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Help! Electrical issue

1953C4Truck

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Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
I have a very cool project going and I just ran hard into a roadblock. The project is a Vette powered 1953 Chevy truck. I've acquired a lightly crashed but totaled 1991 C4, L98 700R4 that ran nice and even drove nice as a donor for just about everything. I've removed the engine, trans, front & rear suspension, steering column, shifter w/wiring and linkages, brakes w/master, gauge cluster, fuel pump & sender, most of the dash and of course the entire wiring harness.

At this point I have the truck about 90% mocked up and was ready to hear the noise of that L98. So, I connected all the plugs into all the computers, gauges & column etc. and connected a ground to all the ground leads just as it was in the car before I took it apart. Right after the install of a new battery that shows 12.7 volts on my meter , I turned the key... NOTHING.

The security light flashes on the left of the cluster but it did that before I took the vette apart because the car had no doors when I got it. When I turn the headlight switch on, the center of the gauge cluster lights up and I see "SYS" flash three times. I get no clicks or noises from the fuel pump or anything else for that mater. It's like the computers aren't waking up at all.

I noticed one other thing. Before I took the car apart, I took a short video of a start-up. In the video, the tach w/key off & out is resting at zero - or the beginning of the scale. Now, w/key off and out the tach is at about 5250 and doesn't budge at all with key in and on, again - like it's not waking up.

The only part I have not hooked back up to the harness (and no longer have) is the ABS control unit that sits behind the left seat. I ran my test meter on the plug and found that there is a pin with 12 volts and am now wondering if the computers need to see something from that part of the harness to tell the computers to run.

I've read a good bit on the VATS but I think my problem is before the VATS

I know this is a complicated mess I have going but I'm determined to learn what I need to figure it out.

Any thoughts, ideas of help would be greatly appreciated.

Regards,

Ken

PS If you're interested, heres a link to a series of project pictures up to this point...
YouTube - ‪53 Chevy truck, "Project Mascot" for Composiclean.com‬‏
 

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vetteoz

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Jan 20, 2004
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1,564
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Not in Kansa anymore
Corvette
1990
, the tach w/key off & out is resting at zero - or the beginning of the scale. Now, w/key off and out the tach is at about 5250 and doesn't budge at all with key in and on, again - like it's not waking up.
Disregard the tacho for now.
90/91 are well known for tacho problems; is commonly a fault in the cluster itself not related to the engine or computer

, I turned the key... NOTHING
I get no clicks or noises from the fuel pump or anything else for that mater. It's like the computers aren't waking up at all..

I would be checking your connections and confirming you have the correct voltage at the right terminals

See ecm pinout link below

ECM Pinouts

90/91 Vette ; bottom of page
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
Hi Veetoz.
The tach did work fine before I took the car apart but I like the idea of forgetting it for now. This site has a lot of great info that i haven't seen yet, I'll dig into this tomorrow. I have done hours of forum and website reading today and can't get my eyes do do anymore.

I'll post after hitting the ECM pins with a meter.

Thanks much.
 

John Robinson

Gone but not forgotten
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May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
I think you have a problem with not having the ABS hooked up. In looking at the FSM for my 93 I see that the ABS unit has input from the ignition, TPS, I am not an electrical guy but I see several pages of wiring diagrams for the abs and they all have at some point or other connection without just about every sensor. It looks like the ABS is the clearing house that puts together the information that goes to the ECM and without it the ecm is idle waiting for input to complete the start up process.

The only thing I have found out on my car is that the electrical side of the car requires all the ducks to be in a row or nothing good happens.
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
Hi John,
Thank you very much for taking the time to look into this for me!! Your news is very good and kind of bad at the same time. The ABS valve unit is "THE ONLY" thing I didn't hang onto because every time I looked at it had a puddle of fluid under it - and I knew I wasn't going to want ABS in the project vehicle.
I do feel better knowing that this the most likely culprit too my issue & I can stop checking the same things over and over. Now I need to figure out how to trick that plug end into thinking it's plugged in. There's got to be a way (I hope) without Frying the computers.
Thank you again.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
I have been told that on some other issues like a knock sensor that you can put a resister of the right value in the ground line and as long as the ecm is getting a value that is in it's programed range it will accept that part of the system as working. So if that is so it tells me that the whole ecm system looks at the ground values from the sensors to determine if they are in the right programmed values to release the things like fuel and spark and crank. I think you will need to have the fsm wiring diagrams for the abs to determine how to tell the ecm what it is looking for. After all the ecm does not know a knock sensor from a temp sensor or any other sensor all it knows is that piece of information is what it needs from the program it has.

Let us know how this turns out for you.

Coincidentally I have a friend who is building a hot rod using the engine and transmission from a c4 corvette. He has bought a whole new wiring harness for his project and will be using the salvaged ecm. I do not remember his saying anything about installing abs with the new suspension he is using. Next time I see him I will ask him what he is doing to eliminate the absl.


Another item you will need to address is the ASR/TPS interface module. I believe that the ASR interface module feeds information from the TPS to the ecm so that the ecm can control wheel spin by shutting down fuel flow etc. And of course nothing happens without a tps signal.
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
Hi John,
If the resistor method is the trick I think there would need to be an "Off" switch there because that 12v in the orange wire is there even with the key off.
I've seen a few posts where guys have pulled the fuse for the ABS for race events. Isn't this the same as unplugging it? Does anyone have experience with this plug (see pic's below, goes to ABS at valve body behind driver seat) and if it's possible to run car (engine) normally if it's "not" plugged in?.
I need to tackle Vetteoz's idea too and check the pinout voltages right out of the ECM. Man, I'm over my head here!
I will definitely keep posting while I (we) figure it out and then I'll use a bunch of happy emoticons when things finally light up.
Thanks again

IMG_1201.jpgIMG_1200.jpgIMG_1197.jpg
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
PS... If your friend is buying a whole new harness for a C4, he'll have that same plug. I'd be very interested to see how he gets it all working. Do you know if he'll use the CCM too. I don't know if it's needed if we're not using all the original car stuff like pwr windows, door locks, hatch light, sound sys etc., etc.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
In my 93 fsm for the asr there is a red wire from the battery to a 20 amp fuse that is for the ABS/ASR - ECU. My diagram shows that red wire being hot at all times. It also shows a brown white wire on one side with a pink /black wire from the 5 amp fuse to the switch on the other side.

