder on 3 different fuel pump relays while having issues with my 88 765 turbo. All 3 of the fuel pump relays still buzzed like a rattlesnake. I found a corroded harness connector to the boost pressure over-pressure switch. I cut the connector off and connected the 2 wires together. No more fuel pump relay issues.
 
Closed it manually, I heard the fuel pump, so that's working. Still won't start.
I am getting connection from the fuel relay to the ECU through the boost over pressure sensor, but could that being bad cause a no-start like this?
I tested the ECU ground pins, and they all show to be good.

The only things I noticed that seemed odd are that pin 12 on the ECU connector (which runs to EZK pin 15) was not showing voltage with WOT, key on. It wasn't showing voltage in any position. According to the page below, I am supposed to be seeing 5 volts.
And apparently I am supposed to be seeing 11-14 volts with ignition on at pin 17, and I am seeing nothing. Even with the relay manually closed and the key on. Interestingly enough pin 17 gets signal from the boost over pressure switch. Continuity exists between this pin and the corresponding fuel pump relay pin, but as I said there is no voltage showing.
 
Pinch the relay shut on the other side, the LH side of the relay. Half powers the fuel pump, the other half of the relay powers the LH system.

Do this and do you have power to your injector relay yet?

If so do you have injector pulse? Sounds like your ECU is dead or not powered up.


Have you checked for power to the ECU?

Use a test light, not a meter, to check for power to a component. 12v can be seen on a meter, but there may not be enough current.
 
Last edited:
The right side of the relay (looking at it from the passenger side) is engaging properly. I have to manually hold the left side.
I have power at the injector/radio suppression relay as well as at the injectors, however I do not have injector pulse. I will check once more for pulse while holding the relay, it's hard to do that without any help. I take it they weren't getting pulse since the car didn't start.

I will check again, but I am pretty sure I have power to the ECU.
 
Another weak point on that car is the hall sensor wiring on the dist. I have seen cars run on brake cleaner with the ignition timing way off due to this. However, I don't see how you would have a spark, but no injector pulse if that were the case.
 
Last edited:
I am getting power to the ECU through pin 18 with the key on. This says it's the signal from the ignition switch.
The only thing I noticed while testing is that I don't have any power on pin 17. According to the pinout I posted I should be getting 11-14 volts on that pin.
Pin 17 is the connection to the fuel relay 86/2 and the boost over pressure sensor. I tried putting a jumper between the terminals of the boost over pressure sensor to bypass it and there was no change.
I thought maybe the 12v comes from the ECU to the relay, so I hooked the ECU back up and tested 86/2 for voltage with the key on. Nothing. Am I supposed to be seeing power here or does that part not apply to the 740?

I am really leaning towards the ECU being a problem at this point. I'm totally stumped.
 
Let me look at a wiring diagram. You say you have now managed to get 12v to the injectors and the main pump, but why not now?

What are you doing to test injector pulse? I recommend a noid light.

A jumper wire can be used in place of any relay. If I had your car I could easily get power to the pumps and injectors, crank and look for injector pulse and spark.

You say it runs on ether. Does it actually stay running if you keep pulsing it into the intake?
 
Okay... pardon my absolute stupidity. I was measuring the wrong terminals on the ECU connector. For some reason I keep mixing up the pinout diagrams.
Pin 17 shows 0.07V when I don't have the relay pinched. When I pinch the pump side I get 0.2V. When I pinch the injector side I get over 11V. That's fine now.
Pin 18 still gets 12V.

The two problems I'm noticing are that the fuel pump relay isn't engaging the pump when it should and the injectors are not getting pulse. I get voltage at the pump when I pinch that relay.
At one point the pump was running when it was supposed to (while cranking and for a moment after letting off), but for some reason it's not anymore. I don't know what changed before or after that. It was only for a few minutes while testing.
I was testing pulse by attaching a wire to the ground clip of my test light and probing the other side. I don't have a noid light set, so that is going to have to do.

I didn't have anyone to help me at the time, so I can't confirm if it would continue to run when continuously spraying ether. I only got it to run for a couple seconds after spraying a generous amount into the throttle before cranking it up.
 
Back
Top