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Old 10-21-2009, 01:09 AM
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Wiring glitch driving me nuts please help!

Hello, i am having what could be a bad ground in my 91 gmc 1/2ton truck. I installed a summit CD ignition box, 454tbi fuel pump, and a guage cluster out of a 95 1/2ton. I also had to wire and install a DRAC box to convert the signal coming from the VSS to digital signal for the speedo to read. Everything worked great until my crus stopped working. Everytime i would try to set it, the motor would cut out really bad, as if there was no spark or too little fuel. Then my speedo started messing up. In park if i reved it, the speedo would come up with the tach, when in gear, at low speed it pulses up and down. the faster i go the faster it goes up and down until anything over 25mph then it goes all the way to 85 pegged out. If i give it a lot of gas it will cut out as if i were trying to use the cruse again when it wasnt on. At first i replaced the DRAC, battery ground cable, swaped out the coil, disconnected the tach, vss, nothing worked. I did notice that while the vss was disconnected, the speedo didnt pulse anymore, but still went crazy at any speed above 25mph, or at high rpm. still cut out, but cruse didnt do anything when activated. The only thing i can think of is a woring problem. But i have no idea how cruse controll can have anything to do with the engine cutting out. If anyone has any ideas or better ways to find out whats wrong, please help

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Old 10-21-2009, 07:19 AM
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Be advised that GM had some problems with gage clusters in full sized pickup and Tahoe's. I am not sure as to what years were affected, but the problem except for the engine cutting out sounds really similar.

Vince
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Old 10-21-2009, 08:11 PM
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But i have no idea how cruse controll can have anything to do with the engine cutting out. If anyone has any ideas or better ways to find out whats wrong, please help [/QUOTE]


Imagine this, its winter and your driving down the road with the cruise on and set at 70 mph, 2100 rpms. You come to a slight grade in the road and at the same time you hit 'black ice' patch. your rpm's go up to 4000 and the speedo goes down to 65 mph. The speedo is telling the engine we need more speed to maintain 70 mph and the ECU is telling the speedo we've increased engine speed to compensate for lack of speed. This just took less than a second to happen. The back of your truck has now started to come around on you at 70 mph......., now what happens?

The ECU senses that there is no ground speed consistent with the forward motion of the front and rear wheels and immediately removes rpms by cutting out the throttle (TPS) and cutting ignition current to the coils. This happens in a nano second! The cruise should have been disengaged now as the vehicle goes in to a limp mode which still allows you full steering and brake use to control the vehicle and slow your speed down.

Now, apply this concept to what is happening to your vehicle. Remove all connections, re-read your instructions and ONLY connect one thing at a time and verify its workability before proceeding to the next item.

Hope this helps.
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Old 10-22-2009, 07:45 AM
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wow i didn't even think the old 7747 ecu could do that. It also cuts out even without the cruse on. Mainly under high throttle. and sometimes it can be just random cutting out. Now my cruse control doesn't do anything now, it use to cause really bad cutting out at any speed, but now does nothing. I replaced what i think is the cruse control unit under the hood next to the master cylinder, but it didn't help. What connections are you talking about when you say "all connections"?
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Old 10-23-2009, 07:18 AM
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I also noticed that when i do something like use the power windows, set the fan to high, everything would go back to normal sometimes. The very little time the speedo does read right, i notice that when it cuts out and misses, it is always pegged out. It can be idling and the tach will go from 1000 to 3000 and stay there for a sec then go back to 1000. Speedo does the same thing. even not in gear or moving it will still go all over the place. I recall that in the first stages of this happening before the cutting out the e-brake light was flashing but not at random, it was almost like it had a patern to it. then the speed started messing up, then tach, then cruise. Now the cruise doesnt work at all anymore, it cuts out at random, I fear I may lose drivability of it as it is getting worse and worse. help
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