Revving Out of Control

[tobyw]

Ok, this officially makes NO sense to me... However, it is 100% consistent and repeatable, so I'll describe it and let everyone have a go at a diagnosis:

'77 EB, 302cid with aftermarket stand-alone TBI system, C4, D20 with twin-sticks...

High Range, no issues to speak of.

Low Range FRONT, no issues to speak of.

Low Range FRONT & REAR, no issues to speak of.

Low Range, REAR, dump it into Reverse, no issues to speak of.

Low Range, REAR, dump it into Drive, and the damn thing immediately jumps to 2500+ RPM!!! At this point, it will no longer shift into any other gear or position (the stalk will move, but the trans will not react), and the only way out is to kill the motor... It does this EVERY time, and only in 2-Low as described.

It's almost like it's a mechanical reaction, but the shifter mechanism is nowhere near the throttle linkage (if you are familair with EB's, you know what I mean). And besides, it only does it in 2-Low???

It's almost like a computer reaction, but there is absolutely no "communication link" between the trans/t-case and the engine management system.

 

[t1h5ta3]

worn motor/tcase mounts? engine torqueing over pulling the throttle?

 

[tobyw]

Nope, rock steady... :booo:

 

[64FJ40]

Shot in the dark, and I probably know the answer, but a C4 doesn't have a vacuum modulator does it?

 

[tobyw]

Yes it does, but I had it disconnected from vacuum and had the manifold port plugged into my vacuum guage instead.

 

[Bill USN-1]

How about the shift indicator sw on the TC is actually hooked to the Pk/N wire going to the ECM.

Who wired it and what wiring diagram was used?

Is this a howell/GM system running a 1227747 ECM?

Is it fuel only or do you have fuel and timing control?

 

[tobyw]

The system is from Turbo City. I did the install, and used the diagrams provided with the kit. No connections to/from the drivetrain to the ECM whatsoever that I recall, but it does have both spark and fuel control...

Edit: here is a link to the Turbo City kit:

http://www.4wdandsportutility.com/tech/03064wd_bronco_tbi_fuel_injection/index.html

 

[Bill USN-1]

yep, GM TBI.

Where are you located?

I still don't think the shift position has anything to do with the rpm....unless there is a wire crossed or you have a broken motor mount.

Connect pin A and B of the aldl connector and turn the key on.
Then listen to the IAC. It should be humming( it doesn't know the words)
Disconnect IAC and turn off the key.
Then remove jumper.

Start motor and repeat your revving test.
Stick a finger over the hole above the IAC and make sure no vacuum is present.

If it fails to rev up in drive then reconnect IAC and start motor back up and try it again.

Here's a pick of the connector.

And this MAP sensor is mounted wrong.

It should be above the vacuum port and the nipple pointed down.

 

[tobyw]

Already tried forcing the IAC closed and then unplugging it to take it out of the equation - no change...

 

[Bill USN-1]

But since you say you can definately repeat the problem, I would like you to verify it and also check for the vacuum in the port above it.

The next thing to do would be to disconnect the timing bypass wire and repeat the test.

The ECM controls idle speed by the IAC and timing.
You need to be able to isolate which is causing the problem.
So repeating test can help narrow it down.
You really can't say it's still the same as last time you did it.

Where's the vehicle setting?

 

[crash2]

Already tried forcing the IAC closed and then unplugging it to take it out of the equation - no change...

I have to agree with Bill USN-1 and even more so with the IAC being closed like you did.. Whats odd is you usually get a chuggle/loading up at idle with the tbi's when you increase the fuel but not the air flow which sounds like the situation

 

[tobyw]

Agreed... Unfortunately, it's about an hour away from me. It's my dad's rig, he's in a wheelchair, and I'm the wrench on his behalf. We have been dealing with this for several seasons, and have pretty much everything ironed out with the exception of the high idle issues. You can imagine how this treats a guy when attempting a techincal obstacle, only to be greeted with a 3000RPM jolt!!! The issue used to be sporatic, but has gotten worse recently and seems to be consistent as mentioned above now.