Timing belt warning light

Timing belt warning light

>the dreaded t belt warning light lit up [without an actual tension problem]

Strange you should ask about this, because I just got a copy of PCNA Technical Bulletin number 9010, dated April 12, 1990 on troubleshooting the belt tension indicator.

If the indicator comes on but the belt tension is OK, they recommend the following steps:

- disconnect the electrical wire for the belt tensioner and bridge it to ground, then start the engine

- if the indicator comes on after three minutes or so of running, stop the engine, then:

-- check the continuity of the wire from the tensioner to the central warning control unit (black plug terminal 9 on '85-88 cars) or the instrument cluster (plug 3, terminal 4 on '89+ cars)

-- if the continuity is OK, replace the control unit or instrument cluster

-- reconnect the wire on the belt cover and verify correct indicator operation

- if the indicator did not come on at all with the wire bridged to ground and the engine running, stop the engine, then:

-- go to the tensioning roller carrier and check the brass spring contact bridge and the wire connector

-- go to the belt tensioner housing and check for leaks and proper oil level

-- reconnect the wire on the belt cover and verify correct indicator operation

The warning system needs a electrical path to ground to keep the light turned off. I'd look at the connection of the wires to the copper "tab" on the tensioner arm, if these are loose....
Also on my 85 I found corrosion between the plunger and tensioner, this might be a prob. if enough resistance built to cause an open circuit. I played around with a ohmmeter when I had mine apart & found the switch would open at about 3 - 3.4 on the belt tension guage...vs. 5-5.3 for correct adj.
Dave P.

False alatms can be caused by a loose connection in the wire to the warning light switch. The warning light comes on some time after the computer senses a continuous or briefly repeated loss of ground connection through the switch on the tensioner arm. A loose connection will fool the computer. There are two connections right at the switch - a round pin on the front of the lower timing belt cover right behind the oil dipstick tube, and a flat connector just inside the cover where the wire attaches to the metal strap on the tensioner arm. These connections can be checked (with some difficulty) by taking the top cover off. I can only assume that either the Porsche tech didn't get the word on what the problem was with the car (i.e., "Tension the belt," rather than "The belt tension light keeps coming on even though the belt has just been tensioned".), or, the tech wasn't very familiar with 928s. We have seen cases where people kept tightening the belt until the extreme tension wore out all of the sprockets. I would go back to the dealer and see if I couldn't get the problem fixed, rather than fixing the symptoms. Especially if it is the same dealer who did all the work, I think that they owe you a solution. The belt warning light system is a valuable addition to the 928, and can save your engine. It should not be ignored.

Wally Plumley
928 Specialists

[In response to an '86 getting occasional false T-Belt lights, Wally wrote:]

The tension warning light works thru a grounding switch. As long as the belt tension is adequate (notice that I didn't say "correct"), the spring on the tensioner push rod is compressed, and the ground connection is maintained. If the tension is lost, the spring expands, and the ground connection is lost.

There are several possible answers for your problem. Since you have rebuilt the tensioner, and presumably it is full of 90 wt gear oil for dampening and heat transfer, the most likely suspect is a loose or high resistance connection in the wire going to the fault warning computer, or a bad fault warning computer.

Go to the central electrical panel under the passenger's floor. Look at the rows of wire connections at the bottom, using a good light. You should find, molded into the plastic, numbers and letters, identifying the connectors. Find L22, which should be a blue/brown wire. Use a jumper and a straight pin or something similar to attach this blue/brown wire securely to a good ground connection. Drive the car. If the tension warning light comes on, it is the connection at the central warning box, or the fault control computer itself. If the light doesn't come on, the problem is the wire to the tension switch.

Wally Plumley
928 Specialists

> I just checked my tension after seeing the warning
> indicator. What I found was that the tension was
> 5.0~ I assume this means I am getting a false
> warning.

I had a seizing water pump cause the warning light to come on - mine did this for about 6 weeks before it
finally went out and neither I or the dealer could figure out why the light was coming on as the tension as OK.

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