89-91 Suburban speedometer

[Wgesnerjr]

I have just about finished up my swap of a 2004 tahoe 5.3/4L60E into our 85 Chevy truck. I decided to use an 89-91 cluster to get the electric speedometer and not have to change the trans to cable Speedo drive. I did have the PCM reflashed by Brandon at LT1SWAP.COM and let him know which speedometer I would be running.
However, took my first drive this past Saturday and found the Speedo is not working. I would like to check my wiring before I declair this the speedometer is bad.
I know the Suburban cluster has 3 leads (power, ground, input) for speed and the PCM has 2 leads. Can anyone tell me how they wired theirs?
Thanks in advance

[ls1nova71]

Originally Posted by Wgesnerjr

I know the Suburban cluster has 3 leads (power, ground, input) for speed and the PCM has 2 leads. Can anyone tell me how they wired theirs?
Thanks in advance

Not sure what you mean by, the PCM has 2 leads? There is one wire, pin 50, a dark green with white stripe on the green PCM connector that is the speedo output. If you have that hooked up to the signal wire for the speedo, and have power and ground then it should work. That is as long as the speedo itself is good and the signal wire has good continuity between the speedo and the PCM. Did you repin the connector for the dash cluster?

[Wgesnerjr]

I did repin the connector. Everything else is working fine.

There were 2 different wires at the PCM that has something to do with the speedometer. (I am using LT1Swap.com instructions) I'm afraid I may have hooked up the wrong one. Will verify pin 50 on green this evening.

How did you hook power and ground at the cluster? Which pin got 12V+ and which one got Ground?

[ls1nova71]

Originally Posted by Wgesnerjr

I did repin the connector. Everything else is working fine.

There were 2 different wires at the PCM that has something to do with the speedometer. (I am using LT1Swap.com instructions) I'm afraid I may have hooked up the wrong one. Will verify pin 50 on green this evening.

How did you hook power and ground at the cluster? Which pin got 12V+ and which one got Ground?

The speedo needs the 4000ppm signal output from the PCM, the other one, the 128,000ppm goes to the ABS module. It's been a while since I've looked at one, but I believe the speedo should be powered up and grounded with all the other gauges in the cluster by the printed circuit board, so you shouldn't need to do anything with them. When you drove it, did the odometer move at all? If it did, then the speedo stepper motor is probably just bad. If it didn't, then recheck your wiring.

[Wgesnerjr]

When I drove it, it was hooked up wrong. Had it wired to the 129K ppm wire. Tested the 4K ppm wire for continuity, wired it in and took it for a ride.

Still nothing. Not even a twitch. (However, it was the first ride in the truck for my daughter and I, Great Times!)

So I checked my Power and Ground wires at the plug: Slot# 17 is 12v+, Slot#2 is Ground, Slot# 18 is PCM 4K signal.



The 17 slot also powers the Fuel, Brake light and SES/Check engine light. These all work normal.

The 2 slot grounds the High beam light, Volt meter and Temp gauge. These all work normal.

At this point I figure the plug is wired correctly so it is either the speedo is dead or the PCM did not get reflashed correctly.

Is there any way to use a digital multi-meter to see the output from the PCM?

[ls1nova71]

Im not aware of any way to test the VSS output from the PCM, but there is no programming that needs to be done for it to work. It does sound like its wired right. If you have a scanner, it would show MPH, and if its working, which I assume it is because you didnt mention the transmission not shifting or doing anything funny, I would lean towards the speedo being dead, since a malfunctioning speed sensor wouldn't allow the transmission to shift right. Might be worth making sure that the connection between the speedo head and printed circuit is good though.