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Jr Gilliam
Jr Gilliam,
Category: Medium and Heavy Trucks
Satisfied Customers: 1
Experience:  Field Technician at Whayne Supply Co
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I have a 1999 GMCc8500 with cat 3126 speedo and tach are not

Customer Question

I have a 1999 GMCc8500 with cat 3126 speedo and tach are not working. Looking for wiring diagrams and possible causes
Submitted: 7 months ago.
Category: Medium and Heavy Trucks
Expert:  Jr Gilliam replied 7 months ago.

Hi there,

Do you have the engine serial number

Thanks

Jr

Customer: replied 7 months ago.
Attached is a pic of the truck serial I won't be back to the truck until tomorrow morning so I can get a engine serial
Expert:  Jr Gilliam replied 7 months ago.

Could you get the serial number info from the tag on the valve cover. May be a bear to get to. Is the engine a 3126, 3126B, 3126E?

Thanks

Jr

Customer: replied 7 months ago.
I cant get any serial number until tomorrow was hoping to get some info tonight. It's a 3126 as far as I know
Expert:  Jr Gilliam replied 7 months ago.

I have attached a 3126E schematic. As far as the truck side the easiest way to check the gauges is to see if you have voltage coming to the gauges from the engine ecm.

Thanks

Jr

Customer: replied 7 months ago.
Thanks but theThe schematic didn't show up.
Expert:  Jr Gilliam replied 7 months ago.

How about now

Customer: replied 7 months ago.
Ok. So I check the + side at the ecm. If I have power where is the speedo and tach located? Is is back of timing cover behind air compressor?
Expert:  Jr Gilliam replied 7 months ago.

The sensor in the timing cover is the speed timing sensor. The sensor that drives the tach is in the flywheel housing(pin 36 at the ecm). The sensor for the speedo is in the tail shaft of the transmission(pin 38 at the ECM). track the wires from the ECM to the connector at the firewall. I have seen failures of the dash cluster that kept the speedo and tach from working. If you have signal to the cluster and it still doesn't work then the cluster needs replaced.

Thanks
JR

Customer: replied 7 months ago.
Yeah I'm thinking it may be the big plug connection the engine wiring harness to the harness in the cab. Pulled it apart and many of the connectors had some corrosion. I cleaned it the best I could but. I will let you know what I find Tomorrow. Thanks
Expert:  Jr Gilliam replied 7 months ago.

10-4 I would say you're onto the problem. With low voltage signals the least amount of resistance can cause a component to have problems. I look forward to know what you find.

Thanks

Jr

Customer: replied 7 months ago.
I haven't had chance to look at it yet today but i got a pic engine serial number..model it just says 3126 pic is attached
Customer: replied 7 months ago.
I looked up engine serial number online I think it's a 3126B..will the info you give me be the same
Customer: replied 7 months ago.
Do I need to check pin 36 and 38 with the engine cranking? Or just key on. With key on I have no power at 36 or 38
Expert:  Jr Gilliam replied 7 months ago.

Yes sir the info is the same. Pin 36 wont have power until the engine is running. Pin 38 will have power when the vehicle is moving.

Customer: replied 7 months ago.
I had power at 36 when running around 3.5 volts having a hard time tracing wire to firewall..can u get a pin out of the plug after the firewall
Expert:  Jr Gilliam replied 7 months ago.
I uploaded the dash cluster harness. It will be easy to verify you have the same signal there that you did at 36 and 38. If so you'll need to replace the dash cluster.Thanks
Jr.