91 firebird 3.1 to 350 tbi speedo not working
05-06-2016, 11:43 AM
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91 firebird 3.1 to 350 tbi speedo not working
I have completed a 3.1 to tbi 350 swap. Everything on the gauge cluster works except the speedometer. I have an ecm out of a 92 Buick roadmaster. Also during the swap I had to replace the transmission as the the original 700r4 had a v6 belhousing. The v8 one had a non electric speed sensor. So I swapped the v6 output shaft to the v8 tranny. I know the cluster is not the problem because I tested it before I pulled the v6 out of the car. Am I missing something? Any help appreciated. Thanks
05-06-2016, 02:03 PM
#2
Re: 91 firebird 3.1 to 350 tbi speedo not working
Just shooting from the hip here, so grain of salt, but...
The 305 TBI cars use an ECM that recognizes a 2000 pulse per mile speed signal. The 3.1L uses the same computer as the TPI V8s that recognizes the 4000 pulse per mile signal from the VSS in the transmission. So on TBI cars there's a VSS buffer box that piggybacks onto the ECM mounting bracket, the 4000ppm VSS signal goes to the buffer box where the signal is converted to 2000ppm and is then sent to the ECM, another wire carries the 4000ppm signal back to the speedometer. The 7730 ECM cars have the speedometer controlled directly from the ECM.
So I would guess that you have something wired wrong.
Assuming the 350TBI ecm you're using is the same technology as the 305TBI ecm, it's not going to have the 4000ppm output needed to run the speedometer. IDK if you'd have to run the buffer box, necessarily but you probably need to tie the speedometer to the 4000ppm signal from the VSS.
The 305 TBI cars use an ECM that recognizes a 2000 pulse per mile speed signal. The 3.1L uses the same computer as the TPI V8s that recognizes the 4000 pulse per mile signal from the VSS in the transmission. So on TBI cars there's a VSS buffer box that piggybacks onto the ECM mounting bracket, the 4000ppm VSS signal goes to the buffer box where the signal is converted to 2000ppm and is then sent to the ECM, another wire carries the 4000ppm signal back to the speedometer. The 7730 ECM cars have the speedometer controlled directly from the ECM.
So I would guess that you have something wired wrong.
Assuming the 350TBI ecm you're using is the same technology as the 305TBI ecm, it's not going to have the 4000ppm output needed to run the speedometer. IDK if you'd have to run the buffer box, necessarily but you probably need to tie the speedometer to the 4000ppm signal from the VSS.