700R4 Speedo Issue

[farm truck]

I bought my 1970 GMC C2500 at the end of last year. The speedo has NEVER worked. The previous owner put a 700R4 and I have been thru about 8 speedo cables (and a new speedometer) trying to get her to work. No speeding tickets yet, knock on wood.

Anyone have a similar situation.... and maybe a fix? The last cable that I bought was from a 82 (or 84) blazer as it has that tranny in it. Every now and then the needle starts jumping around. But not consistently or accurately. I had a lousy tranny shop try to fix it. They charge me..... but it still don't work. They said that they changed the speedo gears in the tranny (forget what they are called). No dice, but my confidence level in there work is lower than whale ****.

Other than that:

New column, new 4 row radiator, 100 AMP chrome alternator, changed out the rotten battery tray. And PAID (oh how I paid) for a complete rewiring.

She' got a GM Performance 350 with a mild cam and roller rockers, hedman headers, glass packs, electronic ignition, new column, and a holley 670 CFM carb (5 miles to the gallon).

Funny thing, I used to buy this same truck for a grand. Now 15K in California gets you a "project".

Thanks in advance - Chris

Edited to add: I put a tach in her and a vac gauge. Tach works, vac don't.

 

[Matt69olds]

Pull the instrument cluster so you can see the end of the speedo cable. Raise the rearend so the wheels are off the ground, start the truck and put it in gear. If the cable spins, the problem is with the speedo head.

 

[farm truck]

Thank you, I will give that a try soon,

 

[GMT830]

Check the drive gear in the 700r4 where the mechanical cable bolts to the trans. You may have a gearing issue. Talk to a tranny shop to advise on the proper gears

 

[Kurnacopia]

I've been researching 700r4's for my 78 Scottsdale 4x4. In one video I saw the gear inside the transmission was worn and was causing a similar issue. I believe its nylon so it can wear out. If Matts advice doesn't pan out I feel GMT830 could be right. The gear inside is shot and will need to be replaced. Check out this video. 1:30

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[Matt69olds]

Another possibility, especially if someone has played around with this truck is mismatched speedo gear housings. If you look on the speedo housing you will see a series of numbers like “34 35 36…..”, that means that housing is for gears with that tooth count.

As the tooth count changes, the diameter of the gear also changes. The different housings position the gear in the right place.

The speedo gear shafts are different diameters to prevent mismatch. I suppose it’s possible to put a smaller shaft onto a larger housing.

 

[farm truck]

Check the drive gear in the 700r4 where the mechanical cable bolts to the trans. You may have a gearing issue. Talk to a tranny shop to advise on the proper gears

Yeah the tranny shop that replaced my column "said" that they replaced the gears. They did lousy work so I do not trust that they actually changed them. Meanwhile I have been staying in the slow lane so as to avoid getting tagged for speeding.

 

[GMT830]

I met a tranny shop owner at a bar one night and he explained his business like this.....
The average person might replace a transmission once in the life of the car or never in their lifetime. I have only one shot to make enough margin on every walk in customer to be profitable.
I'm not saying that I know anything about that business but coming from a shop owner I believe what he said. I've always exchanged and carried out my transmissions.

 

[farm truck]

Sorry for the late update, I changed out the driven gear in the housing on the trans...... works like a charm now!

Man I am lazy! No speedo for a year and it took all of 15 minutes to fix.

Edited to add: next step, EFI Sniper. I have a mechanic buddy that is going to help me with the installation.

Then I just need to save up my Sheckles to swap out the front drums for disc breaks and rebuild the sloppy front end, change out the steering wheel, and add back in air conditioning.

I sure hope my investment in mega millions tickets pan out!

 

[Matt69olds]

No desire to rain on your parade, but if you daily drive the truck, I’d strongly consider sticking with a carb, or going to the better Holley EFI.

I have a sniper on my Olds, it’s great when it works. It SUCKS when it doesn’t. If you have Facebook, join the Holley sniper group. Granted, most people won’t say anything if everything works the way it should, it’s the people who have trouble that complain. But it seems you see a lot of the same problems.

I have had 3 throttle bodies replaced under warranty,a fuel pump damage due to the throttle body, and one Holley distributor. The distributor cost me almost a $1000 for a UHaul truck and trailer to get the car home from Murfreesboro to central Indiana.

The sniper has the ECM, intake air sensor, and map sensor mounted in the throttle body. You can’t service those parts separately, you have to replace the throttle body. In my case, the ECM wasn’t powering the IAT or MAP sensors. Lucky for me, I was within a 100 miles of home, my insurance paid that tow.

I have a 1800 mile road trip planned for later this year, I’m kinda worried. With the Q-Jet, repair parts were a Autozone away. With the EFI, good luck. I’m seriously thinking about buying a spare throttle body for that trip.

 

[Matt69olds]

No desire to rain on your parade, but if you daily drive the truck, I’d strongly consider sticking with a carb, or going to the better Holley EFI.

I have a sniper on my Olds, it’s great when it works. It SUCKS when it doesn’t. If you have Facebook, join the Holley sniper group. Granted, most people won’t say anything if everything works the way it should, it’s the people who have trouble that complain. But it seems you see a lot of the same problems.

I have had 3 throttle bodies replaced under warranty,a fuel pump damage due to the throttle body, and one Holley distributor. The distributor cost me almost a $1000 for a UHaul truck and trailer to get the car home from Murfreesboro to central Indiana.

The sniper has the ECM, intake air sensor, and map sensor mounted in the throttle body. You can’t service those parts separately, you have to replace the throttle body. In my case, the ECM wasn’t powering the IAT or MAP sensors. Lucky for me, I was within a 100 miles of home, my insurance paid that tow.

I have a 1800 mile road trip planned for later this year, I’m kinda worried. With the Q-Jet, repair parts were a Autozone away. With the EFI, good luck. I’m seriously thinking about buying a spare throttle body for that trip.

 

[82sbshortbed]

Don't want a computer on my square. That's one of the many reasons I bought it. Quadrajet works great for me.

 

[Matt69olds]

Don't want a computer on my square. That's one of the many reasons I bought it. Quadrajet works great for me.

I regret the Sniper purchase. I really wish I had left it alone.

Unfortunately, it won’t be cheap or easy to go back. I still have the Q-Jet, but I sold or gave away all the other stuff that was on the car.

If it leaves me stranded one more time, I’m going back to the carb, regardless of the time or expense.

 

[Randy and Easton]

…… the quiet guys in the group will take all that and keep things simple as she came. New edelbrock performer intake, and carb (finally with electric choke) and an HEI ignition…

Learn more by listening on here for sure! Great weekend y'all

Randy and E

 

[farm truck]

I’d strongly consider sticking with a carb, or going to the better Holley EFI.

Which Holley EFI model are you referring to? Mine is Part # 550-510K and she is finally shipping. I can say that I am more than a little trepidatious, pending the install and outcome.

I appreciate your experience and advice.... even if it may not be what I want to hear! Thank you also for the facebook group reference, hopefully I won't be on there much!