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Curtis B.
Curtis B., Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 26028
Experience:  Technician turned service manager on multiple lines of trucks and engines, used in industry.
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We have a 1991 gmc vin# J8DB4B1HXM7000130 and the automatic

Customer Question

We have a 1991 gmc vin# ***** and the automatic transmission works fine if you manually shift it but will not up or down shift automatically
JA: How many miles are on the car? Is it the original transmission?
Customer: 230 000 km on diesel engine and original transmission
JA: What is the model of your '91 GMC?
Customer: w4000
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: yes nothing yet im assuming electrical problem
JA: Anything else you want the mechanic to know before I connect you?
Customer: fuel, temp, and exhaust brake lights and gauges not working thanks
Submitted: 2 months ago.
Category: Medium and Heavy Trucks
Expert:  Curtis B. replied 2 months ago.

Diesel engine. And I assume Allison transmission. Check the modulator that has cable coming from engine to the transmission and the transmission governor. Both are external and control the shifts. Modulator can be broken, or the governor can be stuck.