NHW11 Accelerator Rebuild

I just finished rebuilding another NHW11 accelerator and the following chart shows what happens to the encoder:

As returned, the low end of the accelerator resistance range shows great variability. The upper end is linear. Thanks to Doug's pioneering work, rebuilding restores the low end resistance range. Retensioning the pickup wipers works every time.

This data suggests the low end resistance traces are worn down and certainly you can see metalized traces from the pickup arms. However, there is no evidence of trace wear in the resistance traces. In the half-dozen accelerators I've rebuilt, I keep looking for any wear material but I don't see it.

I'm beginning to think there are two effects:

• weak pickup arms - either they lose tension over time or were not tensioned enough when first assembled.
• metal oxide development - the polished sheen suggests the pickup fingers are depositing a layer of metal, a metal film, that may oxidize into an insulating coating. The weak pickup arms are not strong enough to wipe the oxide off.

I've gotten another failed accelerator so I'm thinking of using a whisker sensor to measure surface resistance across the resistance traces:

You can clearly see the three, metalized traces from the pickup arms on the resistance layers.

Bob Wilson

 

Doug Schafer posted the instructions in the YahooGroup, "toyota-prius-sat2" in the files section.

My standard terms are:

• $250 - covers refurbished accelerator and core charge
• $100 - refunded core charge when old accelerator returned

After testing the returned accelerator, I swap it in my car to make sure it is working. When a new customer comes around, I send them the one in my car and wait a couple of weeks for them to return the failed one. Yes, I have some good accelerators including one from a North Carolina salvage as well my original one.

Bob Wilson

 

In fact here is one of the earliest accelerators, before and after:


You can see how similar it is to this latest one:

It is an important symptom consistent with oxidized, metal traces. The test jig made it possible to quantify the effect:

After each accelerator encoder is repaired, I use the jig to confirm the fix. After all, the next place is my car for quality testing. <wink>

On this last one, I took the encoder out and without touching the arm changed the resistance by pressing on the light gray cover. This let me know we are dealing with small dimensions and/or a coating between the pickup arms and the resistance traces.

I still wiped down the resistance traces with alcohol but the swab came up clean, again. Some metal oxides are transparent so I'll have to figure out a chemical test. The whisker probe may give a clue.

From a typical material data sheets for contact cleaners list:

• Decafluoropentane (HFC-43-10mee)
• Methyl nonafluoroisobutyl ether
• Methyl nonafluorobutyl ether
• 1,1,1,2-Tetrafluoroethane (HFC-134a)

Mostly these look like organic solvents and alcohols. So I suspect they are really used to remove finger prints rather than metal oxide deposits. Please double check a set of the material data sheets for these contact cleaners but this is how I see them. This is one of those times I wish I had my Dad's old microscope.

It may be possible to rig up a 'clamp' (call it a gadget made of deformed washers, model plywood, machine screws and nuts) to hold the cover on the housing. If it works, we should be able to:

1. test the encoder, baseline
2. remove cover
3. replace the cover
4. retest encoder and get similar to baseline values
5. one-by-one, retension the pickup arms and six base contacts

The more we understand about the failure mechanism(s), the better we'll be able to validate the fix.

Bob Wilson