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I recently had the transmission rebuilt in my '87. Now when driving, the car shifts fine from 1st to 2nd, but all other shifts are quick and overdrive and lock up take place at very low speed according my Speedo on the instrument cluster and GPS on my phone. I have connected my OTC Monitor 2000 and monitored vehicle speed according the to ECM. The reading on the scanner is much higher. When drive around 30 mph the scanner indicates 50+. When driving around 55 the scanner indicates around 85+. The transmission shop says it is nothing he did. I did not have these shifting issues before my transmission cratered. The shop says he moved the tail housing from the old transmission shell and installed onto the replacement transmission shell. He thought it was the ECM so I replaced the ECM and still have the same issues. Not sure what to do next other than take the car back to the trans shop and take him for another test drive. We even tried adjusting the TV cable to many different positions, but the position where the cable needs to be, the transmission just doesn't feel right as described above.
You believe your rear axle ratio is what? What size tire do you have on the rear? That's for a start. Using that information it can be determined what drive and driven gear combination should be in the transmission. You can then pull the VSS and check for what's actually there. If the shop says he transferred your driven from the old you could maybe assume that the drive on the output shaft isn't a match for what was on your old. How accurate was the speedometer before the transmission change? I'm somewhat surprised the shop hasn't already done this. Did you keep your old ECM?
**If you're unsure of rear axle ratio, drive 60MPH GPS in 3d NOT OD and note the RPM. Need rear tire size to calculate from there but likely doable for rear axle ratio.
I was looking in the Service Manual and see that the ECM is fed from the IP Cluster. The IP Cluster was replaced a few months ago, but I was not having any issues with car shifting correctly before and after IP replacement.
I was looking in the Service Manual and see that the ECM is fed from the IP Cluster. The IP Cluster was replaced a few months ago, but I was not having any issues with car shifting correctly before and after IP replacement.
When your 'comfortable' with TV Cable adjustment do as I suggested in Post #2. I only asked about ECM because you don't want to surrender a 'thought to be good' ECM foolishly as a CORE!
I have an 86.
It is interesting to note that i swapped in a 91 firebird trans into my car.
the firebird trans did not have the torque arm mount, so i removed the tail housing, only to find that the shafts were different .
i removed the 86 shaft and installed it into the 91 trans. It had a red gear, so i just swapped over everything.
the firebird trans uses a reluctor, not a gear. I bet your new trans has a 4 pulse reluctor instead of a gear driven 2 pulse vss.
get in the guy’s face and request to see the vss. It’s a matter of putting the car up on the rack, and removing the 2 pin plug, and one bolt.
then prying it gently out of the hole.
the reluctor is a black square thing with rounded corners.
the 87 vss has a gear.
you make the call.
Last edited by coupeguy2001; Aug 20, 2021 at 07:56 PM.
Well using what you mentioned GPS 60, IP 61 I'd say the SCANNER IS lying to you!!! Scanner maybe set to 'METRIC' KPH? KPH is something near 96+ a little bit!!
The different readings are what is driving me crazy. With scanner plugged in and watching the RPM and Speed coming from the ECM, I almost have to believe the scanner based upon what the transmission is doing. Driving at 30 MPH on the Instrument Cluster, the ECM is indicating around 51 MPH and the transmission is shifting into OD and going into Lock Up. I replaced the ECM and have the same issues. So with the Instrument Cluster closely matching the GPS I don't know which way to go. The shop says the transferred the VSS and output shaft over. I just know, I was not having any of these issues before the rebuild.
Axle ratio is 2.73
Last edited by TCPower73; Aug 20, 2021 at 10:28 PM.
The different readings are what is driving me crazy. With scanner plugged in and watching the RPM and Speed coming from the ECM, I almost have to believe the scanner based upon what the transmission is doing. Driving at 30 MPH on the Instrument Cluster, the ECM is indicating around 51 MPH and the transmission is shifting into OD and going into Lock Up. I replaced the ECM and have the same issues. So with the Instrument Cluster closely matching the GPS I don't know which way to go. The shop says the transferred the VSS and output shaft over. I just know, I was not having any of these issues before the rebuild.
Axle ratio is 2.73
2,73 and 25.68 tire is 2150 or so. You could certainly have an issue with the trans but GPS, IP and RPM are reasonably close. Scanner is crazy wrong MP! Use a different scanner to confirm yours is working. Start looking for trans issues? Earlier today 'governor' was mentioned for an issue! I'd doubt that to be an issue with yours but I'd say obvious issues. Is the trans in your car an AUX VB build or not? '87 I believe should be.
Originally Posted by TCPower73
Yes, I still have the original ECM.
The TV cable is in the correct position.
I was looking in the Service Manual and see that the ECM is fed from the IP Cluster. The IP Cluster was replaced a few months ago, but I was not having any issues with car shifting correctly before and after IP replacement.
***The RPM you've mentioned is from the scanner OR the IP? If IP and scanner are a 'match' for RPM then it's certainly conceivable a cluster issue relaying VSS to ECM. PROM is an OE or modified?
The RPM from the scanner and IP Cluster closely match each other. The PROM is stock, no performance enhancements.
That would certainly point towards a cluster to ECM communication problem. Was your cluster a recent 'reconditioned' purchase? You might reach out for one to swap 'locally'. If you're confident of the transmission build there's few places to start. If the shop knew the scanner RPM matched the IP etc. their suggestion of the ECM seems pretty legitimate. Diagnostics speed data to ECM seems difficult.
I formed the shop on the differences between the speeds indicated on IP Cluster and scanner. The shop's scanner was not working to verify my information. The shop suggested that I replace the ECM. The ECM was replaced and same issues still occur. The IP cluster was replaced in February of this year, but I never had any issues with transmission shifting and I never had the need to plug in the scanner to verify speed from ECM since speed on IP Cluster closely matched GPS. I called the group that refurbished the IP Cluster and they were not aware of any way of re-calibrating the IP Cluster.
I formed the shop on the differences between the speeds indicated on IP Cluster and scanner. The shop's scanner was not working to verify my information. The shop suggested that I replace the ECM. The ECM was replaced and same issues still occur. The IP cluster was replaced in February of this year, but I never had any issues with transmission shifting and I never had the need to plug in the scanner to verify speed from ECM since speed on IP Cluster closely matched GPS. I called the group that refurbished the IP Cluster and they were not aware of any way of re-calibrating the IP Cluster.
The diagnostics speed data from cluster to ECM seems very difficult to verify. I get the FSM reference but if RPM is close, there's no explanation for faulty speed except that data! Will they provide you with another to 'verify'? Your situation is very different than most. If you can't confirm data to the ECM how do you diagnose the other issues. 6 months I'd think a it a reasonable request to offer up a cluster to confirm operation. If the cluster facility is 'responsive' to you then maybe it speaks well of them. **** does happen. Coincidental in your situation? Certainly possible! To this point you've done much! What's next?
Are you absolutely certain that your tool isn't displaying the speed in kilometers per hour? The speeds that you are reporting are almost exact math for that to be the case. 1 mile=1.609 kilometer.
I switched rear-ends on my '93 and got short-shifting as well. general consensus is it's the governor (2.59 to 3.54). Since you had a rebuild, is it possible that they installed the wrong governor? Or one from another car (corvettes have a special; governor--lighter weights from what I'm told) so any old Camaro won't do.
Vehicle Speed Issues