If you do not have an FSM for a corvette I would suggest getting the Chilton manual for the c4 corvette. It has a section with the schematics for the abs etc. They are small and you probably will need a magnifying glass to read them but they may help you figure out what goes to what and in what order. If nothing else it gives you something to read in your private office.
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
It's not a VATS "key" issue

Just to begin some real eliminations, I have taken the key and key cylinder part of the VATS off the list of possibilities...
There is an orange wire casing (about 1/8" dia.) that comes out of the steering column harness with two small gauge wires in it. It comes out just below the bracket that holds the column. There is a plug about 10" down from the bracket. This comes directly from the key cylinders VATS resistance reader which can wear out and become a problem. I disconnected the plug and put a test meter (set to 20k ohms) on the plug end that goes to the key cylinder - with the key out. Then after sliding the key in, I got the same reading as i do when I test the key alone by reading from one side of the chip on the key to the other.
If the reading was nothing or very different from the key's reading, this is where you could put resistors to match the key's resistance except you'd put them on the other end of the plug that goes to computer. I think there are like 12 or 15 different resistance values used on the keys so you'd have to use a couple or a few resistors to get the right value. Then, your VATS would be tricked into thinking a key with the proper resistance was in the cylinder.
Now what do I do??

IMG_1202.jpg
 

vetteoz

Well-known member
Joined
Jan 20, 2004
Messages
1,564
Location
Not in Kansa anymore
Corvette
1990
I have taken the key and key cylinder part of the VATS off the list of possibilities... , this is where you could put resistors to match the key's resistance except you'd put them on the other end of the plug that goes to computer. ..... Then, your VATS would be tricked into thinking a key with the proper resistance was in the cylinder]
Unfortunately your choice of donor car has enabled a host of problems not found with earlier C4's.
Your car has the CCM ; a master control unit that integrates many systems including non engine operations and results in no start if say ;the air bag (SIR) system displays a fault during the key on self test.

As VATS is part of the CCM in your car , the old resistor trick that works with pre '90 models no longer works because the CCM is looking for a change in resistance ( as well as the correct value ) when key is turned on ; not just a certain fixed resistance valve

Quick fix.
Get a custom chip with VATS disabled
or get a VATS bypass module
Baker Electronix - GM VATS Bypass

to supply the fuel enable signal that would be supplied by the CCM ( sometimes called body control module BCM ) if everything was checking out correctly.

NB
Still does not distract from having correct wiring and power to correct terminals
 

1953C4Truck

Member
Joined
Jun 10, 2011
Messages
7
Location
CT
Corvette
1991 C4 L98 w/ 700R4
Update on research & testing

Hello Everyone.

I found this thread on ASK.com

http://www.justanswer.com/chevy/3awg...nvertible.html

The information here has lead me back to the ABS harness plug that is dangling in the air because I no longer have the pump/valve module. This thread says that the grey/red wire in the plug should show 5v. My grey/red wire has zero volts. I have another wire (orange) in this plug that has 12v. So, I'm thinking that the electronics in the ABS (EBCM) pump/valve somehow converts that 12v down to 5v and sends it back to the CCM or ECM which in turn tells them they can fire up all the systems to let the car start.
Does this sound feasible or am I telling myself a fairy tale? If yes (feasible ), does anyone know of a way I can make 5v and deliver it to this grey/red wire? Or??
Thanks again for taking time to roll this through your heads and give feedback.

ABS-plug.jpg
 

WhalePirot

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May 9, 2002
Messages
2,942
Location
SoCA
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1984 White Z-51/ZF6-40/Shinoda body
UR a bold man

I don't have your answer, but applaud your efforts. I am using an '84 Corvette under my '55 F100; 383 w/carburetor, tho. I say it's Corvette from the waist down.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Talked to my friend about what he does when he uses a corvette for a donor for a hot rod he is building. He said he has done the conversion several times and the secret is this. If you do not reuse the steering column with the original key you have to get an after market wiring harness. The after market wiring harness will have the ABS pigtail but more importantly it will have a resister already wired into the harness that will allow you to use any steering column or key. The ecm is looking for the chip in the key to activate the engine. Basically the new harness already has the correct bypass for the vats.
 

tim1

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Jun 25, 2011
Messages
96
Location
Georgia
John,
I would love to know were the wires with the fuses are located. I have pulled a 26 QDM fualt on my 93 and cant find the problem. Maybe it is one of those fuses. Could you post a pic of the diagram you are looking at so I can reference it under my hood? I am trying to get a FSM but the holiday has slowed the purchase down a day or two.
Thanks for your help.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Tim1 I am not sure what diagram you were referencing but if you have a DTC26 then maybe this is what you really need to have.

scan0001.jpgscan0003.jpg

In the absence of the FSM a Haynes or Chilton may not be as complete as the FSM but they have the wiring diagrams and are better then blind guessing.


You do not say what year your car is and that is important what I have provided is for a 93 and is basically only correct for that model and maybe most of the years for the LT1 engine
 

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