I am working on my 88 944 turbo & need help w/ injector pulse. I am using a noid light. When I crank the motor I'm not getting any pulse. As soon as I let go of the key the noid light will flash. The car will run on it's own w/ the help of starting fluid. I've changed the control unit, all relays, reference sensors, & the ignition switch. Thanks Andrew

 

wiggle the connectors at the fuel injector plugs. they are prone to corrosion inside the boot. may not be the cause of the problem or the solution but every injector harness i've cut apart had the same problem

 

Check for voltage at the injector while cranking. The DME triggers the injectors with a ground signal, the other wire is the 12v source - it should be hot anytime the key is on. It sounds like the 12v may drop out under cranking.

 

Hey Chris I am helping my friend with this car. Yes it is losing 12v while cranking. We can get the car started but sometimes it runs good and sometimes it seems to be running on one or two cylinders.
Thanks
Mitch

 

Chris it is dropping down to around 9v while cranking. It looks like a ground issue. We checked the grounds to the bellhousing all seem good. I know there are some grounds under the dash near the fuse panel we will check these. Any suggestions?

 

Mitch
Are you checking that voltage with a scope or multimeter ?
Multimeter won't do you much good as it's a pulse.

When checking voltage at the injector plugs with a scope make sure at least one injector is still connected so you can see the transistor switching on/off.

 

OP, check 12V directly to ground while cranking.. NOT to the other side of the injector connector.

Check the ground on the connector using a scope as indicated by Mike above.

The 12VDC should be a constant, as this has nothing to do with the DME. The power to the injectors comes directly from the fuse / relay box.

The ground signal for the injectors is what is controlled by the DME, and this is a pulsed signal.. IIRC all 4 grounds are tied together inside the DME, I have the diagrams at home and can try to scan them and load the pics later this evening.

The problem on this car is likely a harness issue, but cant really rule out the DME without a scope, or by replacing all the wiring... neither is fun.

 

Yes, you're right about the 12v being constant - I was confusing the issue by thinking about checking the ground side of the injector to check if it was being pulsed. On a scope you would see this pulse between 0v & ~10v.

 

Still no luck with this car. Not using a scope but I do know there is no pulse while cranking the motor. If you give the car a shot of starting fluid it will run smooth sometimes and real rough sometimes. The injectors are pulsing while the car is running. Not sure what happens when the key is turned to the crank position losing pulse at that time.

 

We did try a LR injector harness on Sunday still same problem.

 

check the ref sensors. also check with the volt meter the 14 pin connector by the brake booster make sure the the injector pin is getting the proper voltage i ran into similar problem this weekend

 

You said it would run if you cranked it with starter fluid? What about the ignition switch?

 

We tried a ignition switch too. I am not sure what changes from the run position to the start position besides activating the starter. All the parts have been tested on my running racecar.

 

Did the car ever have an aftermarket alarm system on it?

 

No alarm. Bypassed factory alarm too.