As with any automotive component, the TPS can wear out or simply become dirty causing the engine to "buck" during exhilaration or cause a rough idle. The rough idle is generally more noticeable when the car is in gear, such as reverse or drive.
Mercedes/Bosch no longer offer new throttle position sensors for the Bosch D-Jetronic fuel injection system used in the early to mid 70's.
If you are in need of a "new" unit, used units are available, but be sure you know they are working and have been checked if going that rout. Several ads on eBay suggested the unit was "working" when it was removed. For me, "working" does not tell me much. Mine is "working" but not as it should. Can I get the car down the road and onto the highway with my unit? Yes, but it's going to be a less than smooth, performance based trip.
In this article I will cover removing, checking, and cleaning the TPS from my 1975 Mercedes-Benz R107 which is not working properly.
If you are going to engage in this, I would have a known working unit on hand incase yours breaks in the process of opening it up. Also, be forewarned, make sure if you are ordering one whether used or NOS, that you order and receive the correct TPS for your car. Bosch produced two unites for Mercedes, one for Federal cars and the other for California cars.
The throttle position sensor is located on the side of the throttle body and held on by a spring clip.
STEP 1
Remove the 2 locking screws, then use a flathead screwdriver and gently pry off TPS at the shaft from the back. NOTE: It will make life much easier to remove the throttle body from the engine. Just don't drop the 4 bolts into the engine compartment. You will not get those back.
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| TPS connected to throttle body with cover removed. |
STEP 2
This is the step that can very easily break the cover so be very careful releasing the cover. Now examine the contacts. The throttle position sensor shown is from my 450SL and as you can see is no longer serviceable. The make-and-break contacts are worn through, causing a very erratic idle and throttle up between 0 mph and 40 mph.
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| Hard to see in this photo, but some of the gold is worn through to the backing. |
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| The red circles highlight where the gold is worn through causing the "short" in the circuit. |
STEP 3
Based on the outcome of your investigation, the unit will either need to be replaced or cleaned. In this case it is no good.
STEP 4
To clean, take a cotton swab with electronic cleaner and gently wipe over the surface of the contact board. If debris remains, take a white vinyl eraser and GENTLY erase the dirt from the contact area of the board. There can be no marks between the contacts or there will be a potential for short-circuiting.
STEP 5
Calibrating unit.
I will do a subsequent article on how to test and adjust TPS.
