Conversions & Hybrids - 90 C1500 LM7/4L60E swap. Speedo question

07-12-2011, 03:45 PM
I swapped an 04 LM7 (5.3L) and a 2WD 4L60E into my 1990 chevy RCSB C1500. I used the 04 truck harness and just modified what I needed for powers and grounds etc.
My question is this:
the stock 90 speedo has the same 4K pulses per mile as the output speed sensor in the 4L60E. If you hook the speedo to the sensor it works fine. If you hook the speed sensor up to the factory 04 harness and PCM it reads fine in the PCM. If you try and splice the 90 speedo harness into the output speed sensor circuits (with the PCM still hooked into it) niether will work. Has anyone run into this and did they find a cure? diodes in line maybe or is it just too much resistance for either to work correctly? Thanks in advance.

07-12-2011, 05:26 PM
I've had that problem with other speedometers. Have you tried hooking it up to the speedo output from the PCM, on C2-50, dark green and white wire?

07-13-2011, 07:00 AM
I had not thought of that. I will check into the schematics and see if it is still the same 4K pulses coming out of the PCM. Thanks for the input.

I checked on the GM SI and this is what it said:

The instrument panel cluster (IPC) displays the vehicle speed on the analog speedometer based on the vehicle speed signal from the powertrain control module (PCM). The PCM converts the data from the vehicle speed sensor to a 4,000 pulses/mile signal. The IPC uses the vehicle speed signal circuit from the PCM in order to calculate the vehicle speed.

The speedometer defaults to 0 km/h (0 mph) if a malfunction in the vehicle speed signal circuit exists.

so it looks like that will work. Thanks.

07-13-2011, 08:26 AM
Yes the PCM out puts 4000ppm to the speedometer. Just don't use the one that goes to the ABS, it's not 4k.

07-13-2011, 08:52 AM
I checked some other years an 88-91 has the DRAC built into the cluster. the 92-94 clusters had a seperate DRAC module. So it seems to me that I can use a 92-94 cluster and just run the 04 speedo output to the new cluster input and be good. I think I have a 92-94 cluster that I will try it on. Or I could bypass the DRAC in the 90 cluster.

04-01-2012, 11:21 PM
what did u use t get it to work

03-09-2014, 07:13 PM
I am interested too about to do 6.0 swap in my 89 c1500. I have 1994 cluster in it now and picked up a drac from wrecking yard