What is the other relay for? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

turbosix · 02-28-2015, 09:22 AM

Does anyone know why some 87's have two relays on the firewall. I know one is for the fuel pump, but what is the other one for?
Thanks

buddy_1 · 02-28-2015, 11:13 PM

If it has an M40 trans it's a downshift relay.

turbosix · 03-01-2015, 08:25 AM

Thanks. I've be a chevy guy for a long time and had never heard of M40. Learn something new everyday.

4x4toy · 03-01-2015, 11:31 AM

M40 is the RPO code for the TH400 transmission.

turbosix · 03-01-2015, 12:54 PM

I'm surprised to see a TH400 in a shortbed 1/2 ton. Seems overkill.

ray_mcavoy · 03-01-2015, 07:29 PM

Yeah, I've seen a few 87 1/2 tons with a TH400 behind a 305. Definitely overkill, but I think TH350's had been phased out by 1987, leaving the TH400 as the only non-overdrive automatic available.

Crabass · 06-14-2025, 09:00 PM

Late to this party, I know, but have a footnote for this topic someone might find of some use. I have a '87 shortbed 2wd with the 4.3 and a TH375 from the factory. Made the discovery when it came time to replace the output shaft seal. Prior to that was curious why the hd tranny was in such a light truck.
Planning to replace it with a manual 5 speed so if anyone needs one it will eventually be available at a really good deal (includes the relay!).

02-28-2015, 09:22 AM	#1
turbosix Registered User Join Date: Nov 2009 Location: Lake Jackson,Texas Posts: 126	What is the other relay for? Does anyone know why some 87's have two relays on the firewall. I know one is for the fuel pump, but what is the other one for? Thanks __________________ Tommy 1 Pt. 3:21 '72 C10 LWB 402

02-28-2015, 11:13 PM	#2
buddy_1 Registered User Join Date: Dec 2013 Location: Humble, Texas Posts: 185	Re: What is the other relay for? If it has an M40 trans it's a downshift relay. Attached Images __________________ I know the "VOICES" aren't real but they have some great ideas. 1978 GMC Sierra Classic C25 1983 Datsun 280ZX Turbo 1997 Chevy Cheyenne K2500 1999 Polaris Sport 400

03-01-2015, 08:25 AM	#3
turbosix Registered User Join Date: Nov 2009 Location: Lake Jackson,Texas Posts: 126	Re: What is the other relay for? Thanks. I've be a chevy guy for a long time and had never heard of M40. Learn something new everyday. __________________ Tommy 1 Pt. 3:21 '72 C10 LWB 402

03-01-2015, 11:31 AM	#4
4x4toy Registered User Join Date: Jan 2009 Location: Southern Md Posts: 228	Re: What is the other relay for? M40 is the RPO code for the TH400 transmission. __________________ 04 Yukon XL 2500 w/ Quadrasteer 61 Chevy C60, 261 inline six 86 Chevy C20 2wd, Banks 6.2 05 GMC Sierra 2500hd Duramax

03-01-2015, 12:54 PM	#5
turbosix Registered User Join Date: Nov 2009 Location: Lake Jackson,Texas Posts: 126	Re: What is the other relay for? I'm surprised to see a TH400 in a shortbed 1/2 ton. Seems overkill. __________________ Tommy 1 Pt. 3:21 '72 C10 LWB 402

03-01-2015, 07:29 PM	#6
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,404	Re: What is the other relay for? Yeah, I've seen a few 87 1/2 tons with a TH400 behind a 305. Definitely overkill, but I think TH350's had been phased out by 1987, leaving the TH400 as the only non-overdrive automatic available.

06-14-2025, 09:00 PM	#7
Crabass Registered User Join Date: Nov 2006 Location: Virginia's Eastern Shore Posts: 92	Re: What is the other relay for? Late to this party, I know, but have a footnote for this topic someone might find of some use. I have a '87 shortbed 2wd with the 4.3 and a TH375 from the factory. Made the discovery when it came time to replace the output shaft seal. Prior to that was curious why the hd tranny was in such a light truck. Planning to replace it with a manual 5 speed so if anyone needs one it will eventually be available at a really good deal (includes the relay!).



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