Drac Module Problems

[TRoy96]

So we recently purchased a lowered 1992 c1500 pickup with a 93 transam LT1 engine in it. Along with the swap was the 4 spd 4l60 tranny and computer. When we bought it the speedometer didnt work and the original owner said that he had a 5spd in the truck before with a tuneport motor and the speedometer worked then, but after the swap it stopped working.

So in attempt to fix it we bought another speedometer, still doesnt work. Then we read up on the drac and figured that ours could be junk. So i got 2 from a salvage yard for $20 and it still didnt work. I then traced the wires going to the drac and for some reason the ground went to a port on the old obd for the truck. So i grounded that to a metal bar in the dash so that i got voltage from terminals c9 and c8. Also noticed that i was only getting continuity from the purple wire that goes to the vss, not the green one. So i traced that back and connected those. As far as the two wires that go to the pcm, (c13,c11) i dont know what to do with those because the truck no longer has the truck pcm, its been swapped to the transam pcm. Do i need the pcm for the drac to work, i have read on certain sites that i did and other sites that i do.
After i made sure that the vss was connected and it was getting power i tested out. Once the truck started i heard a pop. It was the drac.

I measured the voltage from the plug and it was getting 13.6. Is this too high?
Any help or advice to get my speedometer working would be greatly appreciated.
Thanks in advance.

Heres the pinout ive been using.

 

[dyeager535]

The two PCM outs shown on that pinout are for the two different signals used, depending on application. You need the right pinout/info for the LT1 PCM you have though, as later on no "buffer" or DRAC was necessary, and the PCM was able to take the VSS input directly. Dunno if your LT1 PCM is in that category.

I recall that at least one of the DRAC pinouts I ran across was wrong, but it was a difference between 2K and 4K PPM, which would simply skew the readings, not make something inoperable.

Since the DRAC is powered on it's own, and the reluctor generates it's own field, it works fine without an ECM/PCM hooked up to it. The ECM/PCM however is reliant on the VSS input for a multitude of reasons. So yes, most likely the speedometer should work fine regardless of what the PCM is doing, assuming it's similar to the '90/91 R/V speedo setups.

 

[TRoy96]

Do you guys think that the 13.6 volts is whats blowing up that chip.

 

[Fordum]

Do you guys think that the 13.6 volts is whats blowing up that chip.

Short answer: NO.

Long answer: 13.6 volts to ground or in the wrong place is.

The module might not be ruined. You blew a foil trace, but if the overcurrent did not go through a delicate component, its fixable.

Can you tell what the two ends of that trace were hooked to? You might turn it over so that everything is in the same place and take a picture so I can see whats what.
Assuming there is not a cover or something on the other side. If all that is wrong is that blown trace, you can fix it with a small piece of insulated wire soldered from one end to the other.

 

[rampage]

I don't know anything about DRACs, but it looks like the trace that was blown goes between the connector (C8 which is ground) and the large black cylinder in the middle on the right. Hard to tell what that component is, it's not a resistor, capacitor, diode that I can see. If I understand correctly, what you did was connect the DRAC ground (C8) to the dash when it was originally connected to the ecm. If this is what you did then I'd connect it back to the ground on the ecm. This is because there might be large voltage spikes on the chassis ground due to engine noise and the ecm ground is filtered.

 

[Fordum]

Well, its hard to say from that picture. I blew it up a much as I could, but it got fuzzy before it got better.
It looks like the black cylinder may be soldered to the ground plane at the bottom end. Which, if so, and if the other end is hooked to the ground pin, then it would most likely be either a "zero ohm" resistor, or in other words, a jumper, or else maybe a choke.

There is a slight silver band on the very edge, that might indicate a diode, but it does not look likely.

Note that the pin next to that one is the 12 volt positive, and it goes through another one of those things.
Which further makes me think they are chokes. Got to be something that will pass DC, and the plus 12 then goes through a diode.
Not much sense to put two diodes in there in series.

But this begs the question, what blew the foil trace?
If he hooked c8 to ground, where was the short? I'm wondering if he hooked it to a hot lead without realizing it.

 

[rampage]

Well, its hard to say from that picture. I blew it up a much as I could, but it got fuzzy before it got better.
It looks like the black cylinder may be soldered to the ground plane at the bottom end. Which, if so, and if the other end is hooked to the ground pin, then it would most likely be either a "zero ohm" resistor, or in other words, a jumper, or else maybe a choke.

I was thinking it may be an inductor too to filter any noise. A jumper doesn't make sense since that trace could have been extended to a via up to the ground plane.
Something else is going on here. I think he should examine the connections more closely and do more research before blowing something else up.

 

[TRoy96]

Im pretty sure its the foil trace for the ground. Heres a pic.

The wire that went to c8 originally went to the obd1 port that was for the trucks pcm, which is now replaced by a firebird pcm and obd port. So when i would measure the voltage from c8 to c9 i wouldnt get any voltage. Once i grounded the wire from c8 i got 12v between c8 and c9 while the truck was off (key in ON position). The foil trace didnt pop till the truck was running. I read in an article that originally the ground went to the thermostat housing bolt on the engine.
Heres the article:

http://www.atraonline.com/gears/2005/2005-04/2005_04_04.pdf

As for the black cylinder, there is no lettering on it at all.

 

[Stomis]

I believe your work may be in vain. Im not LT1 guy but IIRC the speedo on obd2 and up cars receive their input straight from the ECU at 4kppm. Which basically means the DRAC is misconstruing any info, even if it was working.

For some reason when I screwed with mine I thought that thing was suppose to have 5v to it not 12v. Either way your simplest solution would be to buy a dakota digital box.