Hi y'all. I'm having a problem with the 91 IDI, auto trans, with the shifting when maintaining 35-45mph. Acceleration and deceleration is no problem. Please watch this video I took to see the issue from the driver's seat. Its 15 sec long.

https://www.youtube.com/watch?v=CbY49Z-w_LU&app=desktop

It's like it attempts to downshift but something realizes too late it's not going fast enough yet. What's wrong here?

 

overdrive trans, or C6?
if overdrive trans, it is "hunting" between third and overdrive.
try doing a trans service on it. every time i ever had a trans hunting, a service fixed the problem.

 

That I don't know. How do I find out?

Thanks for the info, I'll call my mechanic and do that. Is it an issue that will escalate from here and quickly get really bad?

 

overdrive trans will shift 3 times, C6 will only shift twice.
also, C6 will rev out around 75-80 MPH, OD trans will go faster.

 

It's an OD trans

 

OD has an OD cancel button on the dash.

 

Mine does not. Anymore at least.

 

40 to 45 MPH is where the trans will shift into OD. so if you are rite there, it will "hunt"
under 40 should stay in third, over 45 should stay in overdrive.
if you do not know when the last time the trans was serviced, it is a goof idea to have it done.
i do filter and fluid every 35,000 miles, as recommended in the service manual.
i have automatic transmissions with over 300,000 miles on them, and one van i serviced years ago had over 450k miles when it was sold, and the engine and trans were all stock parts except for fluids and filters.

 

Ok. It still tries to hunt at just over 35 mph too.

I'll get it serviced as soon as I'm able. Thanks a lot man.

 

it sounded to me by your description that the TPS was probably worn out and needed to be replaced and was causing the converter to lock then unlock on you.
however i was able to search using some of your incorrectly posted video link and found it easy enough;

that there looks as if your tach is going buggy on ya lol.if this is the case,then im gunna guess your tach sensor or it's wires are likely bad.the trans needs the engines rpm info or it can't shift correctly.with a bad tach sensor,it's sending bad info the the trans control unit and so you'll get crazy shifts.

 

Thanks for correcting that, I thought I had it right after the 3rd try...

OK so what's the 'TPS'?

One thing I didn't think of is when I'm at low idle, the tach is flatlined. It doesn't go up to proper level till about 1k rpm. That seems to indicate a buggy sensor to me, am I right?

 

that changes everything.
first thing i would do is figure out what is wrong with the tach.
that may fix the trans issue. but make sure the trans fluid is clean red, and does not smell burnt anyway..

 

Alright. Should have thought of that right off the bat... Tach sensor should be the start then, and a trans service regardless of the issue.

 

i would try the KISS method first. unplug the tach sensor and inspect the connections in the plugs. also inspect the wires between the plug and sensor. dirty connections or loose plugs are a simple fix.
frayed jacket on the wire going into the sensor will cause issues. this usually requires a new sensor to fix though.

 

Alright, does anyone have a tach sensor for sale? If not, I don't want to buy a piece of crap one that won't last or will be buggy, what brand do I need to look at. Its located near the oil fill cap right?