C4 shifts the same with or without vacuum hooked up to modulator. and other transmission mysteries.

[Z Bronco]

My Bronco will shift the same with or without the vacuum hose hooked up to the vacuum modulator. No idle issues with the hose disconnected. It also is shifting early now from 1st to 2nd gear, and the firm shifts aren't happening, it feels stock now (shift kit in transmission). All this happened after a shop worked on my Bronco, I will not be going back to them. The Bronco is drivable, just different. Any ideas would be greatly appreciated. Thanks in advance.

My Bronco's C4 mods, all worked great before:
-A1 transmission lower gear set. 2.92 first gear (18% reduction) and a 15% reduction in 2nd gear.
-Lower than stock stall speed torque converter
-Shift kit
-BCB cable kick down (EFI conversion).

 

[Speedrdr]

You said it shifts the same with OR without the vacuum hose hooked up. Does it shift hard? Does it wind up the RPMs before it shifts? If you haven’t checked the modulator, now is a good time to do that. That’s the best I’ve got, based on a car I had (ford) with a 302 and a C4. Hope this helps some.

Randy

 

[ol gray mare]

If as you said there are no idle issues with the vacuum hose disconnected, check the hose to see if it has any vacuum. It could be plugged or kinked.

 

[Slowleak]

Like they said, check for vacuum and check the module. Sometimes the pin gets lost when people replace the module which means the module does nothing.
Years ago I had a bad vacuum advance can on my distributor. The bronco idled fine since it is on ported vacuum but with the throttle open, it sucked air thru the bad vacuum advance can. Then it didn’t get enough vacuum at the transmission module. It threw the shift points off and it shifted hard, and I mean tire chirping shift kit hard, into second…

 

[Z Bronco]

In answering the above questions.
-It shifts soft.
-It does not wind up the rpms.
-There are no kinks in any of the hoses, I will check to see if the line is plugged. My Bronco is back in storage so it will be a few days.
-A recent transmission shop said the pin was missing in the modulator, which they said they replaced. Unfortunately, I do have my doubts that the work was done. When I can, I will pull the modulator and see if the pin is actually there.
-I have an electronic distributor, so no vacuum advance.

Thanks everyone!

 

[Rustytruck]

watch out for the red arm pit. as far as the vacuum modulator with it not working properly you should have a very late shift as it would be a governor controlled shift. very high hard shift. while your in there make sure the shift valve and spring are under the filter gasket this left out or put in wrong can make for some really weird shift points. these 2 parts get dropped out when replacing the filter screen and for first timers a common failure.

 

[TX-ZACH]

I wanted to chime in as I went through something like this not long ago after many gremlins coming back from paint and the body coming off. Tracing the modulator line back to vacuum source I found the line plugging into modulator was flex and tapped into hard line to firewall then back to flex. I was disconnected in multiple areas. I adjusted the bands and ran all new vac hose, tie strap connections and all good now.

Good luck with finding the issue.

 

[jmhend]

In answering the above questions.
-It shifts soft.
-It does not wind up the rpms.
-There are no kinks in any of the hoses, I will check to see if the line is plugged. My Bronco is back in storage so it will be a few days.
-A recent transmission shop said the pin was missing in the modulator, which they said they replaced. Unfortunately, I do have my doubts that the work was done. When I can, I will pull the modulator and see if the pin is actually there.
-I have an electronic distributor, so no vacuum advance.

Thanks everyone!

I am in the same boat with the same scenario (other than the trans shop rebuild).. Keeping an eye on this one.

 

[Broncobowsher]

This post is nearly a year old. So guessing it was either fixed or given up on.

Some basics. Is there actually vacuum in the hose at the modulator? Rubber can swell and clog internally. Wrong port on the engine, etc.
Did the pin get lost that goes between the modulator and the valve?
Modulator is bad, stuck in position. Valve body has a stuck valve.

 

[Z Bronco]

Adjusted the modulator which helped a little bit, the engine port is correct, there is some vacuum at the hose, pin is there. I am going to replace the modulator when I get back to Oregon (on a road trip now, currently in Wyoming). I have been on a wait list at a transmission shop for a very long time.