My 89 Iroc had a mechanical speedometer(cable). The 85 corvette 350 TPI engine harness, motor, and matching transmission I'm swapping in has an electronic speed sensor and plug on the tailshaft. I'm assuming I could switch the tailshaft piece with my old mechanical unit but would leaving that plug unplugged from ECM harness cause problems? If it ONLY tells gauge to work I guess I'd be fine if it didn't set a code. Not sure, anybody dealt with this before when switching to a TPI harness?

Thanks

 

Your mech speedo drive should fit in place of the electronic VSS

VSS signal also goes to the ECM so it knows what speed the car is doing ; used for functions like convertor lockup for one.
IIRC you should have the required speed signal available at the buffer box that your cable drive goes to?

No VSS signal also gives other well documented tune problems
Read
https://www.thirdgen.org/forums/diy-...-cell-4-a.html

 

If that is a Corvette trans, it won't work in your Iroc, they are shorter in a 'vette.

And, there is no rear mount.

 

Thanks, you confirmed what I was worried about. I'm going to try figuring out where to find that adapter from summit mentioned in the thread you attached. Then I'll have a properly funtioning speedo and a happy ECM.

I have tons of rust repair and welding to do before I drop this drivetrain in. I'm just trying to figure out all the bugs ahead of time. This forum is great!

 

Well crap, I just went out and measured, the vette tranny tailshaft is 6 3/4" long and the output shaft sticks out a half inch. The Iroc tranny tailshaft is 7 1/4" long and the output shaft sticks out a half inch. There's only a half inch difference in the output shafts so if I just swap the tailshafts the corvette output shaft would be flush with the Iroc tailshaft. Just a half inch! Surely I can get by with losing a half inch if it's splined the same. ???

 

I know the 3rd Gen has the torque arm mount on the tailshaft housing but where is the trans mount fitted ?
If it uses the holes on the case ( not the tailshaft housing ) then you need to check the Vette case 1st.
Some of the early Vette T700's didn't have provision for a case fitted trans mount

 

Glad you mentioned that, I'll check it out tomorrow. That would have been awful to discover something like that when I was all done and finishing up the crossmember. Thanks

 

Many folks use the 'vette 700R4 transmission in the f-body. Need to swap the tail housing and move the location of the speedometer gear. On the output shaft there are two locations for it. One for the 'Vette, and one for the others. Otherwise the gears get torn up.

Take note of where the gear is then move it to the other location. The output shaft will have two holes in it for the drive gear retaining clip.

RBob.

 

Sweet! Thanks RBob

 

Forgot to check for provisions for mount first and discovered I didn't have them while I was finishing up project.

I fabricated a bracket utilizing the lower tailshaft bolts to secure it

 

Well I got it all done and test drove and set off a code 24 ( no VSS signal ). Read something in an above post about a buffer box in cluster.

 

I had no idea you were going to put the vette transmission in your car.
I set the memcal to accept a optical vss signal . The brown wire that went to your tbi A10 gets repined to the optical input pin C6. I believe I marked the memcal optical vss. But now you have a 2k ppm magnetic pulse from the corvette and Im guessing no speedometer at all. Send Me a pm with your address I will send you a vss buffer to wire in.
Disregard F it will goto C6 of your 730 ecm.

 

The Vette trans was set up for an electric speed sensor, I switched tailshaft and converted to mechanical gears. My speedometer works good since it's cable driven. I don't have a buffer module somewhere in my dash already?

 

I think you did it right setting it up optical. I found this post on the sticky "TBI to speed density TPI conversion" thread post #80. I'll try to copy and paste.

Okay, fired up the conversion today (and drove it home!)... Some notes for people doing this conversion... All went as planned except for the speedo... Initially I had no speedo input to the ECM (and hence no TCC lockup)... The speedometer on the instrument cluster worked fine though. If you pull the cover off the passenger side (as shown in the pics), and splice wires off of the connecter going to the VSS Buffer box and run the Purple (PPL) to BB9 (VSS Low) and the Yellow (YEL) to BB10 (VSS High), you will have proper speedo operation for the ECM... Easy enough since the buffer box is right there..
Attached Images ltwhat-0011s.jpg (45.9 KB, 503 views)



I had trouble opening the image but this makes sense right? I might already have this buffer box. Just needs monkeyed with right?

 

You won't have that yellow 4-out buffer box. What your car has is a smaller buffer that is mounted in the cluster. It picks up the signal from the back of the speedometer.

I am assuming that your car was originally TBI, someplace in the harness for that ECM is a BRN wire, went to pin A10 of the TBI ECM.

Wire it to pin C6 of the current ECM. Sounds like you went with the '7730? If so then it will get a VSS signal.

If you used the original '85 Corvette ECM, the BRN wire will go to pin A10, same as the original TBI ECM.

RBob.

 

Thanks for all the help! It ended up just being that I didn't specify "optical" for my tune. I failed to mention to Tuned performance that I was switching tailshafts. Just tried out reprogrammed chip and problem solved.