Seat Heater Repair

(1986 Audi 4000 CS Quattro with Velour Sport Seats)

This file provided by: Jeff Goldberg, on or about 9/13/99.
Copy this file freely, credit given to Jeff Goldberg [98%] with the assistance of Huw Powell [2%] and mention of this web site.
You may reach Jeff at: jeffgold@erols.com

The seat heater system description which follows relates to the diagnosis and repair of faults specifically in the seat heater elements and related components internal to the seat. I hope faults in externally related system components can be isolated, based upon the information provided herein, using a process of elimination.

The heater system in each seat consists of three circuits. A heater element in the seat bottom (1) wired in series to a heater element in the seat back (2) and a thermostat circuit which attaches to a small thermostat device which resembles a watch battery in size and shape (3).

My experience has found the heater elements in perfect shape, with all problems at the point where the wires from the under seat plug meet the seat heater pad . The seat bottom has all such connections buried at the rear, center of the seat cushion. The seat back has the connections at the bottom , center of the cushion.

After loosening the wire binding at the lower circumference of the seat upholstery, I peeled back the velour upholstery, removing the hawg ring retainers using a small wire cutter to hold the ring and a screwdriver to spread the ring open. I re-closed the rings with a pliers. A hawg ring tool is said to make this job much easier but "who needs easier"!?

The solder joints I have described as the problem area are at the very rear of the cushion, in the center. Also, the "thermostat" is located in this area, under a circular cloth patch which is glued the seat upholstery and very difficult to remove. Once the patch is removed, you must feel and pinch the patch carefully as the thermostat is sandwiched inside and is small enough to not be easily located. But it's in there for sure

The thermostat is a heat sensitive "resistor" type device that reduces the voltage sent from the relay from 8V down to 0V, at which point the relay opens and eliminates power to the seat heater elements. The relay can be heard clicking at the point at which it opens and closes based upon input from the thermostat. Mine was clicking erratically, which alerted me to the fact the wire connection to the thermostat was faulty. I removed the thermostat "button" from the patch and resoldered newly fashioned wires to each side of the thermostat and connected them to the proper wires from the seat plug. This device is very important because the seat elements can be functioning properly, but if the thermostat circuit is faulty, the seats will not heat. It is a good idea to check this circuit as the first troubleshooting step. Might save you a lot of time.

After checking the thermostat, check all solder connections (2) to the heater element. Small blue heater element wires connect to the solder terminals and have a tendency to develop faults (IME). Look for scorching around the small blue wires and check for voltage in and out of the circuit. If the fault is not at the solder joint, you may have to carefully cut the protective foam covering and look for scorching throughout the heater circuit by carefullyremoving the flannel like element insulator material. Listers have reported that it is best to twist faulty element wires together as they do not solder well. The insulation material should be replaced as well as possible when done.

Reinstallation is the reverse of removal (Bentley).

The seat back upholstery is much easier than the bottom cushion. NO HAWG RINGS. Just a few metal hooks at the seat back base, easily accessible when the seat is fully reclined. Just bend the hooks open and release the steel bars embedded in the seat back upholstery. Then the seat back cushion will swing freely allowing access to the heater pad inside. Checking this element and the solder connections is similar to the seat bottom. The solder terminals are at the bottom center of the pad - (what a relief - easy access).

Reinstallation is the reverse of removal (Bentley).

Note: Voltage and resistance are measured with a multi meter. Voltage tests are done with the under-seat plug connected to the seat. Resistance measurements should be done at the seat plug with the plug not connected to the relay .