700R4 Transmission Cooler 1988-1994

Rusted, weeping cooler lines are one of the most common leaks on a 1988-1994 GMT400 pickup, and once the fluid gets low the transmission is next. This is a pre-bent replacement cooler line set for 1988-1994 Chevrolet and GMC K1500 4WD trucks with the 700R4, 4L60 or 4L60E. It is the Fine Lines TTC8801SS kit: two lines (A and B) in 5/16 inch T304 stainless steel, bent to the factory routing and flared with the threaded fittings in place.

What the cooler lines do on a 700R4 and 4L60E

Oil leaves the transmission from the converter circuit, travels to the cooler in the radiator tank and comes back to feed the lube passages for the bushings, planetaries and output. If a line leaks, the level drops until the pump pulls air. If it is pinched, the fluid runs hot and the gear train runs short of lube. On the GMT400 the lines run along the frame from the radiator back to the transmission, exposed to salt and debris for their whole length, so corrosion is the usual reason they fail. Stainless tubing resists that far better than plain steel.

Symptoms of bad cooler lines

• Wet fluid on the lines, the frame rail or the bottom of the radiator
• Pitting, flaking or pinholes in the tube
• Falling fluid level with a dry pan gasket and dry seals
• Slow engagement into Drive or Reverse, or slipping when the level is low
• Fluid overheating on a long pull because a crushed line cuts flow

How to confirm it before you buy

Wash the lines, idle the truck in Park and trace both lines from the radiator to the transmission looking for fresh wetness. Pinholes often only seep with the engine running and the oil hot. If the level has already been low for a while, read our 700R4 pump failure symptoms guide, because running low starves the pump and can leave you with whine or slow engagement even after the leak is fixed.

Fitment

This kit is made for 1988-1994 Chevrolet and GMC K1500 4WD trucks on the GMT400 platform with a 700R4, 4L60 or 4L60E. GM renamed the 700R4 the 4L60 and later brought in the electronic 4L60E, and the line routing stayed with the truck, which is why one kit covers all three. It is GMT400 only, so it will not fit the older square-body trucks. The kit is written for 4WD, so two-wheel-drive owners should compare the routing before ordering. Aftermarket radiators and auxiliary coolers change the line ends, so check those too, or send us the year, model and VIN. More parts for the family are on our 700R4 parts page, and our 4L60E vs 700R4 guide explains the version changes.

Install notes

• Catch the fluid and loosen the fittings with a flare-nut wrench; old steel nuts round off easily.
• Stainless tube is stiffer than mild steel. Feed the lines in from the end that has the most room and avoid kinking them while you work them over the crossmembers.
• Move the frame clips and isolators to the new lines so they cannot rub through.
• Thread every nut by hand first and tighten it snug; overtightening stainless fittings does not seal them better.
• Flush the radiator cooler if the old fluid was dark, refill with Dexron ATF, and check the level hot in Park.

Our 700R4 cooler line replacement guide covers the job in more detail. If the unit ran low and now slips or flares, it may need more than lines: a 1987-1993 700R4 master rebuild kit covers the soft parts, and a 700R4 TCC solenoid is worth replacing if converter lockup has been erratic.

Do these fit a 4L60E truck as well as a 700R4?

Yes. The kit is listed for 1988-1994 K1500 4WD trucks with the 700R4, 4L60 or 4L60E.

How many lines are in the kit?

Two lines, A and B, covering the run from the transmission to the radiator cooler.

What are they made of?

T304 stainless steel tubing in 5/16 inch diameter, uncoated, pre-bent and flared with fittings installed.

Will this fit my 1987 square-body or a 1995 truck?

It is built for the 1988-1994 GMT400 trucks only. Square-body trucks use different lines. For anything outside 1988-1994, send us your VIN before ordering.

I have a 2WD C1500. Will these work?

The kit is written for 4WD K1500 trucks. Compare the routing with your original lines or send us your VIN so we can check before you order.

What fluid should I use after replacing the lines?

Dexron ATF. Dexron-VI is backward compatible with the earlier Dexron grades. Check the level hot in Park after a test drive.

• 700R4 Cooler Line Replacement Guide
• 700R4 Pump Failure Symptoms and Replacement
• 4L60E vs 700R4 (4L60): What Changed and What Interchanges
• 700R4 Delayed Engagement: Causes and Fix