Jfrahm

It sounded like earlier the new injectors were fired individually and all worked OK. If one has failed now it might be due to powering it for too long, making it a little toaster. For testing they need to be clicked quickly.

I've also heard that a lot of replacement injectors are of poor quality or at least not matched to the application, although some vendors probably do a good job. If they ohm OK they should at least me safe to run.
But at this point maybe due to the earlier injector mismatch the output transistors of the DME might be degraded and, as you suspected, unable to fire the injectors consistently.

T&T Racing

You have isolated the problem. Where are you measuring the +12v? There is a 9 wire connector located above the brake vacuum booster at the firewall. The red/blue is connected to the plug. Separate the plug and clean the contacts, both male and female connectors for the red/blue wire. There should be corrosion in the plug connectors for that wire causing high resistance that drops the voltage to the FI coils so they cannot hold the latch open position.
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orig944

In an early car, pin 4 (87b) of the DME relay powers the fuel pump, and pin 9 (87) powers both the DME via the red/yellow wire, and the injectors via the red/blue wire.

Jman2678

Got it, will do today. Thank you.

EDIT: Sorry, I realized I didn't answer your question. I've measured the 12v off of terminal 87 from the DME/Fuel pump relay as well as on the injector harnesses themselves. I've been able to pull the boots back from the connector and get leads in while the harness is still connected to the injectors.

Jman2678

That's my fear exactly. The only part of it that I still don't understand is that if I switch back to the old injectors, it fires right up. I would imagine if the DME was on the fritz due to my running with the incorrect injectors before, it wouldn't be able to drive those ones? But again I don't really have the level of understanding of this system that I would need to in order to be able to actually know that. I'm really only working with slightly educated guesses.

EDIT: just wanted to add when I manually fired each injector, I fired them as fast as I possibly could by hand.

931guru

Yes, of course, it's right there on the wiring diagram... Learning something new everyday!

orig944

So, without adding any additional information, I cannot proceed with helping you. You have made two conflicting statements, one says the voltage drops to nearly zero when cranking, the other says it doesn't. Let me help you clarify this for me.

I measured the voltage at the (color) wire at the (DME, DME Relay, Injector, Other, please specify where) and the voltage does not drop when cranking.

I measured the voltage at the (color) wire at the (DME, DME Relay, Injector, Other, please specify where) and the voltage drops to nearly zero when cranking.

This will let me know exactly what circuit you have measured, and where in the circuit.

walfreyydo

Coming into this late and have not read all the posts, but are you sure you have the right injectors? 944 came with high and low impedance injectors depending on year...The DME cannot power the wrong impedance injectors.

Jman2678

From the outside the connector looked to be in great shape so I had ignored it until this point, opened it up and it was a mess in there. Got it all cleaned up, took resistance readings on each side of the connector with 0.01ohm resistance. Still didn't solve the overall issue, but great suggestion anyways because I never would have checked there. Definitely could have been a big problem to let sit any longer.

TLDR: Old injectors WERE the wrong ones, 14.5 ohms or so, new injectors are correct 2.5 ohms or so and are not working. Switching to old ones the car will fire right up. Verified I have the early DME so the low impedance injectors should be working, but they won't.

walfreyydo

How is that? Old injectors are wrong impedance but they work... something isnt right here. Are you 100% sure you have the verified you have the correct injectors? There is no way the old injectors were high impedance (14.5 ohms) and would have worked previously. Based on my understanding the drivers in the DME physically cant power high impedance injectors.

I suspect your old injectors were actually low impedance and your new ones are actually high impedance (and your understanding of which is which may be reversed). That would definitely make sense based what you just described above.

See below for part numbers:
https://www.clarks-garage.com/shop-manual/fuel-12.htm

Jman2678

I think you might be confusing voltage dropping to zero when cranking vs dropping to zero if it begins to cough/tries to start. I more than likely messed that up in another post, sounds up my alley lol. Voltage stays when cranking, when it coughs or tries to start it drops to zero. I can't say whether it's a cause or an effect however.


Here's the resistance of the new injectors.
New injectors

Old injectors
And here's the old injectors that the car will still run with. If I could explain it I probably wouldn't have posted in the forum lol.

Jman2678

And just to repost my DME before anyone has to ask, it was a pretty early post on this thread.

walfreyydo

Did you cross check the part numbers on the injectors with those in clarks link I posted? Are they genuine Bosch?

I still cant understand how the car could run perfectly on high impedance injectors. The DME shouldnt be able to power them at all. Can you post the bosch part numbers for both the highs and lows?

orig944

Your injector resistance is correct for early car.

Please fill in the blanks.

I measured the voltage at the (color) wire at the (DME, DME Relay, Injector, Other, please specify where) and the voltage drops to nearly zero when coughing/trying to start.

screamin94Z

This is not true. The DME is a voltage source, it supplies 12V regardless of the fuel injector within the ranges we're talking about.

A "high" impedance injector (12-14ohm) will simply require less current to fire than a low impedance injector (2-3ohm) per Ohm's Law. The OP is seeing an issue when going from a lower current draw to a higher current draw and reporting that the DME doesn't supply anything, implying that it has a current limit built in to protect itself from burning up. That is confirmed by unplugging one injector and getting 3 to fire normally since the circuit resistance will increase proportionally. All of that makes sense IF the DME wasn't rated for low impedance injectors in the first place. However, the injector drivers should have no problem with low impedance injectors since that's what they were originally equipped with.

Here's what I don't know - some vehicles use an additional resistor(s) to limit the current when using low impedance injectors. Is the early 944 one of them?