TH400 kickdown question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gk7771 · 02-02-2009, 08:38 PM

1988 Chevy c/2500 with V-6 and a TH400 (all stock).

I had the TH400 rebuilt and now I have no kickdown. The Tranny guy insists that he looked for a swith by the gas pedal and the intake and no switch. I believe in 1989 the computer triggered the kickdown. Is this true in 1988?

I'm sure the tranny was kicking down before the rebuild. Why woudn't it be kicking down after. How can the circuit be tested? If its computer driven where is the wiring ?

Help

JohnO356 · 02-03-2009, 02:42 PM

I have the same engine and trans in my '88 C1500, and yes, it does "kick down". I'll check the factory manual to see whether kick down is driven by the computer or a switch when I have time this evening.

JohnO356 · 02-03-2009, 08:21 PM

I can't find a definitive answer in the factory manual, and I'm not familiar with auto trans. I think you have to be a trans guy to use the manual, lol. The manual says there are external 12 volt connections, but doesn't go into detail.

About 6 months or more ago, I had a bad throttle position sensor that was causing the transmission to downshift. After I replaced the TPS the problem went away. I was talking with some friends, and the only thing we could come up with was that the TPS was sending the computer an incorrect speed signal thus causing the computer to tell the trans to downshift.

So, my guess is the computer must be involved with kickdown, but hopefully someone who really knows something will respond.

gk7771 · 02-05-2009, 12:29 AM

Thanks for the responses. Problem solved. Here's what I discovered. The kickdown is controlled by the computer; there's a relay on the firewall under the wiring cover.No manual switches. There's is a plunger type relay in the transimission; it plugs onto a round plastic connector which is on the side of the transmission the wire from the computer controlled relay plugs to the outside of this connector. The problem was that the plunger relay wire in the transmissin kept coming loose for the connector on the transmission body. It came loose because the connect needed some crimping for a tighter connection. This connection can be reconnected by pulling the round connector of the side of the transmission; the oil pan doesn't need to be removed. Test internal relay by running 12v+ to the connector. If you hear clicking the relay in the tranmission is good andthe problem is the relay on the firewall triggered by the computer.

JohnO356 · 02-05-2009, 08:05 AM

Glad you got it fixed, and I've learned something, also.

02-02-2009, 08:38 PM	#1
gk7771 Registered User Join Date: Aug 2002 Location: Oregon Posts: 6	TH400 kickdown question 1988 Chevy c/2500 with V-6 and a TH400 (all stock). I had the TH400 rebuilt and now I have no kickdown. The Tranny guy insists that he looked for a swith by the gas pedal and the intake and no switch. I believe in 1989 the computer triggered the kickdown. Is this true in 1988? I'm sure the tranny was kicking down before the rebuild. Why woudn't it be kicking down after. How can the circuit be tested? If its computer driven where is the wiring ? Help

02-03-2009, 02:42 PM	#2
JohnO356 Registered User Join Date: Sep 2005 Location: Linden, NC Posts: 232	Re: TH400 kickdown question I have the same engine and trans in my '88 C1500, and yes, it does "kick down". I'll check the factory manual to see whether kick down is driven by the computer or a switch when I have time this evening. Last edited by JohnO356; 02-03-2009 at 02:43 PM.

02-03-2009, 08:21 PM	#3
JohnO356 Registered User Join Date: Sep 2005 Location: Linden, NC Posts: 232	Re: TH400 kickdown question I can't find a definitive answer in the factory manual, and I'm not familiar with auto trans. I think you have to be a trans guy to use the manual, lol. The manual says there are external 12 volt connections, but doesn't go into detail. About 6 months or more ago, I had a bad throttle position sensor that was causing the transmission to downshift. After I replaced the TPS the problem went away. I was talking with some friends, and the only thing we could come up with was that the TPS was sending the computer an incorrect speed signal thus causing the computer to tell the trans to downshift. So, my guess is the computer must be involved with kickdown, but hopefully someone who really knows something will respond.

02-05-2009, 12:29 AM	#4
gk7771 Registered User Join Date: Aug 2002 Location: Oregon Posts: 6	Re: TH400 kickdown question Thanks for the responses. Problem solved. Here's what I discovered. The kickdown is controlled by the computer; there's a relay on the firewall under the wiring cover.No manual switches. There's is a plunger type relay in the transimission; it plugs onto a round plastic connector which is on the side of the transmission the wire from the computer controlled relay plugs to the outside of this connector. The problem was that the plunger relay wire in the transmissin kept coming loose for the connector on the transmission body. It came loose because the connect needed some crimping for a tighter connection. This connection can be reconnected by pulling the round connector of the side of the transmission; the oil pan doesn't need to be removed. Test internal relay by running 12v+ to the connector. If you hear clicking the relay in the tranmission is good andthe problem is the relay on the firewall triggered by the computer.

02-05-2009, 08:05 AM	#5
JohnO356 Registered User Join Date: Sep 2005 Location: Linden, NC Posts: 232	Re: TH400 kickdown question Glad you got it fixed, and I've learned something, also.

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