Archived speedometer/pcm - overclocking

[jdwayland]

We have a 1992 D350 club cab dually with the stock driveline that we bought in January 2003 as a tow vehicle for a landscape trailer. It had been out on a farm and apparently nickel and dimed on maintenance and not taken care of the last few years . The speedometer was disconnected and the problem was allegedly a need for new gear to tie into side of transmission. Have put that in but speedometer reads about 70 mph when really running 30 mph+/-. Dealer has truck now and thinks it is PCM problem. ( maybe a newer pcm than should be on truck) Could this have the wrong PCM and be in metric mode and feeding speedometer too high a rate? (5/8mile per kilometer)



Questions: 1. Any experience - thoughts on the speedo / PCM issue? 2. If we leave the situation as is and let it overclock the speedo,. will it do damage to the drivetrain through improper engine settings?



JDWayland

 

[Mark_Kendrick]

I had similar problems with mine.



The wires on the + battery terminal were corroded internally. I found one of them bad and cut them all and reterminated them. all is good now.



check the fusable links too. they get old and brittle. I cut them out and put a fuse block at the battery.



Everything seems to work fine now.



Later,

Mark

 

[Paychk]

On the side of the transmission there should be a distance sensor with some wires plugged into it. The sensor in turn is mounted on an adapter (only if your transmission has overdrive) this adapter is a 90° drive adapter and inside are changeable gears to calibrate the speedometer. Maybe the adapter is damaged or has been messed with or completely missing.



Can you tell which transmission you have on your rig? it will help the diagnosis.



PS the PCM on this year truck has no input to fuel control, that function is totally inside the injector pump and mechanical.

 

[dpuckett]

I would be more inclined to think along the lines of speed sensor malfunction. I recently had this experience with the 91 I just got. If the speed id fairly consistent, get an adaptor made. I had one made at a speedo shop for $54 and it is DEAD ON. Dealer wanted $94 for the same part. Look on the side of the transmission, and hook up the sensor directly to the case- no adaptors, etc. Run it down the road over a given known distance. Highway markers are good. Run 5-10 miles, and then see how far it has gone. Divide number of miles actually driven by number showing on odometer. This is the ratio of adaptor you need. But if the speed is erratic, it is most likely the sensor. I dont think the PCM would be a primary culprit. The dealer could just want to sell you a $450 REBUILT part that may or may not work. Volt regulator on my 93 (pcm controlled) went out after 10 months. I didnt get the computer, but it had a tag on there that said Reman 10/01 or so. So I paid a shop $40 to install an external volt regulator.



Daniel

 

[jdwayland]

The transmission is a 5 spd manual Getrag. The dealer said the snesor and gears check out okay. Pickuing it up toady 4-3-03 - turned down the dealers $750. price to replace the pcm. Will still be looking for ways to make it right. Keep ideas coming and thanks for ideas so far.





JD

 

[Mark_Kendrick]

I had the same problem but mine was intermittent. I tried greasing the zurk at the sender, cleaniing connection, etc.



I finally fixed the electrical stuff and it went away.



I'm betting that the problem is crosst6alk in those low voltage circuits. When all the voltage woes went away the problem went with it.



3-5 ring terminals are way less $$$ than a PCM.

 

[Paychk]

JD



There is a splice in the speedometer circuit and it is located in the main wire harness over the power brake booster near the wiper motor. This bundle is notorious for wearing on the booster and shorting out. The splice will be 3-5 wires, twenty gauge size, white with orange tracers. Also the main ground for the PCM is located in front of the battery on the radiator support. It will be 2 wires with ring terminals, black in color. Also check the PCM connector and make sure it is not corroded and tight to the PCM (behind battery)